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.
Resistors may be connected either way round and they are not damaged by heat when soldering.
Resistance is measured in ohms, symbol (omega). 1 is quite small so resistor values are also given in k and M:
1k = 1000
1M = 1000k = 1000 000.
Most resistors are too small to show their resistance as a printed number. A colour code is used instead.
For information on resistors connected in series and parallel please see the resistance page.
Rapid Electronics: Resistors
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 1000 000 if the letter was M. The letter R means multiply by 1.
Resistor values are usually shown using coloured bands, each colour represents a number as shown in the table. Most resistors have 4 bands:
This resistor has red (2), violet (7), yellow (4 zeros) and gold bands so its value is 270000 = 270k (usually shown as 270K on circuit diagrams).
Make your own Colour Code Calculator.
The standard colour code cannot show values of less than 10. To show smaller values two special colours are used for the third band:
The first and second bands represent the digits in the usual way.
red, violet, gold bands represent 27 × 0.1 = 2.7.
green, blue, silver bands represent 56 × 0.01 = 0.56.
This calculator can be used to identify resistor values. It consists of three card discs showing the colours and values, these are fastened together so you can simply turn the discs to select the value or colour code required. Simple but effective!
There are two versions to download and print on A4 white card (two calculators per sheet):
To make the calculator: cut out the three discs and fasten them together with a brass paper fastener. The Black & White version must be coloured manually and it is easiest to do this before cutting out.
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 390 resistor with a tolerance of ±10% will have a value within 10% of 390, between 390 - 39 = 351 and 390 + 39 = 429 (39 is 10% of 390).
A special colour code is used for the fourth band tolerance:
Tolerance may be ignored for almost all circuits because a precise resistor value is rarely required and where it is a variable resistor will usually be used.
You may have noticed that resistors are not available with every possible value, for example 22k and 47k are readily available, but 25k and 50k are not!
Why is this? Imagine that you decided to make resistors every 10 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.
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.
A similar arrangement is used for money: the step size of coins and notes increases as the value increases.
For example UK currency (£1 = 100p) has 1p, 2p, 5p, 10p, 20p, 50p, £1 & £2 coins (plus £5, £10, £20 & £50 notes).
The E6 series has 6 values for each multiple of ten, it is used for resistors with 20% tolerance. The values are: 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 has 12 values for each multiple of ten, it is used for resistors with 10% tolerance. The values are 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 each gap.
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 have a tolerance of ±10%.
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 is 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 300) or high voltages (more than 15V).
Rapid Electronics: Power Resistors
High power resistors
(5W top, 25W bottom)
Photographs © Rapid Electronics
The power, P, developed in a resistor can be worked out using these equations:
|P = V² / R or P = I² × R|
P = power developed in watts (W)
I = current through resistor in amps (A)
R = resistance of resistor in ohms ()
V = voltage across resistor in volts (V)
Rapid Electronics have kindly allowed me to use their images on this website and I am very grateful for their support. They stock a wide range of resistors and other components for electronics and I am happy to recommend them as a supplier.