Circuit symbol: 
Also see: Resistance | Ohm's Law
Example:
Circuit symbol:
Colour Code | |
| Colour | Number |
| Black | |
| Brown | |
| Red | |
| Orange | |
| Yellow | |
| Green | |
| Blue | |
| Violet | |
| Grey | |
| White | |
.
is quite small so resistor values are often given in
k
and M.
= 1000
1 M
= 1000000 .
Resistor values are normally 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
= 270 k.
On circuit diagrams the
is usually omitted and the value is written 270K.
Find out how to make your own Resistor Colour Code Calculator
.
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:
red,
violet,
gold bands represent
27 × 0.1 = 2.7
green,
blue,
silver bands represent
56 × 0.01 = 0.56
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:
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.
For example:
= 2700
= 1000 k
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. 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 390
could vary by ±10% × 390
= ±39,
so it could be any value between 351
and 429.
 |
 |
| High power resistors (5W top, 25W bottom) Photographs © Rapid Electronics |
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).
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) |
resistor with 10V across it, needs a power rating P = V²/R = 10²/470 = 0.21W.
resistor with 10V across it, needs a power rating P = V²/R = 10²/27 = 3.7W.
