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 sensible DC voltage if you have a correct value resistor fitted.

Resistors are relativley inexpensive and basically restricts the current passing throught the LED, as the LED's has no current limit itself. If you put an LED on a battery with no resistor it would be extremly bright for a fraction of a second before it blows.

To connect LED's into a circuit, it is advisable to know a little about Ohm's Law, a few basic formuals is all thats needed.

The figures you need to calculate the formula is 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 in the data sheet for that particular LED, it is the value of Vf, around 3.4V for White, different colours 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 and one resistor

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

R = (12 - 3.4 - 3.4) / 0.020

R = 260 ohm resistor.

Calculating resistor power

Power is measured in Watts, things like light bulbs are measure in Watts, most of the power is heat as a light bulbs is effectivly a resistor.

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

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.

Most of the time the rating of the resistor dosn't matter as they are usually 0.5watts which is plenty, but if you were to use surface mount resistor you would need to be aware.

Connecting LEDs in series and parallel

The above methods of connecting LED's was in series, if you wished to have many more LED's in the circuit you can basically multiply up the circuit across your power source to have as many LED's as you desire, this is where you may need to take care of the resistor heat, if you have many LED's in a small box with the resistors, all of it could get a little warm.