A Quick Guide to Wiring LED's in a Circuit

LED's are quite different to light bulbs. Light bulbs are designed to work on a voltage, LED's can work on almost any DC voltage if you have a correct value resistor fitted.

As LED's have no current limit, a resistor restricts the current passing through the LED. Resistors can be fitted at a minimum cost. If an LED is not correctly fitted, it is common for them to blow up. This is the case if an LED is connected to a battery with no resistor.

It is advisable to know a little about Ohm's Law when connecting an LED to a circuit, in order to do this a few basic formulae are required.

To calculate the formula, the following information is needed: the voltage you wish to work from, the current you wish to pass through the LED and the forward voltage of the LED. The forward voltage can be found on the data sheet for each LED, this is the value of Vf, for example around 3.4V for a White LED. Different LED's will have different values.

Ohms law formula is V = I * R.

As we wish to calculate resistance it can be arranged to say R = V / I.

Connecting One LED and One Resistor

For example, lets say we have a 12V supply, and wish to run one Blue LED. We know the forward voltage Vf is 3.4V, and we wish to pass 20mA (0.020Amps) through the LED.

R = (12 - 3.4) / 0.020

R = 430 ohm resistor.

Connecting Two LED's and One Resistor

As we have plenty of volts to drop across the resistor, we could add another LED into the circuit which would make the circuit more efficient. At the moment the resistor will get warm depending on power dissipation of that resistor. The reason for the heat is that you are converting energy into heat which is considered to be wasted power. By using two LED's there are less volts to loose across the resistor. It will still be the same current passing through one LED, in effect you are getting extra light as there are two LED's, and less heat is being generated.

R = (12 - 3.4 - 3.4) / 0.020

R = 260 ohm resistor.

Calculating Resistor Power

Power is measured in Watts, items such as light bulbs are measured in this way. Most of the power generated from the light bulb is heat, therefore this is effectivly known as a resistor.

It is possible to calculate the power rating of the required resistor.

P = (V drop) * I

For one LED and one resistor at 12V

P = 0.172 watts

For two LEDs and one resistor at 12V

P = 0.104 watts.

In most cases the rating of the resistor does not affect the LED in any way. As LED's are usually 0.5watts there is plenty of available power, however should you choose to use a surface mount resistor, you should be aware of the amount of power being generated.

Connecting LED's in Series and Parallel

The above methods of connecting LED's has been described using a series circuit. If you wish to increase the number of LED's being used, you can multiply the circuit across your power source. It is important to take precautions regarding the resistor heat, as a large number of LED's contained in a small box can generate an excessive amount of heat.