Introduction to Resistors

Resistors are devices with two terminals that act to maintain Ohms Law:

That is: Voltage over the two terminals = Current through the resistor (amps) times The resistor's resistance (ohms)

In other words, a resistor maintains a voltage over its two terminals proportional to the current flowing through it. This can be used for specifying a voltage drop when the current is known, or limiting current flow over a voltage drop.

The symbol for a resistor is either a rectangle or a zig-zag:

The coloured bands on some resistors are codes to indicate their resistance value.

In an example, with a 100 Ohm resistor in the following circuit, powered by 5 Volts:

we could calculate how much current is flowing through the resistor by using Ohms Law:

$V = I \times R$

5 volts across the resistor, 100 ohm resistor:

$5 = I \times 100$

divide by 100, re-arrange:

$I = 0.05A\cr$

the current flowing through the resistor is 0.05A or 50 mA (milliamps)

If we have a device, for example a LED, which requires a certain voltage drop and a certain current we can use a resistor to limit the current to the device and create the proper voltage drop. Calculating what value resistor to use can be done with Ohms Law.

E.g. With the following circuit:

We'll say that we want a 2.4 Volt drop across the terminals of the LED, with a current of 0.02 Amps (20 milliamps) runnning through the LED. These values are specific to different types of LED and too much current will burn out an LED.

First we work out the voltage drop we require across the resistor: from the source voltage (5 Volts) down to the LED supply voltage (2.4V)

We need a 2.6 Volt drop over the resistor.

So we use Ohms Law to calculate the necessary resistance to create this voltage drop at the required 0.02 Amps:

2.6 voltage drop, 0.02 amps current:

$2.6 = 0.02 \times R$

so we have found that we need a 130 Ohm resistor to run the LED at the required current and voltage.

Resistors can be used to set the brightness of an LED or lamp (or limit the current, so it doesn't burn out), set the speed of a motor, etc.

Placing resistors in series creates a total resistance of all the resistor values added up. Placing resistors in parallel reduces the total resistance: see Wikipedia on Resistors for more info.

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