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.
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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)
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Units, values and symbols
The symbols for resistors in circuits diagram are shown below. Notice that American symbols are different.
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Resistor, European and American
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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
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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.
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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%)
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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
220Ω1.4 kΩ
1.5kΩ620 Ω
680_or_560kΩ13 kΩ
12kΩ1.35 MΩ
1.2_or_1.5MΩ
995 Ω
1kΩ13.5 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.
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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: |
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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. |
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And so on...
Example:
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2 resistors of 10 kΩ 3 resistors of 330 kΩ 4 resistors of 100 Ω
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R = 5 kΩ R = 110 kΩ R = 25 Ω
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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).
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For reading the colour code the band of the tolerance lays always right (here gold).
|
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Colour
|
1st
colour band
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2nd
colour band
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3rd
colour band
|
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black
|
0
|
0
|
-
|
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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
|
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grey
|
8
|
8
|
00 000 000
|
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white
|
9
|
9
|
000 000 000
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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.
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560 Ω
330 Ω
2.2 KΩ
470 Ω
100 KΩ
270 Ω
10 KΩ
100 Ω
4.7 KΩ
1 Ω
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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.
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5 W wire resistor
7 W wire resistor, both with cement body.
More seldom and expensive 50 W resistor in metal housing
|
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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.
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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%)
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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
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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 €
|
Sources and additional information
Resistor:
http://en.wikipedia.org/wiki/Resistor
Resistor, practical:
http://www.kpsec.freeuk.com/components/resist.htm
Potentiometer:
http://en.wikipedia.org/wiki/Potentiometer
Thermistor:
http://en.wikipedia.org/wiki/Thermistor
Potentiometers:
http://sound.westhost.com/pots.htm
Colour code calculator:
http://www.ese.upenn.edu/rca/calcjs.html