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An electronic circuit is a combination of electronic devices like resistors, capacitors and transistors. The function of a circuit can be amplifying, oscillating, switching, controlling etc. These basic circuits or stages can be combined to an electronic equipment.
The electronic circuit runs with low voltage DC but can control high voltage AC applications. A power supply which transforms the AC high voltage to to the needed DC low voltage is always needed. To understand an electronic equipment it is important to identify the basic circuits and know their functions.


目录（直接点击目录直接到达）

• Principles
• Resistors
• Ohm's law
• Varistors
• Car batteries
• Halogen bulbs
• Capacitors
• Diodes
• Rectifiers
• Zener diodes
• Power supplies
  

孟想成真

Ohm's Law and Resistor Application
DC voltage
In electronics we usually deal with low voltage DC (Direct Current). Supply voltages between 5 V and 24 VDC are common. Very often electronic circuits are designed for 12 V.
Supply cable in electrical installation are called phase (P) and neutral (N). In DC the two supply cables are plus (+) (red) and minus (-) (black).
In general minus is the common connection. Often it is grounded and connected to the metal housing of the equipment.
In order to run such a circuit on mains voltage a power supply is needed. The power supply converts the high voltage (e.g. 230 V) mains AC (Alternating Current) into low voltage DC, for example: 230 VAC → 12 VDC.

AC voltages can be transformed (making a voltage smaller or bigger) by means of a transformer without much losses. A powerless transformation of a DC voltage is not that easy. An AC voltage can also easily converted into a DC voltage by a rectifier. The other way round, converting a DC voltage into an AC voltage is also not that easy.

The voltage V is measured in V (Volt). Electronic circuits usually run on DC.

A battery is a typical DC power source. A battery is a connection of several single (battery) cells.

Measuring a voltage
The voltage can be measured with a voltmeter or a multimeter which includes a voltmeter. Digital voltmeters are easier to read but not necessarily more accurate. Only a calibrated voltmeter is precise, no matter if it is analogue or digital, cheap or expensive.

Clamp and tips for measuring voltages.

The measurement current through the voltmeter is extremely small so that the measurement leads can be thin and the the tips or clamps small.
Because an existing voltage is a condition for an electric circuit, the first measurement in a defective circuit is always the checking of the operating voltage.

Electric circuit
An electric circuit consists of a voltage source and a load. If the circuit is closed a current can flow.

Simple circuit with voltage source and a light bulb.

The battery delivers the voltage V to the bulb. A current I can flow. The current depends on the voltage and the resistance R of the bulb.

Here the equivalent circuit with a resistor.

The voltage of the resistor is measured across the resistor.
The voltage of the power supply is measured across the power supply.

In this case the voltage across the resistor and the voltage of the supply supply is the same.

Current
The first condition for a current is an existing voltage, second the closed circuit.
The current in a closed circuit is everywhere the same and can be measured everywhere.
Is the voltage DC also the current is DC.

In a simple circuit the current is everywhere the same.

The current I is measured in A or mA (Ampere).

Measuring a current
The current can be measured with an ammeter which is always integrated in a multimeter.
Compare to voltmeters the quality of the leads, the contacts and the clamps are import because the current of the whole circuit flows through the ammeter, the leads and connectors. The higher the current, the bigger the connectors, cable and clamps have to be.

Appropriate clamps for measuring currents.

A small voltage measurement clamp was used for current measurement. The high current destroyed the clamp.

Tips and clamps for voltage measurement must not be used for current measurement.

In electrical engineering clamp meters are common. The measurement happens contact-less through the electrical field around the conductor which depends on the current. Clamp meters are not very precise and can be only used for high currents.
In electronics the measurement of the current through a resistor is not very common. Because the current depends on the voltage and the resistor, the current can be measured indirectly by the voltage across a resistor (Ohm's law).

Resistors
Every electrical or electronic device has a electrical resistance. Even cable have resistances. Those cable resistances depends on the lengths, the diameter and the material of the cable.

In electronic circuits resistors are found everywhere. They are available in different values, sizes and shapes.

The resistance R is measured in Ω, kΩ or MΩ (Ohm).

For troubleshooting electronics we are interested in the resistance of a resistor, the voltage across the resistor and the current through the resistor. The current can be determined by measuring the voltage across the resistor and the calculation with the resistance itself (I = VR / R).

The resistance R has influence on the voltage across the resistor (VR) and the current through the resistor (IR).

Measuring a resistor
The resistor can be measured with an ohmmeter which is always integrated in a multimeter.
For the measurement the resistor has to be disconnected from the circuit. Otherwise the rest of the circuit with its resistances influences the measurement result.
The same applies when holding the resistor in hands. On one side the probe and the resistor connection wire can be touched but not on both sides because the body resistance will be in parallel with the resistor on test.

Defective resistor
Typically resistors get burned. They get high-ohmic (open connection) because of excessive current. This means: When a resistor got burned, there must be a reason for the higher current somewhere. Often it is a semiconductor (transistor, FET...) which got a short and produced the abnormal high current.

Ohm's law
The resistance R of a resistor, the current I trough the resistor and the voltage V across the resistor are related together and are expressed in Ohms law:

V = R x I

If two of the three values are known, the third can be calculated.
The three variations of this formula are:

V = R x I        or         I = V / R        or        R = V / I

Important:

The values for the units are V, A and Ω.
kΩ can be used if the current is rated in mA.

Ohm's law is the most important formula in electronics. Everybody who works in the field of electronics has to know the law by heart. To remember the formula there is a simple method:

Imagine or draw a pyramid with the three values. The voltage V has to be up.

If you now cover with a finger the value you are looking for, the correct constellation of the formula will be shown.

Example 1:   A resistor at a power supply of 12 V creates 1 A.
                   What is the value of the resistor?

                  R = V / I            R = 12 V / 1 A            R = 12 Ω

Example 2:   What current creates a resistor of 120 Ω at the same power supply?

                  I = V / R            I = 12 V / 120 Ω          I = 0.1 A or 100mA

Example 3:   Through the same 120 Ω now flows only 41.7 mA.
                   What has happened to the power supply?

                   V = R x I           V = 120 Ω x 0.05 A        V = 6 V
                   The voltage of the power supply has changed to 6 V.

Short circuit
In the following circuit the resistor is bridged. The resistor now is 0 and the current is maximum. The battery will get damaged and wires and battery can burn. To prevent this dangerous situation every power supply and every equipment has a fuse.

The resistance is 0. The current is maximum.                      The voltage is 0.

Note:
A blown fuse has a reason. Do not only change the fuse. Find the reason for the high current first.

Open circuit
In an open circuit no current can flow and no current can create a voltage drop across the resistors.

The current is 0. The voltage across the resistor is 0. The voltage in front of the resistor and behind is the same.

Series connection of resistors
In a circuit with two or more resistors in series there is only one path for the current to flow: Through both resistors. The current through both resistors is the same.
But because the resistors might be different the same current creates different voltage drops across the resistors. The voltage drops across the resistors are different.
The voltage drops can be easily calculated by using Ohm's law.

The current through both resistors is the same. The voltage drops across the resistors can be calculated when the resistor values are known.

The partial voltages can be added. V1 + V2 = 12 V

For the circuit above we know the overall voltage and the two resistors.

First we can calculate the current through both resistors:   
I = V / Rtotal         I = V / (470 Ω + 220 Ω)        I = 17.4 mA

Then we calculate the voltage drop across each resistor:
V1 = R1 x I        V1 = 470 Ω x 0.0174 A        V1 = 8 V
V2 = R2 x I        V2 = 220 Ω x 0.0174 A        V2 = 3 V
(or V2  = V – V1    V = 12 V – 8 V)

In practice the minus probe of the voltmeter is connected to ground or common and remains there. Measurement are only made by using the plus probe of the multimeter. We are measuring against ground. Because the voltage drops are part of the overall power supply voltage can be easily calculated.

The voltage V1 is the difference of the power supply voltage and V2. In other words: The voltage in front of a resistor minus the voltage behind the resistor makes the voltage drop across the resistor.

Parallel connection
In a parallel circuit all resistors are parallel to the voltage source. The voltage drops across the resistors are the same.
But because the resistors might be different the currents through the resistors are not the same. The currents through the resistors are different.
These currents can be easily calculated by using Ohm's law.

The voltage across both resistors is the same. The currents through the resistors can be calculated when the resistor values are known.

The partial currents are added to the total current. I1 + I2 = Itotal

For the previous circuit we know the overall voltage and the two resistors.
The total current is not known.
To get the total current we have to calculate the 2 partial currents and add them.

I1 = V / R1        I1 = 12 V / 470 Ω        I1 = 25.5 mA

I2 = V / R2        I2 = 12 V / 220 Ω        I2 = 54.5 mA

Itotal = I1 + I2        Itotal = 25.5 mA + 54.5 mA    Itotal = 80 mA

The total current is 80 mA.

We can also calculate the total resistor and then the total current.

Rtotal = 1/ (1/ R1 + 1 / R2)        Rtotal = 1/ (1/ 470 Ω + 1 / 220 Ω)         Rtotal = 150 Ω

Itotal = V / Rtotal             Itotal = 12V / 150 Ω             Itotal = 80 mA.

Also here: The total current is 80 mA.

LED project 1
A LED always needs a series resistor for limiting the current because LED do not work with an operating voltage like bulbs but with an operating current.

A series resistor is a must for every LED application.

In this example a 3mm LED with 20 mA at 2.5 V is used.
When we want to use this LED as a indicator for a 12 V application the series resistor has to be calculated like this:

The overall voltage is 12 V. The current is everywhere the same and should be 20 mA. The voltage across the LED should be 2.5 V.

For calculating the value of the resistor we need the voltage drop across the resistor and the current through the resistor.

The current is already known: 20 mA
The voltage across the resistor can be calculated: 12 V – 2.5 V     VR = 9.5 V
Now the resistor:     R = V / I        9.5 V / 0.02 A        R = 475 Ω

According to the E-12 series the next suitable resistor has 470 Ω.

This small circuit can be used as a voltage indicator. It can show if a voltage exists or not. This is sometimes helpful in electronics when just have to be checked rather than exactly measured. In digital electronics for example high or low signals just have to be detected and a digital multimeter is often to slow for fast voltage changes. A simple LED with the series resistor mounted in an old ball pen is a great help for detecting this voltages or signals.

The LED and the series resistor are integrated in the ball pen housing. The old ball pen tip acts as a probe and a short cable with a clamp closes the circuit and is connected to ground.

Such an indicator is also a cheap and good help in car repairs where only 12 V voltages have to be detected and traced.

Power law
A current through a resistor or a load does not only creates a voltage drop but also power loss. This power loss is heat. The higher the voltage drop or the current the bigger the heat.
A bigger resistor (in size, not in value) is able to transfer heat better to the surrounding air. It gets less hot. The power dissipation (wattage) depends on the size of the resistor.
Beside the resistor value we also have to make sure that the resistor size or the power rating of the resistor is correct.
In practice a resistor calculation is always both: A resistance calculation according Ohm's law and a power rating calculation according the following power formulas:

P = V x I        or         P = V2 / R        or        P = I2 x R

Unfortunately these three formulas have also three variations each. That means 9 formulas have to be learned or the technique of converting a formula has to be known.
But there is a little trick. Because of Ohm's law every missing value can be expressed by the others. That means, we only have to know one power formular:

    P = V x I

If e.g. the resistor is given but not the current, we first calculate the current with Ohm's law and then take the power formula.

Also here a pyramid can be used – with P in the top.

Example 1:    Back to our LED and the series resistor.
                   The current was 20 mA and the voltage across the resistor was 9.5 V.
                   The power dissipation of the resistor is:

                   P = V x I        P = 9.5 V x 0.02 A        P = 0.19 W

                   A common ¼ W resistor can be used.


Example 2:    A light bulb for 230 V has 100 W.
                   How high is the current?

                   P = V x I    I = P / V    I = 100 W / 230 V    I = 0.43 A


Example 3:    A heating element for 230 V has 353 Ω.
                   What wattage has the heating element?

                  P = V x I   
                  I is not known, but can be calculated:     I = V / R     I = 0.65 A
                  P = V x I        P = 230 V x 0.65 A            P = 150 W

LED project 2
The power supply of a microscope is defective and can not be repaired. The alternative can be a conversion from halogen bulb technology to a white LED system.
A high power LED Luxeon Star has an integrated lens which focusses the light beam perfectly for this purpose. The LED has the following specifications: 350 mA at 3.42 V.
It is planned to use a common Nokia mobile phone charger as a power supply (5.7 V, 4 W)

Problem: Can the charger be used and what series resistor has to be taken?

1. The maximum output current of the power supply can be:
    Imax = P / U        Imax = 4 W / 5.7 V        Imax = 0.7 A

    The charger can deliver 700 mA. The LED only needs 350 mA.
    Yes, the charger can be used.


2. For calculating the value of the series resistor for the LED we need the voltage across the
    resistor and the current through the resistor.
    The current is already known: 350 mA

    The voltage across the resistor can be calculated: 5.7 V – 3.42 V     VR = 2.28 V
    Now the resistor:     R = V / I        2.28 V / 0.35 A        R = 6.5 Ω

    According to the E-12 series the next suitable resistor has 6.8 Ω.


3. The resistor value is found, but will a standard ¼ W type be big enough?

    The power dissipation of the resistor is calculated by the current through the resistor
    multiplied by the voltage across the resistor.

    P = V x I        P = 2.28 V x 0.35 A        P = 0.8 W

    A standard resistor is NOT big enough. We need a 1 W type.


LED project 3
For a lot of examinations with the microscope, it is necessary to reduce the brightness of the light. For reducing the light we simply can add another resistor or better we make it variable and take a potentiometer.
Please note that we add the pot and not replace the existing resistor. We still need the series resistor for limiting the current through the LED when the pot is set to the minimum
(0 Ω).

A pot in addition to the resistor-LED-combination reduces the brightness by reducing the current through the LED.

The question now is: What is the value of the pot?
The answer depends on the voltage and the current of the LED under reduced conditions. We have to test it. By taken a variable power supply and a multimeter we have found out that the specification for the lowest brightness are:
I = 50 mA at a voltage of 3.3 V across the LED and the series resistor.
For calculating the maximum value of the pot we need the voltage across the pot and the current through the pot.

    The reduced current is: 50 mA
    The voltage at the LED together with series resistor is: 3.3 V

    The voltage across the pot can be calculated:    5.7 V – 3.3 V     VR = 2.4 V
    Now the resistor:     R = V / I        2.4 V / 50 mA        R = 48 Ω

    According to a catalogue the next available potentiometer has 47 Ω.


Unfortunately such a small value is not very common and we might not get such a pot. But maybe it is not necessary to limit the brightness continuously. Maybe three or four different brightness steps are enough. In this case a rotary switch with different resistors is a good solution.
If we have a position left we can even use it for switching off the light.

A rotary switch with 6 positions gives 5 brightness steps and an off position. Turning the switch to the right gives more brightness (the resistors get smaller). To the very right no additional resistor is in use. The brightness is maximum and just limited by the 6.8 Ω resistor.

Here the result is the same but the realisation is different. The resistors are added up as far we turn to the left.

孟想成真

Resistors

A resistor is an electronic device which has a specified amount of electrical resistance. The resistor has two terminals and works in both directions. It has no polarization.
The primary characteristic of a resistor is its resistance (Ω) and the power rating (W).

Resistors are usually made out of carbon. Resistors for higher wattages are made out of resistance wire and a body of cement. High precision resistors are metal film resistors.

Wire resistor with 11 W. Wire resistor with 5 W. Carbon resistor with 2 W. Common carbon resistor ¼ W. Chip resistor or SMD (Surface mounted device)

Units, values and symbols
The symbols for resistors in circuits diagram are shown below. Notice that American symbols are different.

Resistor, European and American

In formulas the letter R is used for the resistor and the unit is Ω (Ohm). To keep large numbers small and handy the units are used in conjunction with the SI prefixes.

1 000 Ω is 1 kΩ
and
1 000 kΩ is 1 MΩ

In circuit diagrams very often the dot is replaced by the R or Ω.

47K    = 47 KΩ
1K5    = 1.5 KΩ
1M0    = 1.0 MΩ
2R2    = 2.2 Ω
0Ω22    = 0.22 Ω

The resistors R9 and R14 have the value of 4k7 or 4.7 KΩ. All resistor without any wattage information are common ¼ W resistors. Otherwise it is mentioned. Like the two 5W-types R12 and R13.

Exercises:

To see the answer just the space behind the values.

Transfer in KΩ:  1 MΩ   1000KΩ
                        2K2   2.2KΩ
                        560 Ω   0.56KΩ
                        3,300 Ω   3.3KΩ

Transfer in Ω:    2.7 KΩ   2700Ω
                        56 KΩ   56,000Ω
                        120 KΩ   120,000Ω
                        2Ω7   2.7Ω

Preferred values
Resistors are not available in all possible values and gradations but only in selected values. The industry provides a specific range of standard values, known as preferred values. The most common group of preferred values is the E12 series with 12 different numbers and their multiples. The gradations are:

10  12  15  18  22  27  33  39  47  56  68  82

Example:    Available resistor are: 33 kΩ, 150 Ω , 2.2 MΩ, 82 Ω
                But the following resistors do not exist: 74 kΩ, 14 MΩ, 460 kΩ, 21 Ω

All resistors of the E12 series are common types with 5%.


Beside the E12 series a E24 with 24 values and even a E48 with 48 values exist. Because the gradation is smaller the series consist of only precise resistors with smaller tolerances. The resistors are metal film resistors with 2% or 1%.

E-12 Series (5%)
1 Ω 1.2 Ω 1.5 Ω 1.8 Ω 2.2 Ω 2.7 Ω 3.3 Ω 3.9 Ω 4.7 Ω 5.6 Ω 6.8 Ω 8.2 Ω	10 Ω 12 Ω 15 Ω 18 Ω 22 Ω 27 Ω 33 Ω 39 Ω 47 Ω 56 Ω 68 Ω 82 Ω	100 Ω 120 Ω 150 Ω 180 Ω 220 Ω 270 Ω 330 Ω 390 Ω 470 Ω 560 Ω 680 Ω 820 Ω	1 kΩ 1.2 kΩ 1.5 kΩ 1.8 kΩ 2.2 kΩ 2.7 kΩ 3.3 kΩ 3.9 kΩ 4.7 kΩ 5.6 kΩ 6.8 kΩ 8.2 kΩ	10 kΩ 12 kΩ 15 kΩ 18 kΩ 22 kΩ 27 kΩ 33 kΩ 39 kΩ 47 kΩ 56 kΩ 68 kΩ 82 kΩ	100 kΩ 120 kΩ 150 kΩ 180 kΩ 220 kΩ 270 kΩ 330 kΩ 390 kΩ 470 kΩ 560 kΩ 680 kΩ 820 kΩ	1 MΩ 1.2 MΩ 1.5 MΩ 1.8 MΩ 2.2 MΩ 2.7 MΩ 3.3 MΩ 3.9 MΩ 4.7 MΩ 5.6 MΩ 6.8 MΩ 8.2 MΩ

Exercises:

The result of resistance calculations are the following. Which resistors can be used?
To see the answer just the space behind the values.

235 Ω   220Ω
1.4 kΩ   1.5kΩ
620 Ω  680_or_560kΩ
13 kΩ   12kΩ
1.35 MΩ  1.2_or_1.5MΩ
995 Ω  1kΩ
13.5 kΩ 12kΩ_or_15kΩ

Resistor Combinations
There are two different ways to connect resistors: Serial and parallel connection. In addition to that a combination of this two principles is possible, the serial-parallel connection.

Resistor in Series
Two or more resistors can put together like a chain. The values of the single resistors simply have to be added to get the value of the whole combination.

In series connection the total resistance is always higher than the highest value of a single resistor.
Example:    The total value of this resistor combination is:   10 Ω + 22 Ω + 33 Ω = 65 Ω

Respect the prefixes Ω, kΩ, MΩ. Do not mix them.

Resistors in Parallel
The calculation of a resistor combination in parallel is slightly more difficult.
But in general one can say:

In parallel connection the total resistance is always lower than the lowest value of a single resistor.
Example:    The total value of this resistor combination is:

               



If only two resistors are put in parallel a more simple formula can be used (Fig.11).
Then, the total resistance is the product of the two resistors, divided by the sum of the two resistors.

Example:

Much easier is the calculation when resistors with the same resistance are taken.

For two resistors the result is half the of resistor value. For three resistors the result is one third of resistor value. For four resistors the result is one fourth of the value.

And so on...

Example:

2 resistors of 10 kΩ 3 resistors of 330 kΩ 4 resistors of 100 Ω

R = 5 kΩ                   R = 110 kΩ R = 25 Ω

Colour code
The resistance and the tolerance of the resistor are printed on the body of the resistor with a colour code. The power rating is determined by the physical size of the resistor.
Common carbon resistors have four colour bands (three for the value, one for the tolerance) and metal film resistors have five colour bands.
In the common four band system the first two bands represents the number of the value and the third band the multiplier or easier number of zeros. The last band shows the tolerance (mostly gold) and also indicates the direction of reading (always right).

For reading the colour code the band of the tolerance lays always right (here gold).

	Colour	1st colour band	2nd colour band	3rd colour band
	black	0	0	-
	brown	1	1	0
	red	2	2	00
	orange	3	3	000
	yellow	4	4	0 000
	green	5	5	00 000
	blue	6	6	000 000
	violet	7	7	0 000 000
	grey	8	8	00 000 000
	white	9	9	000 000 000

The resistor above (brown-black-red) has the following value:

brown   = 1
black    = 0
red       = 2 x 0 = 00

= 1000 Ω or 1 kΩ

The 4th colour band indicates the tolerance of the resistor value or the precision of the resistor value. The smaller the value the more precise the value. The following tolerances exist:

silver    = 10%    (no more common, in old equipment)
gold     = 5%    (most common)
red      = 2%    (for measurement purposes)
brown  = 1%    (for precise measurement purposes)

Example:    A 100 KΩ with a golden band has a tolerance of +/- 5%. The value will be
                 between 95 KΩ (100 KΩ – 5 KΩ) and 105 KΩ (100 kΩ + 5 KΩ)

With this system all resistor values can be outlined, as long as they are not under 10 Ω. Brown–black–black is the smallest value which can be expressed with the system.
If a resistance of less than 10 Ω has to be outlined, then the 3rd band is gold. The golden band in this case stands for a dot between the 1st and 2nd band.
The colour code of red–red–gold stands for 2.2 Ω.

But those resistors are uncommon and in practice resistors with small resistance values are bigger wire-wound resistors where the value is printed in numbers on the body.

Problems reading the colours
Very often the colour is not easy to define. Green could be blue and orange maybe red. A short look at the E-12 preferred value list helps.

Example:   Is the first band green the second must be blue
                Is the first band red the second can only be red or violett

Exercise:   
Which value have the following resistors?
To see the answer just the space behind the resistors.

560 Ω 330 Ω 2.2 KΩ 470 Ω 100 KΩ 270 Ω 10 KΩ 100 Ω 4.7 KΩ 1 Ω

Wattage
The wattage of a resistor is identify by its size.
Smaller resistance values are needed where higher current flows. The wattage which is produced by the resistor gets higher and the produced heat has to be delivered to the surrounded air. The resistors gets bigger.
The high power resistors are wire-wound resistors with a body of cement or ceramic. Wattages of 5 W, 7 W, 11 W and 17 W are common.

The common resistor has a power rating of ¼ W.

5 W wire resistor 7 W wire resistor, both with cement body. More seldom and expensive 50 W resistor in metal housing

The resistor R 77 is a bigger 2 W-type. The wattage of the other resistors is not mentioned. In this case they are common carbon resistors with ¼ W.

Metal film resistors
In measurement or reference circuits (e.g. digital multimeter, ECG and other measurement equipment) high quality resistors with low tolerance are needed. Metal film resistors with 2% (red) or 1% (brown) from the E24 or E48 series are used.
Because the values get more precise and the numbers get bigger an additional colour band is needed. With a 5th colour band a value of 432 kΩ (E48) can be expressed.

Metal film resistor with 2% or 1% have five colour bands. The last colour band indicates the tolerance: red = 2 %, brown = 1 %

The gradation of the E24 series are the following:

10  11  12  13  15  16  18  20  22  24  27  30  33  36  39  43  47  51  56  62  68  75  82  91

The metal resistor above has the following value:

yellow    = 4
violet     = 7
black     = 0
orange   = 3 x 0 = 000
= 470 000 Ω
= 470 kΩ

The 5th colour band is brown. The resistor has a tolerance of 1 %.

E-24 Series (2%)
1 Ω 1.1 Ω 1.2 Ω 1.3 Ω 1.5 Ω 1.6 Ω 1.8 Ω 2.0 Ω 2.2 Ω 2.4 Ω 2.7 Ω 3.0 Ω 3.3 Ω 3.6 Ω 3.9 Ω 4.3 Ω 4.7 Ω 5.1 Ω 5.6 Ω 6.2 Ω 6.8 Ω 7.5 Ω 8.2 Ω 9.1 Ω	10 Ω 11 Ω 12 Ω 13 Ω 15 Ω 16 Ω 18 Ω 20 Ω 22 Ω 24 Ω 27 Ω 30 Ω 33 Ω 36 Ω 39 Ω 43 Ω 47 Ω 51 Ω 56 Ω 62 Ω 68 Ω 75 Ω 82 Ω 91 Ω	100 Ω 110 Ω 120 Ω 130 Ω 150 Ω 160 Ω 180 Ω 200 Ω 220 Ω 240 Ω 270 Ω 300 Ω 330 Ω 360 Ω 390 Ω 430 Ω 470 Ω 510 Ω 560 Ω 620 Ω 680 Ω 750 Ω 820 Ω 910 Ω	1 kΩ 1.1 kΩ 1.2 kΩ 1.3 kΩ 1.5 kΩ 1.6 kΩ 1.8 kΩ 2.0 kΩ 2.2 kΩ 2.4 kΩ 2.7 kΩ 3.0 kΩ 3.3 kΩ 3.6 kΩ 3.9 kΩ 4.3 kΩ 4.7 kΩ 5.1 kΩ 5.6 kΩ 6.2 kΩ 6.8 kΩ 7.5 kΩ 8.2 kΩ 9.1 k Ω	10 kΩ 11 kΩ 12 kΩ 13 kΩ 15 kΩ 16 kΩ 18 kΩ 20 kΩ 22 kΩ 24 kΩ 27 kΩ 30 kΩ 33 kΩ 36 kΩ 39 kΩ 43 kΩ 47 kΩ 51 kΩ 56 kΩ 62 kΩ 68 kΩ 75 kΩ 82 kΩ 91 k Ω	100 kΩ 110 kΩ 120 kΩ 130 kΩ 150 kΩ 160 kΩ 180 kΩ 200 kΩ 220 kΩ 240 kΩ 270 kΩ 300 kΩ 330 kΩ 360 kΩ 390 kΩ 430 kΩ 470 kΩ 510 kΩ 560 kΩ 620 kΩ 680 kΩ 750 kΩ 820 kΩ 910 k Ω	1 MΩ 1.1 MΩ 1.2 MΩ 1.3 MΩ 1.5 MΩ 1.6 MΩ 1.8 MΩ 2.0 MΩ 2.2 MΩ 2.4 MΩ 2.7 MΩ 3.0 MΩ 3.3 MΩ 3.6 MΩ 3.9 MΩ 4.3 MΩ 4.7 MΩ 5.1 MΩ 5.6 MΩ 6.2 MΩ 6.8 MΩ 7.5 MΩ 8.2 MΩ 9.1 M Ω

Other fixed resistors
Fixed resistors sometimes appear in other versions. Modern electronic boards are often equipped with SMD devices. SMD stands for Surface Mounted Devices. SMD are very small and have not connection wires. They are mounted directly on the board.

SMD resistors and capacitors (below) in comparison with common resistors (above).

SMD resistors are not marked with a colour code. But the numbers which are printed on the body follows the same rules as the colour code. The first two are numbers and the third numbers indicates the number of zeros.

Example:    564 = 5 6 0000 = 560 kΩ
                222 = 2 2 00 = 2.2 kΩ
                105 = 1 0 00000 = 1 MΩ

When a lot of resistors of the same value are needed electronic manufactures sometime use resistor network. Several resistors of the same value are conflated in one package.

Two resistor networks in an electronic board of a UPS device.

Sometimes resistors with only one black band can be found. These resistors do not have a resistance. Their value is 0 Ω. They are used when robots assemble the boards because robots can not handle wire bridges.

The lower resistor really is a 0 Ω resistor!

Variable resistors
Beside the fixed resistor there are also variable resistors.
All variable resistors have three pins. Two ends with the resistor in between and one wiper. The wiper can take a resistor value between zero and the maximum according to the position.

Variable resistors which are set with a little screwdriver are called trimmer. They are mounted on the electronic board and made for the technician to calibrate the circuit. Where fine calibration is needed multi-turn trimmers are used. From one end to the other the adjustment screw then has to be turned 10 turns or more.

Resistors which can be set from outside by the user are called potentiometer or just pots.
For audio purpose (e.g. volume control) a stereo potentiometer is used.

Potentiometer (pot) in stereo version for audio purpose and trimmer. The last trimmer is a 10-turn trimmer for fine calibration.

The symbols for variable resistors in circuits diagram are shown below. American symbols again are different.

Trimmer European new and old, American Potentiometer European new and old, American

Potentiometers are available in two different versions: Linear or logarithmic (lin or log)
The final value is the same but the change of resistance compare to the position of the control shaft is different. In general all pots which set voltages and DC applications are linear and pots for audio use, especially for volume control, are logarithmic ones.

The change of the resistor compare to the rotation angel. The blue line shows a lin pot, the yellow line a log pot.

Applications
Trimmer or pots have 3 connecting pins (1) and can be connected in different ways for different purposes.
The most common method is shown in (2). The resistor is variable and has 2 pins.
For audio applications the variable resistor is always connected as a voltage divider (3). Input and output are related to ground and the input resistor is always stable.
For stereo usage 2 pots have to be used. Both are working against ground (4)

Function check
Resistors can checked directly with an ohmmeter or multimeter. Therefore the equipment has to be switched off and one connector of the resistor has to be disconnected from the board. Otherwise other devices on the board can distort the measurement result.

When R5 would be measured while connected with the board, the resistances of R2, R5, T1, T2 will deliver a wrong result.

Also checking a removed resistor by holding the probes with the fingers will lead to a wrong result. The body resistance distort the measurement.

It is allowed to touch one terminal of the resistor during the measurement, but the second must not be touched.

With some experience it is sometimes easier and faster to check the function of a resistor by switching on the equipment and measuring the voltage over the resistor. A voltage drop indicates that the resistor works.

In general a measurement is really not necessary. Because a defective resistor is usually burned the defect is visible. Bigger power resistors always get warm during operation. Just touch the resistor. If heat is produced the resistor is OK.

Common problems
When resistors get broken they always become high-resistance or interrupted. Compare to capacitors the resistance never get smaller. Interrupted in practice means burned and burned resistors are easy to spot. It is always a good idea to do a thoroughly optical inspection of the board first.

Keep in mind that a burned resistor always has a reason. The reason is an unusual high current which can not produced by the resistor itself. Check the following device (specially transistors) for shorts. After replacing and switching on the equipment be prepared to switch off immediately when the resistor gets hot again. Sometimes it is a good idea to disconnect the following stage or device first to surround the fault.

Bigger power resistors get hot and can produce cold solder joints. Very often the solder joints of power resistors are the source for faults. It is good practice to resolder all poor solder joints.

Defective resistors are easy to spot. This resistor is burned. It of course has to be replaced but the resistor is NOT the cause of the problem. A burned resistors always means: There is a problem somewhere else.

For changing a resistor not only the value is important but also the wattage and the tolerance. That means also the size and the last colour band must be respected.

Sometimes for a repair the needed value is not available. Then two or more resistors can put together according to Ohm's law. And because also the wattage increases it is also possible to put several small resistors together to get a resistor with a higher wattage.

Pots very often are 'jumping' or in audio amplifiers 'cracking' and 'scratching' when turning. Just dirt inside the pot housing is the cause. Contact spray helps or just some fast turns from one end to the other.

Broken SMD resistors or network resistor can replaced by common carbon resistors.

Prices
Resistors are cheap and a selection of standard values of the E-12 series belongs in every workshop. Here are the average prices for resistors in Europe:

Standard carbon resistor ¼ W	0.05 €
SMD resistor	0.05 €
Carbon resistor 2 W	0.30 €
Metal film resistor 1 %	0.10 €
Wire resistor 5W	0.40 €
Wire resistor 17W	0.80 €
Trimmer	0.20 €
Multi turn trimmer	0.50 €
Pot	0.70 €
Pot stereo	1.40 €

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Power Supplies

A power supply converts 230V (125V) AC mains into a low and stable DC voltage. A simple power supply consists of a transformer, a rectifier, a capacitor and a simple stabilizer.
For complex medical equipment, better power supplies with a more sophisticated stabilization are needed. Often these power supplies deliver two, three and more different voltages.
Many failures of medical (and electronic) equipment due to power supply defects. Therefore a lot of devices can be repaired just with the knowledge of the functioning of power supplies.

Types
Power supplies are 'voltage sources'. That means, that the output voltage is stable even if the output current fluctuates.
Battery charger for example are 'current sources'. They deliver a stable current and the voltage changes depending on the state of charge of the battery.
A stable voltage is very important in electronics. A (biomedical) measurement equipment will run out of range with a poor stabilization and will deliver wrong diagnostic results.
Nowadays power supplies with expensive and heavy transformers are replaced by cheaper switched-mode power supplies. Switched-mode power supplies have smaller transformers but therefore more electronics. As a result of the smaller transformers they are cheaper but also more difficult to repair.

The parts of a simple power supply
In different stages the high AC voltage is converted into a stable DC low-voltage.
First the mains voltage has to be reduced (transformer), then converted to DC (rectifier), filtered (capacitor) and finally stabilized (zener diode and transistor or voltage stablizer IC).

The transformer transforms the mains AC voltage (230V) to low AC voltage. This has always to be done at first because transformers only can transform AC.

The smaller AC voltage gets to a rectifier. The rectifier converts the negative part of the wave into a positive signal.

A (small) capacitor is added. The ability of voltage storage of the capacitor makes the signal smoother.

Is the capacity high enough the output signal is completely flat. We have created a DC voltage.

Under a bigger load the DC voltage breaks down. Instead of a open-circuit voltage of e.g. 20V we now have less. This is unacceptable because a voltage fluctuation has big influence to the connected stages. It is very important to stabilize the output voltage now.

The output voltage now is the voltage across the zener diode. That means: absolutely stable within the range of the diode's specifications. A series resistor is always needed where the (unstable) voltage difference can drop.

With this little circuit we produce a very clean and stable DC voltage. But unfortunately only a small current can be taken from this zener diode circuit. For running electronic applications it is not enough.
For a practical usage, this current has to be amplified. This is the job for a transistor. The stable voltage now controls just the input of a transistor and the transistor makes sure that a much higher current can be taken from the circuit.

Power Supply with Transistor
Transistor Principles

The three pins of a transistor: Base, Emitter and Collector.

A transistor is a three-terminal semiconductor device. The three pins are named: Base, Emitter and Collector. Transistors are used to switch or to amplify signals, voltages or currents.
The three terminals are used for input, output and for the common connection. Which terminal is what depends on the wiring. Three variations are possible.
In general the Base of a transistor is the input lead. The input current flows from Base to Emitter. When a current flows the voltage drop across BE is like the voltage drop of a diode, always 0.7V. That also means that always a base resistor is needed which limits the base current and let drop the excessive voltage.
This Base current now controls the CE path of the transistor which means a much higher Collector current. The transistor acts as an amplifier: A small Base current causes a big Collector current. For example a Base current of 10mA can control a load current of 1A.
In principle the Base current controls the CE path. The CE path opens or closes depending on the Base current. The higher the base current, the smaller the CE path (CE voltage drop) and the higher the Collector current will be.
With the maximum base current the transistor is fully controlled, the current is maximum and the CE voltage is minimum. The transistor acts like a switch or relay.

A small Base current controls a much bigger Collector current. The higher the Base current, the higher the Collector current. The higher the Collector current, the smaller the CE voltage drop.

Function of a power supply with a transistor
For a power supply the transistor is used as a current amplifier. The right transistor mode for this operation is called common-collector mode. This means the Base is used as control input, the Collector as power supply input and Emitter as the controlled output.

The stabilized voltage of the zener diode is used to control the transistor. The zener-voltage is connected to Base. This is possible because the needed Base current is low enough not to effect the zener voltage.
An additional base resistor is not needed because the series resistor of the zener diode also acts as a series resistor for the transistor.

Collector: Unstable input voltage Base: Stable control voltage Emitter: Controlled (stable) output voltage

This Base current now controls the much bigger load current C to E. In our case a stable voltage at the base keeps the output voltage stable or more precise controls the CE voltage until the Emitter-to-ground voltage is stable. The output voltage has to be stable because the BE voltage drop is always fixed to 0.7V and it is in series with the also fixed zener voltage (12V for example). If both voltages are fixed the resulting voltage must be also fixed. The resulting output voltage is the zener voltage minus the BE voltage:

12V - 0.7V = 11.3V.

or

Vout = VZ-diode - VBE

The output voltage is stable because the zener voltage and the BE voltage are stable. Both voltages are in series.

What ever the input voltage is, if it is drifting up or down, the output voltage is always 11.3V. What changes is the CE voltage across the transistor. This is of course the difference of input voltage and output voltage.

Vout = Vin - VCE

When the input voltage changes only the CE voltage of the transistor changes because the Base voltage is fixed. The Emitter voltage (output voltage) is also fixed because it depend on the fixed Base voltage minus the fixed 0.7V Base-Emitter voltage.

Now the power supply is stabilized or regulated. The output current can be much higher because it now depends on the specifications of the transistor and not any more on the small zener diode.

In practice an additional capacitor is always connected to the output in order to buffer the voltage against fast current peaks which could courses fast voltage drops.
The only thing which is missing now, is a mains switch and a fuse. Then the power supply is complete.

The current through the transistor now is stabilized and high enough to supply small electronic applications.

More power
In the above shown circuit the limiting device now is the transistor. The parameters of the transistor defines the output voltage (or more precise the maximum EC voltage) and the maximum current which can be taken. Important is always the situation between Collector and Emitter. Here the high load current flows and together with the CE voltage drop the heat loss of the transistor is created.
If the power supply has to deliver a higher output current or the difference between input and output voltage is too big (VCE) a bigger transistor is needed. Unfortunately a bigger transistor also needs a bigger base current which again stresses the zener diode and thus the stabilization. What we need in this case: An additional transistor. A transistor which controls the main transistor. Two transistors in series. One controls the other.


Now the smaller transistor takes the zener voltage and gives this stable voltage (minus 0.7V) to the bigger output transistor. The Base current for the bigger one now flows through CE of the lower one and does not effect the zener diode.
The upper transistor is always much bigger than the other one, because the main load flows through this transistor, while the lower transistor only has to deliver the small base current for the big one. Such a power supply can deliver some Amps. But note, together with the CE voltage drop this high current creates a big power loss, which means heat. The load transistor always has to be mounted on a heat sink.

Again a look at the voltages:

- Zener voltage is fixed at 12V
- Voltage drop BE of the first (smaller) transistor is also fixed at 0.7V
- Voltage at E: (12V - 0.7V)= 11.3V
- Voltage drop BE of the second (bigger) transistor is also fixed at 0.7V
- Voltage at E which is the output voltage: (11.3V - 0.7V) = 10.6V
- The output voltage is stable but only 10.6V
- Or the other way round: If we need a 12V output voltage the zener diode has to be one for
  13.4V (12V + 0.7V + 0.7V)

Power loss
Now a look at the power loss:
The current through the transistor together with the voltage drop between C and E makes the power loss. In case of the upper load transistor there can be several watts of power loss, which means heat. The transistor gets hot. That is why the load transistor of a power supply is always mounted on a heat sink or directly to the metal housing of the equipment. The rule of thumb is: Every semiconductor with a power loss bigger than 1W needs a heat sink.
The power loss or heat is the product of the VCE voltage drop and the load current through the transistor ICE

P = Iload × VCE

Negative Voltage
Now something confusing.
Power supplies can also generate negative voltages. The technology is the same as for positive voltages. It is just a matter of grounding or where the reference point for our measurement is.
Negative voltage means, that the output voltage is more negative against the ground.
Is the positive terminal of a battery connected to the ground, then the negative terminal is more negative than the ground. The output voltage is negative.

Imagine two 9V-batteries in series. First we connect the minus connection of the lower battery (and our measuring cable) to the ground. In the center we would measure 9V at the top 18V. Now we put the center point to the ground (and also our measuring cable). On top we would measure 9V and at the minus connector of the lower battery -9V. We get two voltages, a positive and a negative one.

In the same way a power supply for a positive and a negative voltage works. The + connection is more positive and the - connection more negative compare to the ground.

Power Supplies with Stabilizer-IC
Beside voltage stabilization, often a short circuit protection and an overhead protection for power supplies are demanded. Nevertheless the circuit should be as simple, small and cheap as possible.
The solution is a special IC (Integrated Circuit), which contains all these functions. The most common stabilizer is the 78xx series. This IC contains the whole stabilization and all the safety circuits.

Positive stabilizer 78xx
The IC has three pins and is build into a transistor housing. The output voltage is fixed. Different types for different voltages are available.

It looks like a transistors but it is complex integrated circuit. The 78xx type (left) is a stabilizer for up to 1A and the smaller 78Lxx (right) for up to 100mA.

The IC is available for different output voltages. The output voltage is expressed by the name. An 7812 is a 12V stabilizer for a positive voltage.

Output voltage Stabilizer 5V 7805 6V 7806 8V 7808 10V 7810 12V 7812 15V 7815 18V 7818 24V 7824

78xx for these voltages exist.

The pin connection depends on the case type. Good to know that the metal part of 78xx is ground. The IC can mounted directly to a heat sink without any isolation.

The pin connection for the positive 78xx type. The most common type is the 1A type in the TO-220 case. The connection pins are: left - in center - ground right - out

The application is simple. Only a input capacitor and a small output capacitor is needed for a fully stabilized power supply. The power supply is short circuit protected and delivers up to 1A.
Negative stabilizer 79xx
Beside the positive 78xx also a stabilizers for negative voltages exist. It is the 79xx series. The stabilizer look similar but the connecting pins are different.

Here the pin connection for the negative 79xx type. The most common type is the 1A type in the TO-220 case. The connection pins are: left - ground center - in right - out

Important: This time the metal case is NOT ground.

Also the negative stabilizer is available for different output voltages. A 7912 is a -12V stabilizer.

Output voltage Stabilizer -5V 7905 -6V 7906 -8V 7908 -10V 7910 -12V 7912 -15V 7915 -18V 7918 -24V 7924

79xx for these negative voltages exist.

The following power supply circuit from an oxygen concentrator combines two power supplies, one for a positive and one for the negative voltage.

The upper part provides the positive voltage (+5V), the lower part the negative voltage (-5V). Note that the rectifier is drawn inverted. The positive lead is down, negative up. Also the following capacitor is upside down. The input voltage of the IC is negative (more negative than ground). After stabilization the two reference potentials are put together to ground.

Below a similar power supply of a spectrometer.

The transformer is drawn somewhere else, but anyway the AC voltage gets to the points AC-15-2-15V and AC-15-2-0V, which obviously means 15V AC (at the diodes) against the ground. The rectification is done by just two diodes (D5,D6). The ground is now drawn in the middle, the upper part shows the part for the positive voltage, the lower part is for the negative voltage.
(By the way, there is a fault in the circuit. Look at the voltages specially in the negative part...)

Troubleshooting and common problems
Reasons for defects in electronic circuits in general are always high currents, high voltages and power losses with the development of big heat. All this applies to power supplies. That is why troubleshooting in electronic equipment should always start with checking the voltage(s) of the power supply.

In theory voltage regulators should never fail because they are protected against short circuit and over heating. But in practice they sometimes gets broken. (Why? - I don't know.)

A function check has to be done under power. Even if stabilizers look like transistors, they are ICs. You can not check a stabilizer with an ohm-meter!

A voltage check is very easy:
   Connect your multimeter to the ground (metal housing, minus of biggest capacitor...)
   Left pin is input voltage (up to 30 V), center is ground (0 V) and the right pin is thecommon 78xx type)
   The pin connection of negative regulators (79xx) is different (ground - input - output).

Think about the short circuit protection when there is no output voltage. No output voltage can mean, that the stabilizer is defective and delivers no voltage. But it also can mean, that there is a short circuit after the power supply and the integrated protection pulls the voltage down. Therefore, always disconnect the load from the stabilizer if there is no voltage. Just take off the cables to the connected stages or cut the output leg of the IC with a small cutter. Now you can check the output voltage directly at the IC. Later you can solder it again.


This is the power supply of a spectrometer. Clearly to see the big charging capacitors at the left and in the center, two rectifiers in between and three stabilizer mounted on small heat sinks.
First step for checking the board is: Connecting minus of the voltmeter to the ground (minus of the capacitors, the largest conductor track or the center pin of the stabilizer 78xx). The unstable input voltage is at pin 1, the stable output voltage is at pin 3.
Remember, negative 79xx stabilizer have different pin connection.

Here again the pin connections of a 78xx and a 79xx

If a stabilizer is defective and the right one is not available, maybe another one can be taken. The trick is to take a stabilizer for a lower voltage and putting up the ground by a zener diode. Zener voltage and stabilizer voltage make the output voltage.

A 9V-output voltage (8.9V) can be created by a common 5V-stabilizer and a 3.9V-Zener-diode.

Do-it-yourself power supply with stabilizer IC
A power supply is often needed. A battery powered equipment shall run on mains voltage or the defective external power supply of e.g. a microscope is not repairable. In this case a power supply can be build by yourself. But for the construction some experiences are needed and some calculations have to be done.
Here some universal hints to calculate the values of the needed parts:

Transformer: Output voltage should be 3-5V higher than the needed (unstable) DC voltage. The output current should be 10-20% higher than the needed DC current.

Rectifier: The proof voltage must be at least 1.4 x transformer output voltage.

Capacitor 1: Charging Capacitor as big as possible. 470µF per 100mA is perfect. Proof voltage at least 1.4 x UTransformer

Stabilizer: Power loss bigger than 1W always with heat sink. P = (Uout - Uin) x I

Capacitor 2: Output capacitor. For Audio applications 220µF, for all others 10µF. Proof voltage at least 1.4 x UStabilizer

Often you find two small bipolar capacitors C2,C3 in the input and output path of the stabilizer. Their task is to suppress unwanted oscillating of the IC. They should be mounted close to the stabilizer. The values are not critical. 0.1µF are common.

Prices
Stabilizer ICs are cheap and standard electronics spare parts. Some types specially the 7805 and the 7812 should be present in every workshop.

78xx, 79xx (TO-220)	0.30 €
78Lxx, 79Lxx (TO-92)	0.20 €
78xxK, 79xxK (TO-3)	1.50 €

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Principles
Quantity, units and symbols
To avoid misunderstandings it is important to use the correct physical expressions.

Physical quantity	Symbol	SI Unit	Symbol	Measurement device
length	l	meter	m	ruler, caliper
mass or weight	m	gram	g	scale
		pounds	lb
current	I	ampere	A	ammeter, clamp meter
voltage	V	volt	V	voltmeter
resistance	R	ohm	Ω	ohmmeter
wattage, power	P	watt	W	wattmeter

Examples:  A current I is 2 A (Ampere).
                 The voltage V is 12 V (Volt).
                 The resistor R has 10 kΩ (kilo ohm). The resistor R13 in the circuit has
                 56 kΩ (kilo ohm).

SI Prefixes
In practice the values of measurements are often too big or too small to manage with the basic units only. The following prefixes are used in electronics:

Prefix	Symbol	Factor	Power of 10
giga	G	1 000 000,000	109
mega	M	1 000 000	106
kilo	k	1 000	103
milli	m	0.001	10-3
micro	µ	0.000 001	10-6
nano	n	0.000 000 001	10-9
pico	p	0.000 000 000 001	10-12

Examples:     1 A = 1 000 mA
                    100 kΩ = 100 000 Ω (or 0.1 MΩ)
                    97.6 MHz = 97 600 kHz (or 97 600 000 Hz)
                    100 nF = 0.1 µF (or 100 000 pF)

Usage
When converting a unit or a formula write down all steps carefully.
Do not do it mental! A 0 is lost and a . is put at a wrong place easily.


-  Learn the sequences. For numbers which are bigger than the units: kilo – mega – giga
    and for number smaller than the unit: milli – micro – nano – pico
-  Remember that the steps between the following or the previous prefix is the
    factor 1 000
-   During converting the value remains the same. When the prefix gets bigger
    (mega instead of kilo) the number must get smaller (1 instead of 1 000)

Examples: 1 A = ? mA
                the difference of A to mA is 1 000
                mA is smaller than A. So the number must get bigger (x 1 000)
                1 A = 1 000 mA


                2.2 KΩ = ? Ω
                the difference of KΩ to Ω is 1 000
                Ω is smaller than KΩ. So the number must get bigger (x 1 000)
                2.2 KΩ = 2 200 Ω


                3 000 V = ? kV
                the difference of V to kV is 1 000
                K is bigger than V. So the number must get smaller (/ 1 000)
                3 000 V = 3 kV


                94.5 MHz = ? Hz
                the difference of M to the Hz is 1 000 000 (kilo is in between)
                Hz is smaller than MHz. So the number must get bigger (x 1 000 000)
                94.5 MHz = 94 500 000 Hz (or 94 500 kHz)

Exercises
Please convert the following values.
To see the answer just the space in front of the target value.

1 000 V =                  1 KV
1 000 mV =                1 V
2.2 KΩ =              2 200 Ω
3.3 MΩ =             3 300 KΩ
250 mV =               0.25 V
25 mV =               0.025 V
545 V =               0.545 KV
11 KV =              11 000 V
11 KV =        11 000 000 mV
500 µA =                 0.5 mA
50 µA =                 0.05 mA
50 µA =           0.000 05 A
0.230 KV =              230 V
220 nF =               0.22 µF
10 000 µF =             10 mF
10 000 µF =  10 000 000 nF
22 pF =               0.022 nF

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Varistors (MOV)

A varistor or metal oxide varistor (MOV) is a special resistor that is used to protect circuits against high transient (short term) voltage. These surges and spikes attacks the equipment by the power line and will destroy the power supply of the equipment. A varistor is able to short these surges and spikes and keep them away from the following application.
A varistor is also known as Voltage Dependent Resistor or VDR.

	Different varistors. The short circuit voltage is printed on the housing.
	Schematic of varistor.

Surges and Spikes
A power surge or a spike is an increase in voltage significantly above the standard voltage of 230 volts. The precise definition is:

    When the increase lasts 3 ns or more, it's called a surge.
    When it only lasts for 1-2 ns it's called a spike.

However, if the surge or spike is high enough, it will damage a device or machine. And in fact power-line surges can easily reach 6,000 volts.
Even if the increased voltage doesn't immediately break your machine, it may put extra strain on the components and wearing them down over time.

Spikes on a AC voltage.	Surge on a AC voltage.

A cause of surges and spikes on the power line is the operation of high-power electrical devices, such as, air conditioners, refrigerators and elevators. These high-powered equipments require a lot of energy to switch on and off motors and compressors. This switching creates sudden, brief demands for power, which upset the steady voltage flow in the electrical system.
These surges and spikes can damage electronic components, immediately or gradually and are a common problem in most building's electrical systems.
Beside power lines also telephone lines and antenna cables are affected by high voltage pulses caused by strokes of lightning.

It's a good idea to use surge protectors for all sophisticated electronic devices electronic equipment, such as computers, entertainment center components and of cause biomedical equipment. A surge protector will generally extend the life of these devices..

FunctionUnder normal conditions the resistance of the varistor is very high. When the connected voltage gets higher than the specification of the varistor the resistance immediately gets extrem low. This circumstance is used to protect electronic applications from over-voltage. The varistors is simply added to the power supply input. When high voltage surges and spikes appear the varistor will short them and protect the following application.

Characteristic curve of a MOV. Is the voltage low also the current is low (the resistance is high). When the voltage reaches the voltage of the varistor the current gets high very fast (the resistor is extrem low. The connectors are short.

Specifications
Varistors are a kind of resistors but their specifications are not resistance ῼ and wattage W. For varistors the most important specifications is the clamping voltage.

Clamping voltage
This is the voltage which short circuit the varistor. A lower clamping voltage indicates better protection. But on the other hand the voltage must not be that low, that smaller power changes destroy the varistor. For 230 V mains a varistor of 275 V clamping voltage is a good choice.

Energy absorption / dissipation
This rating is given in joules and shows how much energy the varistor can absorb. A higher number indicates greater protection. Varistors with 200 to 400 joules offer good protection, better protection is given with devices of 600 joules or more.
For extending the energy absorption two or three varistors can put parallel.

Response time
Varistors switch fast but not immediately. There is always a very slight delay as they respond to the power surge. The longer the response time the longer the connected application is exposed to the surges. A response time of 1 ns or faster is fine.

Application

	Varistor at the input of a power supply.
	The varistor is simply connected between line and neutral but after the fuse. If the varistors gets a short circuit the fuse will blow and disconnect the main from the following application.
	Simple solution for effective protection. The original high current fuse should be replaced with one matching with the equipment.
	A better protector contains three varistors: One across each of the three pairs of conductors (line, neutral and ground).

Problems
Varistors can be destroyed by too many surges. They wear out a little with each surge above the threshold and some day they are completely destroyed.
Over-voltage is also a common problem. The varistors burned but also let the fuse blow and so save the connected equipment.

	Defective varistor. Too many surges over long time destroy varistors.
	The normal failure of MOV is overheating. This can cause fires.

Alternatives
A gas discharge tube or gas tube is a kind of spark gap which contains air or a gas mixture.
When the voltage surges reaches a certain level, the gas will ionize the gas, making it a very effective conductor. It passes the current to the ground line until the voltage reaches normal levels.
Compare to varistors gas tubes have higher breakdown voltages. They can handle significantly higher fault currents and withstand multiple high-voltage hits without self destruction. On the other hand the response times is longer.
Gas arrestors are commonly used in telecommunication equipment to protect against lightning strikes.

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Lead Acid Battery (Car Battery)

Lead acid batteries are used as car batteries and in power backup systems.
The connectors and terminals of a common lead acid battery are made out of lead metal. The electrolyte consists of 35% sulfuric acid and 65 % of water. The ability of this typ of battery to provide extreme high current makes the battery perfect as starter batteries in cars.
The typical acid battery is of the "flooded cell" type with liquid electrolyte. It is indicated by
six plastic caps for refilling distilled water. Maintenance (check of liquid level) is essential.
Maintenance free batteries or gel batteries are completely sealed. They contain no liquids, so there is no need to control the level of liquid and no possibility of adding water. Maintenance free batteries operate in any position.

Working on car batteries
Car batteries provide extreme high current. NEVER short a battery. Be careful with tools while working on the terminals. A spanner which touches both connectors will be welded on immediately and the battery can explodes.


In cars the negative electrode is connected to the body. If you want to take out a car battery, disconnect the minus connector of the battery first. If your spanner gets contact to any metal part of the car while unscrewing, noting will happen. If you would start with the plus terminal and you get contact with the body, you would short-circuit the battery with dangerous consequences (no fuse, extremly high current, overheating and danger of explosion). After disconnecting the minus terminal you are allowed to disconnect the plus connector. A contact with the body now has also no consequences.
Installing the battery has to be the other way round.

For removing a battery out of a car always disconnect minus first and always connect minus at last.

Charging
The charging voltage is very important. Is the voltage too high (over 14.4 V) water will evaporate, explosive gases will develop, and the battery gets warm or even hot. The battery gets destroyed.

   The correct charging voltage is 13.8 V - 14.4 V and the charging time should be 10 - 16 hours.
   The typical (daily) charging should be 14.2 V - 14.4 V
   During charging the battery must never get hot.
   Continuous-preservation charging: 13.4 V for gelled electrolyte and 13.8 V for flooded cells.

After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V.

Maintenance
Water loss has must be replaced immediately with distilled water. Never use tap water. It contains too many minerals. Dry cell plates which are exposed to air loose capacity rapidly and get damaged for all times.
   Battery must not get hot at any time.
   Charging voltages must be checked.
   Battery terminals must be clean. Terminals can be cleaned from corrosion and sulfateswith a wire brush and a solution of baking soda and water. Wash the removed dirt with water.
   Contacts can be protected against sulfates by applying petroleum jelly (no grease) around the terminals. Be careful, lead sulfate is toxic by inhalation and skin contact.

Testing
Before testing separate parallel connected batteries. Every battery has to be checked alone.
The open circuit test only works if the battery is disconnected and is not charged or discharged for some hours. The best way is to charge the battery for 8 – 12 hours, disconnect charger and load and leave the battery for 24 hours. After this time of rest you can check the voltage without any load just with a voltmeter. The expected voltage of a good battery is about 12.6 V. A battery with only 11.9 V or less is defective. Here the results in details:

Open Circuit Voltage	Approximate charge
12.65 V	100 %
12.45 V	75 %
12.24 V	50 %
12.06 V	25 %
11.89 V or less	0 %

All voltages are measured at 20 °C.

Defects
A swollen, hot battery is very dangerous. Disconnect the charger and let the battery cool down before handling the battery. A hot battery probably needs water and the plates are already damaged. The capacity is poor and if the open circuit voltage is below 11.9 V the battery is defective and has to be replaced.
Charging with over voltage (> 14.4 V) and complete discharging must be avoided in any case and are the reasons for most of battery defects.

Repair
In general a battery can not be repaired.
Only with liquid acid batteries there is a small chance to reactivate the capacity by doing the following procedure: Charge fully for 8 - 12 hours, wait 24 – 48 hours and charge again. Repeat it this procedure if necessary.
This does not work with maintenance free batteries.

New batteries
New acid batteries are precharged but empty. They have to be filled up with acid before usage. Then, after waiting for half an hour the battery is ready for use without charging.
Motorcycle batteries are often based on a different (older) technology. They have to filled up with acid and charged some hours before using.
The life span of a car battery depends on the charging and discharging cycles and the surrounding temperature. Under perfect conditions a battery can live for 10 years under 25°C and only 5 years under 33°C.
The storage of batteries should only be done with fully charged batteries.

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Halogen Light Bulbs
Light bulbs in general
A light bulb creates not only light but also a lot of heat. Only 5% of the input energy is used for emitting light. The rest is heat. So the efficiency of a standard light bulb is very poor.

To improve this energy efficiency (or more precise the luminous efficacy) the industry nowadays provides different types of bulbs or different technical solutions to produce light (halogen lamps, energy saving bulbs, LED lamps, fluorescent lamps...)

Bulbs react as PTCs. That means that the resistor is very low at the first moment after switching on. For this short moment the bulb takes a much higher current than usual. That is why most of the bulbs get broken while switching on. Expensive halogen bulbs with an electronic brightness control very often also have a soft start function which limits this input surge current.

Life expectancy of light bulbs
The life expectancy depends on the voltage. The higher the voltage the shorter the life expectancy. Regions with many overvoltage periods will have a higher consumption of light bulbs.



In the diagram you can see the dependence on voltage and life time.
Is the voltage 100% the life expectancy also is 100% (graphs are crossing). If I increase the voltage of only 10% the life expectancy drops to 1/3 (at 110% voltage).
Or if I could reduce the voltage of 10% the bulb will live 4-times longer (400% at 90% voltage).
By the way the brightness of the bulb will not change very much. See the yellow graph.

Halogen lamp
The advantages compare to a normal bulb are: smaller size, higher efficiency (brighter with lower wattage) and higher color temperature (light is whiter) than a normal bulb.
Halogen bulbs are made in general for 6V, 12V, 24V. To run the lamps on mains a transformer is needed. A rectification is not needed because the bulbs work with AC and DC. Nowadays the industry produce also lamps for 230V direct use.
Wattages of 10W, 20W, 50W and 100W are common.


In medical equipment halogen lamps are used in microscopes, slit lamps, endoscopes and spectrometers.
It is also possible that one can find halogen lamps in vaporizers. In this case the bulb is used as a heating element.

Problems and solutions
A new bulb should not be checked with the main voltage in the shop – only with an ohm meter. Once the light bulb was hot it is sensitive for vibrations.


Any surface contamination, especially fingerprints, can damage the quartz envelope when the bulb gets hot. Contaminants will create a hot spot on the bulb surface when the bulb is turned on. This extreme, localized heat causes the quartz to change which leaks gas and the bulb will be destroyed.
Halogen lamps must be handled without touching the clear quartz, either by using a clean paper tissue or carefully holding the porcelain base. If the quartz is contaminated it must be cleaned with alcohol and dried before use.


An additional problem could be the connecting pins. Never bend the pins. They brake easily.
Because the pins are often silver-plated the pins of used and even new bulbs are very often oxidized. Clean the pins carefully with a glass fiber brush or steel wool. Do not forget to clean the bulb surface after that work.


Consider to reduce the voltage of the power supply if it is possible. The life expectancy will increase, specially when you often have problems with overvoltages.
A reduction from 12V to 11.4V (5%) will double the life expectancy. The loss of brightness is hardly visible.

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Capacitors

A capacitor is a passive electronic component which basically consist of two parallel metal layers separated by an insulator. The types of capacitors are named after this dielectric. We use capacitors with dielectrics made of ceramic, mica, polyester, tantalum etc.
Capacitors are used to block or to store voltages and to filter out signals.
Capacitors always have two pins. Some are bi-polar others mono-polar.
Mono-polar capacitors have two different leads, one positive and one negative.

Capacitors in different shapes and sizes. Mono-polar capacitors are usually cylindric while bipolar have disc or rectangular shape.

Units, values and symbols
The formula letter of capacitors is C.
The symbols for capacitors in circuit diagrams are shown below. Specially for electrolytic capacitors several different symbols exist.

Non-polar capacitor (left) and three mono-polar capacitors.

The capacitor is characterized by the capacitance, which is measured in farad (F).
In practice the units are µF, nF, pF.

1,000 pF = 1 nF
1,000 nF = 1 µF

Non-polarized Capacitors
Capacitors of this type have no positive and negative terminal and can be mounted in both ways in the electronic board.
Common non-polarized capacitors are made of ceramic, mica or polypropylene. Ceramic capacitors are small, cheap and are used for high frequency applications.
The main characteristic for anon-polarized capacitors is, that they block DC and let pass AC. They are also able to store voltages for a short moment.
Capacitors in electronics are mostly used in AC applications like signal filters and timing circuits.
Unlike the dielectric in polarized capacitors, the dielectric in non-polarized capacitors is a solid material that make the device durable and reliable. Failures of this type are rare.

Different non-polar capacitors. The small disks are ceramic capacitors.

Beside capacitors with fixed capacity there also exist variable capacitors. But in hospital equipment they are not common.

Polarized Capacitors
Some capacitors such as electrolytic and tantalum are polarised. They have two different leads, plus (+) and minus (-). That means that they have to be connected in the correct way. The leads are always clearly marked.
Polarized capacitors are mostly electrolytic capacitors. The construction is cylindric with connection lead on both ends (axial) for horizontal mounting or at one side only (radial) for standing mounting position.
For smaller voltages and capacities polarized capacitors made out of tantalum are often used. They are smaller and look different. They are drop-shaped.

Electrolytic capacitors offer very high capacity. The value of electrolytic capacitors is always µF.
Electrolytic capacitors are always marked with their maximum working voltage. The voltage across the terminals must never exceed this value.

In contrast to non-polarized capacitors the electrolyte is a liquid. In practice this fact is a source for a lot of problems.

The polarization is always mentioned. Often the negative (-) lead is marked. Capacitors are available for vertical and horizontal mounting. Vertical (or standing) mounting is also called radial. Horizontal (or laying) mounting is also called axial.

Standard values
Similar to resistors the available capacitor values are standardized in the E-series. The most common series is E-12:

10   12   15   18   22   27   33   39   47   56   68   82

Example:  Available capacitors are  33 pF, 220 nF, 0.68 µF

Electrolytic capacitors have a higher tolerance. They are available only in E-6 or even E-3 graduation.

Example:  10 µF, 220 µF, 4.700 µF

Voltage
The second important characteristic of a capacitor is the proof voltage. This is the maximum voltage the capacitor can be used. This applies especially for electrolytic capacitors.

Bipolar capacitors for electronic purposes (low voltages) often do not show the proof voltage because the voltages for electronic boards are much smaller than the proof voltages of the capacitors. Only for mains application (e.g. 230 V) the proof voltage has to be noticed.

Capacitor for mains. Here the proof voltage is very important (275 V AC)

Tolerance
In addition to the capacity and the proof voltage the tolerance of the value is mentioned on the body of the capacitor. A single letter indicates the tolerance:

J ±5%   K ±10%   M ±20%

Example:  A capacitor which has the following text on its body: 105  K  330 V
               has the following specification:
               1 µF (explanation next chapter), tolerance of ±10%, maximum voltage of 330 V.

Usually the tolerance of an electrolytic capacitors is higher than the tolerance of a non-polar capacitors. Tolerances of electrolytic capacitors are not important and so they are not mentioned on the capacitors. Tolerances of ±20% or more are common.

Capacitor ReadingIf you are lucky, capacitance and maximum usage voltage are clearly marked on the capacitor.

µ47 means 0.47 µF or 470 nF J stands for 5 % tolerance 63 is the maximum usage voltage in V

Often the reading of the values is not very clear. Too many numbers and letters can confuse you. Always look for numbers from the standard values.

Only the number 10n on top of the capacitor indicates the capacity: 10 nF K stands for 10 % tolerance and the 100 probably means the proof voltage. 1829 or 93 or 30 are not numbers of the standard values. They can mean everything but not the value.

The reading of the value is often not easy because the units specially on bipolar capacitors are often missing. In principle then the value means µF.

The value 0.33 means 0.33 µF or 330 nF

Only ceramic (disk) capacitors are different. Because their value is always very small, the value now means pF.

A ceramic capacitor without the unit. 27 in this case means 27 pF.

To make it more confusing, sometimes the value is expressed as a 3 digit number code specially on ceramic capacitors. The first two digits are the base of the value and the third number indicates the multiplier or more simply, the number of zeros.

Another ceramic capacitor without the unit. Again the unit must be pF. 47 expresses a part of the value (E-series) and the 3 the number of zeros of the value. This capacitor has a value of 47,000 pF or 47 nF.

683 K means 68 (3x 0) = 68 -000- pF or 68 nF with a tolerance of ±10%

Example:  102 = 10 00 = 1,000 pF or 1 nF
               224 = 22 0000 = 220,000 pF or 220 nF or 0.22 µF
               471 = 47 0 = 470 pF

Exercise:  What do the following characteristics of capacitors mean?
              (To see the answer just the space behind the values)

               104 K 50V     0.1µF, ±10%, 50V
               473 M 100V   47nF, ±20%, 100V
               68 K 50V       68pF, ±10%, 50V

For electrolytic capacitors it is more clearly. The value is always µF and this is also always mentioned.
The polarization is also always clearly indicated.

Capacity and voltage are printed clearly on electrolytic capacitors. 1000 µF 25 V (-) pin is down

CombinationsSimilar to resistors several capacitors can be connected in parallel or in series. But in contrast to resistors the capacity in series gets smaller and in parallel bigger.

Capacitors in series. The capacity gets smaller but the proof voltage gets bigger.

Most common combination: Capacitors in parallel. The capacity can be simply added. The capacity gets bigger. The proof voltage remains the same.

In practice the parallel combination is sometimes useful: The capacitor you need is not available but two of a smaller capacity. The capacities are simply added. The proof voltage of each capacitor must be as high (or higher) as the original.

Example:  A 1000 µF/25V capacitor is needed but not available. But two capacitors of
               470 µF/50V are available. In parallel the value will be 940 µF which is about 6%
               less than the original. Because tolerances of ±20% are common this combination
               can be used. This solution is even better than the original, because of the higher
               proof voltage.

Applications
The two main characteristics of capacitors are storage of voltages and filtering.

DC-Applications: Storage
Storage of voltage is a typical DC-application. DC voltage is stored for some time in a capacitor. The time of storage depends on the capacity and can be milliseconds or some seconds. Typical applications are power supplies where capacitors buffer the DC voltage to keep it stable and timer circuits where capacitors determine a switching time.

For voltage storage applications the capacitor is connected to ground (drawn always vertically). After switching off the DC voltage slowly falls off.

The storage time depends on the capacity. The bigger the capacity the longer the time. For storage or buffering polarized electrolytic capacitors with high capacity are used.

After switching off the LED slowly gets dark. The bigger the capacity the slower the time.

In power supplies high capacitive electrolytic are used to buffer and smooth the voltage. The capacitors clean the DC voltage from fluctuations and irregularities.

This is a part of the power supply of a pulse oximeter. The device in the centre is a voltage stabilizer IC. Input voltage and output voltage are filtered by the capacitors.

AC Applications: Filtering
A decoupling capacitor is a capacitor used to separate one part of an electronic stage from another. That is important because different (analog) stages run by different DC voltages. The stages have to be separated DC- wise. DC has to be blocked but an AC signal has to pass. The capacitor filters out the AC-part of a signal.
In schematics decoupling capacitors are usually drawn horizontal. The signal direction is from left to right (left = input, right = output).

A capacitor blocks the flow of DC. A DC voltage on one side as no DC effect on the other side of the capacitor.

AC can pass through a capacitor. The loss (the AC resistance) depends on the capacity and the frequency of the AC-signal.

In electronics, AC-signals (sounds, heart beats, video images...) very often have to be amplified or converted. The electronic stages need power supply (DC) for operation. During the process AC signal and DC voltage are layered. Capacitors are needed to separate the stages DC-wise and to connect the stages AC-wise.

This is a small pre-amplifier. The microphone needs a certain DC voltage as well as the transistor. The DC voltages have to be decoupled but the microphone signal (AC) has to pass. C1 does this job. Also capacitor C2 leads the output signal to the next stage without any DC potential. The stages are AC-coupled and DC-isolated.

Testing
A capacitance meter is an electronic test equipment used to measure capacitors. Upmarket digital multimeter often contain a function of measuring capacities. But in practice a capacitance measurement function is not really necessary because defectives on capacitors are usually visible.
For measuring electrolytic capacitors keep in mind that they suffer from poor tolerances.
Tolerances of ±20% are common.

If you do not have a capacitance meter the function of electrolytic capacitors can be checked by connecting and disconnecting a voltage and measuring the stored voltage with a voltmeter. Depending on the capacity the voltage will drop more or less quickly.
With some multimeter you can switch on Ω range to charge the capacitor (by using the internal battery) and then switching over to the V range to see the voltage drop.

Troubleshooting
Most of the troubles with capacitors are coursed by electrolytic capacitors. Bipolar capacitors in electronic boards usually last for ever.

The reasons for defective electrolytic capacitors are leakages, heat and poor production quality. Very often the cheapest quality is used with proof voltages very close to the operating voltage. After some time of working on the limit the capacitors become damaged. Electrolytic capacitors can leak, crack or even explode. In most of the cases the defect is visible. It is unusual that electrolytic capacitors loose capacity without any signs of damage.

This loss of capacity is often difficult to discover. The current does not get bigger, fuses are not triggered and nothing gets warm. The equipment seems to work somehow but not correctly. Voltages are not buffered, signals not filtered and other strange effects can appear.

Reasons for the defectives is the electrolyte inside the capacitor. Often the capacitor is not sealed perfectly and the capacitor leaks. The dielectric liquid also can vaporize under high temperature, can create a pressure on the capacitor case and makes the capacitor swell or even explode.

The leaked electrolyte can also corrode the printed circuit board where the capacitor is installed. Look for corrosion, clean the board and renew the solder points.

Defects of electrolytic capacitors are usually visible. Here the body is burst and the dielectric comes out.

To prevent an explosion electrolytic capacitors have a perforation to let escape gases or the dielectric liquid in case of a failure.

When changing a capacitor bear in mind the following:

   Make sure that the polarity is correct.
   Electrolytic capacitors store voltages for a long time. Discharge electrolytic capacitors.by shorting the two terminal leads briefly. High voltage capacitors should be shorten by a resistor (e.g. 1 KΩ). Check the voltage with your multimeter.
   Choose capacitors with a proof voltage which is as high or better higher than the original.

Prices
Defects of non-polarized capacitors are rare. There is no need having them in stock. But some electrolytic capacitors should be available in every workshop.
Here are the typical prices for capacitors in Europe:

Ceramic	0.10 €
MKS 630V	0.20 €
SMD capacitor	0.30 €
Tantalum 10 µF/25V	0.30 €
Electrolytic 10 µF/40V	0.20 €
Electrolytic 1,000 µF/40V	0.80 €
Electrolytic 4,700 µF/63V	4.00 €

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Diodes

A diode is a semiconductor device which conducts only in one direction. This effect is used for rectification when the positive part of an AC-signal can pass while a negative part is blocked.
A diode has two different terminals. The positive electrode is called anode and a negative cathode. The cathode is always clearly marked on the body of the diode as a ring.
The function of all this diodes is the same. The differences are the maximum operating voltages and the maximum currents.
On electronic boards and in circuit diagrams diodes are often marked with a D.

Different sizes means different operating voltages and/or different currents.

Symbols
The symbol expresses the one way function of the diode. The arrow in the diagram shows the direction of the current flow.

Current can only flow in one direction: From Anode to Cathode - in the direction of the arrow.

Types
As all electronic devices also diodes have losses while working. But compare to resistors the voltage drop across the diode does not depends on the resistance and the current. The voltage drop over a diode is fixed. It is always 0.7 V, no matter which current flows. (Some people say it is 0.6V).

The voltage drop across a diode always is 0.7 V.

Applications
In electronics the one-way character is very often used. DC voltages can be blocked or added and AC voltages are rectified.
But also the fact that the voltage drop is always the same and stable can be used as a reference voltage in stabilizer circuits and in measurement stages.

When the current only goes in one direction (from anode to cathode) and the voltage drop across the diode is always 0.7V (or 0.6V), then the voltage at the anode has to be about 0.6V higher than at the cathode. We say the diode is in forward bias.

Forward biased. The voltage at anode is more positive than at the cathode. The voltage drop is 0.6V.

When the voltage at anode is smaller than at cathode the diode blocks. No current flows through the diode. The voltage at cathode comes from another source but not through the diode. The diode is in reverse bias.

Reverse biased. The voltage at anode is more negative than at the cathode. A current can not flow through the diode. The voltage at cathode comes from another source.

Diode in forward direction. The bulb glows. The voltage across the bulb is 11.3V because the voltage drop across the diode is 0.7V.

Diode in reverse direction. No current flows. The bulb does not glow.

The bulb glows when a voltage from the battery or from the external power supply is available. When both are applied the current flows from the power supply because the voltage is slightly higher (12V), than from the battery (12V - 0.7V = 11.3V). The diode also prevents the battery by destroying from the external voltage. In this case the diode works in reverse direction.

The sine wave of the AC input signal is cut. Only the positive part passes through the diode. More information under Power Supplies

A reverse polarity protection. The current only flows when the polarity of the battery is correct. Advantage: No fuse will get triggered Disadvantage: 0.7V voltage loss, the maximum current has to be respected.

An other reverse polarity protection. When polarity is correct the diode has no influence. Is the polarity in reverse, a short circuit current flows and blows the fuse. Advantage: No voltage loss, operating current must no be respected. Disadvantage: Fuse will be destroyed and has to be exchanged in case of wrong polarity.

Testing
A diode does not have a certain ohmic resistance, because the voltage drop is fixed and independent from the current. The measurement result of the ohmmeter depends more on the ohmmeter itself than on the diode. Do not use the ohmmeter range of your multimeter. Use always the special diode-range.
How ever, the value in the display is not important. The multimeter is only used to test if the diode has conduction or not.

Multimeter in Diode-range. Plus-lead to anode. Current flows. The display shows a value.

Plus-lead to cathode. Now current must not flow. The display shows open circuit. The diode is OK.

As always when working with an ohmmeter on a board, a correct measurement result you only get after disconnection of at least one diode lead from the rest of the circuit.

Under power the diode can be checked by measuring the voltage drop.
The voltage at anode has to be 0.7V higher than at cathode.
Is the voltage the same the diode has a short.

In operation the voltage drop is 0.7V. (Anode to cathode)

Thus the voltage at cathode is 0.7V lower than at anode.

Troubleshooting
In practice defective diodes always have a short. In theory it is possible that a diode first gets a short and then it bursts because of the much higher current and becomes high-resistance. But in practice a fuse is triggered or a resistor is burned before that happens.

Under power a diode not only creates a voltage drop of 0.7V but also can separate two different voltages. A voltage at cathode must not necessarily be the voltage coming from the anode. It also may come from another voltage source. In general, is the voltage at the cathode higher than at anode, the voltage comes from anywhere else and the diode keeps the voltages separate. The diode is OK.

As always in electronics heat is a big problem. Diodes get overheated and/or create cold soldering points. Check all solder points of the board carefully and resolder the joints in case of a doubt.
When a diode is defective choose a bigger type if possible.

List of common diodes
Diodes differ in their maximum operating voltage and their maximum allowed current.
Types reaches from a few mA (1N914) to several Amps (BY550).
Here some common diodes and their specifications:

Type	Voltage (maximum)	Current (maximum)
1N914	100 V	75 mA
1N4148	75 V	200 mA
1N4001	50 V	1 A
1N4002	100 V	1 A
1N4003	200 V	1 A
1N4004	400 V	1 A
1N4005	600 V	1 A
1N4006	800 V	1 A
1N4007	1000 V	1 A
1N5400	50 V	3 A
1N5401	100 V	3 A
1N5402	200 V	3 A
1N5404	400 V	3 A
1N5406	600 V	3 A
1N5407	800 V	3 A
1N5408	1000 V	3 A
BY 133	1300 V	1 A
BY 255	1300 V	3 A
BY550-400	400 V	5 A

Prices
Diodes are very cheap and standard types should not be missing in every workshop.
Here are the typical prices for diodes and rectifiers in Europe:

1N4148	0.02 €
1N4007	0.02 €
1N5408	0.06 €

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Rectifiers

Rectifiers simply consist of four diodes which are connected in a special way. They are used in power supplies to convert AC into DC. This conversion is called rectification.
Rectifiers always have four leads: Plus (+), minus (-), AC (~), AC (~)

Rectifiers for different voltages and currents. Both, maximum voltage and maximum current is printed on the body as well as the polarization.

Rectifier are simply four diodes in one housing. The way of connection is always the same. It is a ring circuit with two cathodes to plus (+) and two anodes to minus (-).

Types
Rectifiers vary in their maximum voltages and currents. Both specifications are printed on the body of a rectifier. In practice we will find two types of rectifiers: The B-type and the KMU-type.

B-types
The printing of the very common B-type is for example the following:

B40C1500
B is the voltage, C is the current in mA

Example 1: B40C1500
                40V, 1.5A

Example 2: B380C800
                380V, 800mA

KBU-types
The voltage on a KBU-type is expressed in a letter code.
The number after KBU is the maximum current in A followed by the voltage code:
A=50V, B=100V, D=200V, G=400V, J=600V

Example 3: KBU4A
                4A, 50V

Example 4: KBU6G
                6A, 400V

Applications
Rectifiers are used in power supplies to convert AC to DC. This process is call rectification.
The function of power supplies and the process of rectification is shown in details in the Power Supplies chapter.

Simple power supply. The transformer transforms the 230 V mains AC into low voltage AC. The rectifier converts the low voltage AC into DC. The capacitor smooths the output voltage.

Testing
Rectifiers contain simple diodes. These diodes can be checked with an multimeter in diode-range. It is helpful to make a simple sketch of how the diodes are connected before measuring.

Troubleshooting
The reason for defective rectifiers usually is overheating. But it is very likely that a short behind the rectifier is the reason for overheated rectifier. Always check the charge capacitor right behind the rectifier and the connected stages before changing the rectifier.

Sometimes the polarization is not clear because of missing symbols on the body. The + lead is always marked. Often the corner of the housing is cut off and marks the + lead. Then, the - lead is opposite of the + lead and the other two are AC input.

Rectifiers always contain four diodes. When a rectifier is needed but not available it can be easily build by four common diodes.

Left the original rectifier. Right the replacement made out of four diodes.

Prices
Here are the typical prices for rectifiers in Europe:

Rectifier 1 A	0.20 €
Rectifier 5 A	1.00 €
Rectifier 25 A	1.50 €