Colour Code | Tolerance | Real Values (E6 & E12 series) | Power Rating
Also see: Resistance | Ohm's LawExample:
Function
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.Colour Code | |
Colour | Number |
Black | |
Brown | |
Red | |
Orange | |
Yellow | |
Green | |
Blue | |
Violet | |
Grey | |
White |
Resistor values - the resistor colour code
Resistance is measured in ohms, the symbol for ohm is an omega1
1 k
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.
So its value is 270000
On circuit diagrams the
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 10red, violet, gold bands represent 27 × 0.1 = 2.7
green, blue, silver bands represent 56 × 0.01 = 0.56
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 390silver ±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
Real resistor values (the E6 and E12 series)
You may have noticed that resistors are not available with every possible value, for example 22kTo 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 390
Resistors in Series and Parallel
For information on resistors connected in series and parallel please see the Resistance page,Power Ratings of Resistors
High power resistors (5W top, 25W bottom) |
The power, P, developed in a resistor is given by:
P = I² × R or 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 ( V = voltage across the resistor in volts (V) |
Examples:
- A 470
resistor with 10V across it, needs a power rating P = V²/R = 10²/470 = 0.21W.
In this case a standard 0.25W resistor would be suitable. - A 27
resistor with 10V across it, needs a power rating P = V²/R = 10²/27 = 3.7W.
A high power resistor with a rating of 5W would be suitable.
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