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Resistors are components that are used in electronics circuits (such as you'll find on this website) to limit currents to a safe value and to drop voltages to the level that is required for a particular device.

Resistors have a unit of measurement called the Ohm, named after the Georg Simon Ohm.

His law is:               V = IR

The law is a simple one but extremely useful in electronics, every electronics enthusiast should have this law imprinted on their memory. If you know how to use Ohm's Law you can do almost anything in electronics.

What Ohm's law means:

The Voltage measured across a resistor measured in volts, is equal to the current flowing through it measured in amperes, multiplied by the value of the resistance measured in Ohms.

Very often two of the quantities are known and the third has to be found using the formula.

The law is probably best understood by trying out lots of examples that occur in practice.

Example 1 - A 6 volt lamp is required to be illuminated from a 12 volt D.C. supply. The lamp operates with a current of 0.5 amperes.

What value resistor would be necessary to ensure that the lamp operates safely?

Answer - a resistor would need to be connected to the lamp as shown (Fig. 1).

There is 12 volts to be supplied across the lamp and resistor combination. 6 volts across the lamp and 6 volts across the resistor.

All that remains is to use the known quantities and put them into Ohm's Law

V = I x R

6 = 0.5 x R

The formula has to be rearranged to make R the subject (see Maths section on transposition of formulae)

After a little manipulation it can be shown that:-

R = V / R

R = 6 / 0.5

12 ohms 

A 12 ohm resistor is a standard resistor that could be bought off the shelf, but the rating of the resistor has to be taken into account also.

Resistors are rated in terms of how much power they can handle measured in watts. Most projects use quarter watt resistors, but they would burn up and be destroyed very quickly if one was attempted to be used in this example.

Resistor Power Ratings - another formula takes into account the power rating a resistor needs in order to operate safely. It is measured in Watts.


P = VI

Power measured in Watts is equal to the voltage measured in volts that is across the resistor multiplied by the current measured in amperes that is flowing through it.

We already know these quantites there is 6 volts across the lamp and 6 volts across the resistor and 0.5 amperes flowing through both.

hence the power rating the resistor must have is:-

P = 6 x 0.5

Answer 12 Watts

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