An LED driver circuit is required in electronic circuits to maintain correct operating conditions for the Light Emitting Diode (LED). Maintaining the correct conditions ensures the LED is operating at it’s optimum condition and helps to prevent damage from over heating due to too much current flow.
Simple LED Driver Circuit
The simplest circuit is a resistor in series with the LED. The circuit acts as a voltage divider which intern reduces the current flow to the LED. The drive circuit looks like this:
Calculating the Resistor Value
Before we can do any calculations we need to know the typical current and voltage requirement for the LED. This information can be found in the manufacturer’s product datasheet which is usually available from the manufacturer’s website and sometimes the retailer’s website.
Example – Blue LED Driver Circuit
We have a blue LED that requires a typical voltage of 3V and a current of 20mA. The supply voltage will be 5V.
We use ohm’s law (V=IR) to calculate the resistance R.
Ohm’s law formula rearranged to find R looks like this:
R = V/I
V is the voltage across the resistor, so if we have a 5V supply and the LED requires 3V we have 2V across the resistor.
Resistor voltage is 5V – 3V = 2V.
The current in ohm’s law needs to be converted from milliamps to amps. This is done by dividing 20mA by 1000 which is 0.020A.
Getting back to the resistor calculation,
R = V/I
R = 2V/ 0.02A
R = 2/0.02
R = 100 ohms
So our resistor value is 100 ohms. By using a 100 ohms resistor in our circuit the LED will be supplied with 20mA from the 5V supply.
Resistor Power Rating
A very important thing we must not overlook is the resistor power rating that is required. If the power in the resistor is too high it could burn out. For a low power low voltage circuit like this a 1/4W resistor should be Ok. Let’s just have a look quickly at a simple power calculation to see what power will be dissipated through the resistor. This time, the formula is Power equals Voltage multiplied by Current. The voltage and current both going through the resistor is V= 2V and I = 0.02A. So we put these values in the formula below:
P = VI
P = 2V x 0.02A
P = 0.04 W
P = 40mW
So a 1/4W resistor is suitable. If we wanted to we could use a 1/8W resistor but a 1/4W resistor is more commonly found at retailers so we’ll stick with a 1/4W resistor.
So there we have it, a simple calculation for a simple LED driver circuit. This circuit can be used to power up a single LED on it’s own or in circuit such as a microcontroller circuit as we used in our previous post LED control sequencer using Arduino board.
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