How does a Resistor Work?
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Imagine water flowing through a pipe. If we make the pipe narrow then this will restrict the flow of water. If we force the water (current) through the narrow gap by increasing the pressure (voltage) then energy will be given off as heat. In addition, there will be a significant difference in pressure (voltage) above and below the restriction.
In electronics we use a resistor when we need to reduce the voltage applied to a circuit.
On the right is the symbol used to represent a resistor. You may also see it drawn as a zigzag line.
A resistor is defined by several parameters:
Resistance in Ohms (½)
Heat Dissipation in Watts (W)
Manufacturing tolerance (%)
A resistor drops voltage by turning excess power into heat. The amount of power turned into heat can be calculated from
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The value of a resistor is either printed in normal characters or, more usually, as coloured bands. Here is an example.
The first band is red, indicating the number 2.
The second band is also red, indicating 2.
The third band is yellow, indicating 4 zeros.
The fourth band is gold, indicating 5% tolerance.
(Silver would indicate 10%, brown = 1%, red = 2%)
This resistor is 220000 Ohms in value, often written as 220k½
As the tolerance is 5%, the actual resistance lies between 209000 and 231000
or 209k½ and 231k½ due to manufacturing inaccuracies.
0 = Black
1 = Brown
2 = Red
3 = Orange
4 = Yellow
5 = Green
6 = Blue
7 = Violet
8 = Grey
9 = White
The colour code is essential and the only way to learn it is by practice. Take a box full of resistors. Work out the value of each then check with a meter to see if you are correct.
Note that the last band on the resister indicates the tolerance. (The first band is usually slightly broader than the rest).
In a 1% resistor, there may be an additional band if more accuracy is needed.
Thus a 1% 220k½ resistor would be coloured:
This time the first THREE colours indicate actual numbers.
The fourth colour indicates the number of zeros.
The fifth colour indicates the tolerance.
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Here are two resistors connected "in series". The total resistance from end to end is equal to the sum of the values of both resistors. So, if each resistor has a value of 2200½ (2k2) then the total value will be 4k4.
Resistors connected "in parallel" have a total resistance that can be calculated as
Two resistors can be used to set a specific voltage. For example, if two resistors are connected as shown (left) and a voltage of 10 volts is applied to the ends, then if both resistors are of equal value, the voltage at the centre connection will be 5 volts. The voltage is divided between the two resistors.
There is a very important equation known as "Ohms Law".
I = V/R
Current (in mA) = Volts divided by Resistance (in k½) or
Current (in Amps) = Volts divided by Resistance (in ½)
We can turn this around to calculate voltage so
V = I x R
W = V x I
Watts = Volts x Amps
Substituting for I from Ohms Law in this equation gives
W = V x V/R or
W = V2/R
Or substituting for V in the above equation gives
W = I x I x R or
W = I2R
From these equations we can work out the required "Wattage" of any resistor provided that we know the value of any two of the three variables, Voltage, Current and Resistance.
Suppose we have a 10½ resistor with 10 volts across it.
W = V2/R gives 10 x 10/10 = 10 Watts
or, from Ohms Law, I = V/R = 10/10 = 1 Amp W = V x I gives 10 x 1 = 10 Watts
Q. Is a resistor polarity conscious? Does it matter which way round I connect it?
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