Quick Guide to Wiring LED's in a Circuit
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.
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.
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
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.
law formula is V = I * R.
we wish to calculate resistance it can be arranged to say
R = V / I.
One LED and One Resistor
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.
= (12 - 3.4) / 0.020
= 430 ohm resistor.
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.
= (12 - 3.4 - 3.4) / 0.020
= 260 ohm resistor.
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
is possible to calculate the power rating of the required
= (V drop) * I
one LED and one resistor at 12V
= 0.172 watts
two LEDs and one resistor at 12V
= 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
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.