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A resistor as its name might suggest, is a passive component used in electronic circuits to resist the flow of electrical current. As resistors restrict the flow of electric current, a resistor may placed in series with another component to limit the current passing through it.

The potential difference (voltage) across a conductor is proportional to the current through it. The constant of proportionality is called the "resistance". Resistance is measured in terms of Ohms. (Ω)

Ohm's Law states V = IR where V = voltage, I = current and R = resistance.

Typically resistors are classified by function, construction (i.e. material used in their manufacture) and by shape.

Some common forms of resistors include:-

  • Carbon Film Resistors - used for low power applications
  • Chip Resistors - used in surface mount technology
  • Metal Film Resistors - used where predictable temperature characteristics and accuracy are required
  • Resistor Networks - a composite made up of a number of resistors. Extensively used where space is at a premium on the PCB.
  • Variable Resistors - available as thick film, carbon film or metal film. They all have electrical resistant properties.
  • Wirewound Resistors - used for power and high precision applications

Resistor Colour Coding

Leaded axial resistors have colour bands on the resistor body. They can typically have 4 bands or 5 bands. It is possible to tell the resistance of axial resistors from the sequence of the colour bands.

How to read a resistor value

The first two numbers are the main digits of the resistance value, the third is what is known as a multiplier (this is how many zeros need to be added after writing down the first two digits), and the fourth is the tolerance of the resistor.

It is more than likely the tolerance band will be either Brown 1%Gold (5%) or Silver (10%). If no fourth band is shown, the tolerance will be 20%. Any further band after the multiplier (the fifth band) is likely to denote a quality characteristic.

Each colour corresponds to a number as per the table below.

Colour Band One Band Two Multiplier
Black 0 0 0
Brown 1 1 x 101
Red 2 2 x 102
Orange 3 3 x 103
Yellow 4 4 x 104
Green 5 5 x 105
Blue 6 6 x 106
Violet 7 7 x 107
Grey 8 8 x 108
White 9 9 x 109
Gold N/A N/A x 10-1
Silver N/A N/A x 10-2

For example a four banded resistor with the following sequence of colours:

Red, Green, Yellow and Gold would represent a 250000 Ω or a 250K resistor with a 5% tolerance.

Surface mount resistors have their resistance printed on their body. In this case, the third or fourth digit is the multiplier. Therefore for example a component with the markings 473 would be a 47000 Ω or a 47K resistor.

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Guide Written by Paul Wilson

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