Resistors restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED.

Connecting and soldering

Resistors may be connected either way round. They are not damaged by heat when soldering.
The Resistor
Colour Code

Resistor values - the resistor colour code

Resistance is measured in ohms, the symbol for ohm is an omega ohm. 
1 ohm is quite small so resistor values are often given in kohm and Mohm. 
1 kohm = 1000 ohm     1 Mohm = 1000000 ohm.
Resistor values are normally shown using coloured bands.
Each colour represents a number as shown in the table.
Most resistors have 4 bands:
  • The first band gives the first digit.
  • The second band gives the second digit.
  • The third band indicates the number of zeros.
  • The fourth band is used to shows the tolerance (precision) of the resistor, this may be ignored for almost all circuits but further details are given below.
This resistor has red (2), violet (7), yellow (4 zeros) and gold bands.
So its value is 270000 ohm = 270 kohm.
On circuit diagrams the ohm is usually omitted and the value is written 270K.
Find out how to make your own Resistor Colour Code Calculator 

Small value resistors (less than 10 ohm)

The standard colour code cannot show values of less than 10ohm. To show these small values two special colours are used for the third band: gold which means × 0.1 and silver which means × 0.01. The first and second bands represent the digits as normal.
For example:
redvioletgold bands represent 27 × 0.1 = 2.7 ohm
greenbluesilver bands represent 56 × 0.01 = 0.56 ohm

Tolerance of resistors (fourth band of colour code)

The tolerance of a resistor is shown by the fourth band of the colour code. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390ohm resistor with a tolerance of ±10% will have a value within 10% of 390ohm, between 390 - 39 = 351ohm and 390 + 39 = 429ohm (39 is 10% of 390).
A special colour code is used for the fourth band tolerance:
silver ±10%,   gold ±5%,   red ±2%,   brown ±1%.
If no fourth band is shown the tolerance is ±20%.
Tolerance may be ignored for almost all circuits because precise resistor values are rarely required.

Resistor shorthand

Resistor values are often written on circuit diagrams using a code system which avoids using a decimal point because it is easy to miss the small dot. Instead the letters R, K and M are used in place of the decimal point. To read the code: replace the letter with a decimal point, then multiply the value by 1000 if the letter was K, or 1000000 if the letter was M. The letter R means multiply by 1.
For example:
    560R means 560 ohm 2K7  means 2.7 kohm = 2700 ohm 39K  means 39 kohm 1M0  means 1.0 Mohm = 1000 kohm

Real resistor values (the E6 and E12 series)

You may have noticed that resistors are not available with every possible value, for example 22kohm and 47kohm are readily available, but 25kohm and 50kohm are not!
Why is this? Imagine that you decided to make resistors every 10ohm giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits. In fact it would be difficult to make resistors sufficiently accurate.
To produce a sensible range of resistor values you need to increase the size of the 'step' as the value increases. The standard resistor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.
The E6 series (6 values for each multiple of ten, for resistors with 20% tolerance)
10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc.
Notice how the step size increases as the value increases. For this series the step (to the next value) is roughly half the value.
The E12 series (12 values for each multiple of ten, for resistors with 10% tolerance)
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, ... then it continues 100, 120, 150 etc.
Notice how this is the E6 series with an extra value in the gaps.
The E12 series is the one most frequently used for resistors. It allows you to choose a value within 10% of the precise value you need. This is sufficiently accurate for almost all projects and it is sensible because most resistors are only accurate to ±10% (called their 'tolerance'). For example a resistor marked 390ohm could vary by ±10% × 390ohm = ±39ohm, so it could be any value between 351ohm and 429ohm.

Resistors in Series and Parallel

For information on resistors connected in series and parallel please see the Resistance page,

Power Ratings of Resistors

Resistor 5W
Resistor 25W
High power resistors
(5W top, 25W bottom)
Photographs © Rapid Electronics
Electrical energy is converted to heat when current flows through a resistor. Usually the effect is negligible, but if the resistance is low (or the voltage across the resistor high) a large current may pass making the resistor become noticeably warm. The resistor must be able to withstand the heating effect and resistors have power ratings to show this.
Power ratings of resistors are rarely quoted in parts lists because for most circuits the standard power ratings of 0.25W or 0.5W are suitable. For the rare cases where a higher power is required it should be clearly specified in the parts list, these will be circuits using low value resistors (less than about 300ohm) or high voltages (more than 15V).
The power, P, developed in a resistor is given by:
P = I² × R
P = V² / R
where:P = power developed in the resistor in watts (W)
I  = current through the resistor in amps (A)
R = resistance of the resistor in ohms (ohm)
V = voltage across the resistor in volts (V)