Resistor values in colour
Resistors are electrical devices that are used in circuits
to vary current. Because they can have very low values or very
high values, it is not easy to write the resistance on the resistor
itself. Instead, a system of colour coding has been devised.
Resistors are very cheap and can be bought at any electronics
shop. A valuable maths and science project can be to work out
resistance values from the colours on the resistors.
The colour convention
The colour bands around the resistor each represent a number.
The colours represent:
Black
= 0
|
Brown
= 1
|
Red
= 2
|
Orange
= 3
|
Yellow = 4
|
Green
= 5
|
Blue
= 6
|
Violet
= 7
|
Grey
= 8
|
White = 9
|
|
|
You might find it easier to remember if you notice
that the colours for numbers 2 to 7 are also "colours of
the rainbow". There are two other colours that shows how
precise the resistor value is: a gold band means the tolerance
is +/- 5%; silver means +/- 10%; no fourth band means +/- 20%.
On some resistors you may see a brown band (+/- 1%) or a red
band (+/- 2%) for the tolerance. Basically, the lower the percentage
tolerance, the closer the resistor will be to the stated value.
View the resistor as shown in the diagram and read off the numbers
from left to right. The precision band is the closest to the
centre of the resistor.
As an example, the resistor here has bands of brown, yellow,
red and silver.
The first two bands are the first and second
digits, in this case 14. The third band shows how many zeros
there are. In this case there are two zeros, so the value of
this resistor is 1400Ω. The silver band indicates a tolerance
of +/- 10%, so this resistor can be anywhere between 1260Ω
and 1540Ω.
Mega value
Sometimes numbers have lots of zeros, so a prefix (fixed to
the front) is used to make it easier to write. Prefixes usually
used for resistors are kilo- and mega-. Kilo means one thousand
and mega means one million. One thousand ohms is called a kilohm,
and a million ohms is called a megohm. The units are kΩ
and MΩ.
As an example 15 000Ω = 15 × 1000W
= 15 KΩ.