Electrical principles - Properties

Resistors

Resistors are devices which are manufactured to a predetermined resistance value. They are made in many shapes and sizes and from a range of materials including:

• carbon
• metal film
• nichrome.

Carbon resistor and electrical symbol

A resistor can be used in a circuit to restrict the amount of current flow or decrease the voltage.

Colour codes

The colour bands on carbon and metal film resistors are used to identify the resistance value (in ohms) and tolerance (accuracy in percentage) of the resistor. The resistance value colour bands are crowded towards one end of the resistor and the last band is for the tolerance.

• Four colour bands have three value colour bands and then a fourth for the tolerance. Some have no fourth tolerance band, therefore the resistor has a tolerance of plus or minus 20.
• Five colour bands have four value colour bands and then the fifth band for tolerance.

The following table can be used to identify the value of a resistor. Read the table:

• from the top down for a four band resistor
• from the bottom up for a five band resistor.

 

Colour codes for identification of resistance values

Read the table from the top down for four band resistor
Colour		1st band	2nd band		Multiplier	Tolerance band	Colour
Black		0	0	0	1
Brown		1	1	1	10	± 1%		Brown
Red		2	2	2	100	± 2%		Red
Orange		3	3	3	1k
Yellow		4	4	4	10k
Green		5	5	5	100k	± 0.5%		Green
Blue		6	6	6	1M	± 0.25%		Blue
Violet		7	7	7	10M	± 0.10%		Violet
Gray		8	8	8		± 0.05%		Gray
White		9	9	9
						± 5%		Gold
						± 10%		Silver
						± 20%	(None)
Colour		1st band	2nd band	3rd band	Multiplier	Tolerance band	Colour
Read the table from the bottom up for five band resistor

 

Example

The resistor shown has four bands. Using the table allows us to work out the value.

1. The first band is brown so the first digit is one.
2. The second band is black, so the second digit is zero. Putting the two and the zero together gives a value of ten.
3. The third band, which is the multiplier, is brown so it has a value of one. This means that one extra zero will be added to the value.
   The resistor is rated at one hundred ohms.
4. The fourth band is gold which indicates that the resistor has a tolerance of plus or minus five percent.

The resistor is rated at 100Ω but would have an actual resistance between 95Ω and 105Ω.

 

Variable resistors

As the name suggests, a variable resistor is a device that allows a user to vary the amount of resistance. An example of this is the volume control on a radio. Variable resistors come in a variety of shapes, sizes and current carrying capacities. They have three terminals and one is joined to a wiper that can be moved along the resistance material.

 

Two types of variable resistor and associated symbol

 

Variable resistors are either:

• potentiometer – used for voltage adjustment
• rheostat – used to vary current flow.

They are shown in the following circuit diagrams.

Potentiometer and rheostat circuits