Resistors resist the flow of current and do so by using various materials to give the resistor it’s properties.

The 2 most common resistors encountered are the ceramic coloured ‘carbon film’ 5% tolerance resistors and the blue coloured ‘metal film’ 1% tolerance resistors.

The Standard and Advanced qualification only requires understanding of the older 5% carbon film types.

A resistor works by using various materials that impede the flow of current and convert the excess into heat (thermal) energy.

Resistors are designated by the body colour and coloured bands around the body.

The carbon film resistor shown in the table above is a good example. We can also discuss a variation with different colours to provide a different example to the one pictured.

Say we have a YELLOW, VIOLET, ORANGE, GOLD resistor.

1^st band Yellow = 4

2^nd band Violet = 7

3^rd band Orange = 10³ (Usually the number of zeros to add, 3 zeros in this instance)

4^th band Gold = Tolerance of +/- 5%

This is read as 47,000 ohms with a tolerance of +/- 5% or 47K in its abbreviated form.

Lastly on the odd occasion one might encounter the third band as gold or silver.

In these case we divide the value rather than multiply it.

For example – BLACK, BROWN, GOLD, GOLD

1^st band Brown = 0

2^nd band Black = 1

3^rd band Gold = 10^-1 (divide by 10)

4^th band Gold

This is read as 0.1 ohm +/- 5%

The schematic symbol for a resistor has traditionally been the zigzag, but the IEC standard version sees an increasing usage is the simple ‘box’ with the value shown inside.

Examples below show both symbol formats.

Common Resistor symbols