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Identifying Resistors

Introduction

Resistors are physically quite small most of the time and so it is inconvenient to simply write the value and tolerance on the resistor. A clear but compact code is needed!

On circuit diagrams it is important not to miss the decimal place and use a 47kΩ resistor instead of a 4.7kΩ so clarity is required.

The Resistor Colour Code uses coloured bands to make it easy to identify resistor values and tolerances and the Resistor Printed Code (BS1852) gives a standard for printed markings on resistors and on circuit diagrams.


The Colour Code

The resistor colour code uses 10 different colours to represent the value of the resistor. A different set of colours is used to represent other information such as the tolerance or thermal stability. It is worth knowing what the colours represent as it makes life easier when building circuits.

For the bands representing the value, the colour code is:

Black = 0

Brown = 1

Red = 2

Orange = 3

Yellow = 4

Green = 5

Blue = 6

Violet = 7

Gray = 8

White = 9


For the tolerance band, the colour code is:

Brown = ± 1%

Red = ± 2%

Gold = ± 5%

Silver = ± 10%

Blank = ± 20%


Using the Colour Code

In many cases, such as in schools, the resistors used are all ± 5% or worse. Therefore there is no point in giving the value to a high level of precision using many significant figures. Resistor values are usually given to 2 significant figures. The value is given as the 2 most significant digits and then the relevant number of zeros. For example, a resistance of 127.5Ω is simple given as 130Ω to keep things simple.


Examples

220R

The resistor has a value of 220Ω ± 5%


10k

Note: 10,000Ω = 10kΩ

The resistor has a value of 10kΩ ± 5%


4k7

The resistor has a value of 4700Ω ± 5%

Note: 4700Ω = 4.7kΩ but the decimal place is easy to miss so the decimal place is represented by the position of the ‘k’ in the value. Therefore, 4.7kΩ is written as 4k7Ω

Therefore, the resistor has a value of 4k7Ω ± 5%


BS1852 Printed Code

The BS1852 Printed Resistor Code uses letters and numbers to signify the value of the resistor.

The position of the letter indicates the position of the decimal place.

Tolerance is indicated by adding a letter at the end:

Given the number of letters available, it is a bit surprising that the standard code uses the same letters for value and tolerance but there you go!


Examples

220R A 220Ω resistor with a tolerance of ± 5% is written as 220 R J


4k7 A 4700Ω resistor with a tolerance of ± 5% is written as 4 K 7 J


10k A 10,000Ω resistor with a tolerance of ± 5% is written as 10 K J


More Examples


Small Value Resistors

Small value resistors - between 10Ω and 100Ω - are often identified wrongly. The third band in this case is always black which means that there are NO zeros following the value. It is all too easy to read the third band as zero and assume that the zero follows the value ... it does not! Black means there are zero zeros after the value!

10R

The resistor has a value of 10Ω ± 5% (not 100Ω)


56R

The resistor has a value of 56Ω ± 5% (not 560Ω)


10R

Both resistors have a value of 10Ω ± 5% but shown in different ways


Low Value Resistors

Low value resistors - those less than 10Ω - use the THIRD coloured band to reduce the value of the resistor by dividing by either 10 or 100. The first two coloured bands give the value of the resistor and the third coloured band divides the value to make it smaller.

Gold ÷ 10

Silver ÷ 100

For example, a 4.7Ω ± 5% resistor is 47Ω ÷ 10 and would therefore be Yellow, Violet, Gold, Gold

Similarly, a 0.56Ω ± 1% resistor is 56Ω ÷ 100 and would therefore be Green, Blue, Silver, Brown

1R

The resistor has a value of 1Ω ± 5% (not 10Ω)


Zero Ohm Resistors

A resistor with a value of 0Ω is useful as a wire link. The reason why a zero ohm resistor is better than a simple piece of wire is that the same machines that fit resistors into circuit boards on production lines can also be used to fit the "wire links" to the circuit boards. A zero ohm resistor has a single black band.


High Precision Resistors

For resistors with a tolerance of better than ± 5%, more than two significant figures are needed to represent the value.

High precision resistors have THREE bands for the value and then one band for the number of zeros - four bands in total to represent the value. The colour code otherwise works in exactly the same way. Note: 5th band gives tolerance where Brown = ± 1%

130k

The resistor has a value of 130kΩ ± 1%


Surface Mounted Resistors

SMDAn entirely different approach is adopted for Surface Mounted Device (SMD) resistors - and, as it happens, low value capacitors

The idea of 2 significant figures and then a 3rd digit representing the multiplying factor (number of zeros) is retained but the numbers are simply printed on the device / resistor and there is no tolerance indicated. The first two numbers represent the value and the 3rd number represents the number of zeros. Therefore, if the 3rd digit is 4, multiply by 10,000 (4 zeros) but if the 3rd digit is 1 just multiply by 10. If the 3rd digit is 0, multiply by 1 - there are no extra zeros.


E12 Series

Resistors with a tolerance of ± 10% (Silver) form a series of 12 values, each approximately 20% bigger than the last. This series is called the E12 series.
The E12 series is:

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

... and all the subsequent decades / powers of 10

Examples of E12 values include: 100Ω, 15kΩ, 220Ω, 330kΩ, 4.7Ω, 6.8MΩ, 10kΩ, 1kΩ


E24 Series

Resistors with a tolerance of ± 5% (Gold) form a series of 24 values, each approximately 10% bigger than the last. This series is called the E24 series.
The E24 series is:

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

... and all the subsequent decades / powers of 10

Examples of E24 values include: 110Ω, 13kΩ, 200Ω, 360kΩ, 4.3Ω, 6.2MΩ, 11kΩ, 1k3Ω