LED Current Limiting Resistor for LCD Module Backlight

Selecting an LED Current Limiting Resistor

Never connect an LED directly to a battery or power supply. The LED will be destroyed instantly by excessive current flow A series resistor is therefore needed to limit the current to a value that will not harm the LED.

Selecting the current limiting resistor for any LED or LED backlight brings us back to the very beginning or our electronics training, namely Ohms Law. Nothing more is needed.

There are two pieces of data that you will need from the data sheet of the LED. They are:

Forward Voltage, usually designated V_F and,
The Forward Current at the desired brightness, usually designated I_F.

The following example will show how to select the value, and how to hook it up to the LED. (Note that in the data sheet, the cathode of an LED may be designated by a K instead of a C as in our picture!)

In our circuit V_S = Supply Voltage = 5V
V_f= 3.9V
I_F = 70ma

LED Current Limiting Resistor

R_L = (V_S - V_F) / I_F
R_L = (5V - 3.9V) / 70 ma
R_L = 15.7 Ohms (Feel free to round this value, but watch I_F)
The power dissipated will be (5V - 3.9V) * 70ma = .077 Watts

For a circuit with multiple LED's, it best to wire them all in series. In addition, all the LED's connected in series pass the same current, so it is best if they are all the same type. To work out a value for the resistor you just add up all of the LED voltages and use that value for V_F (You can see in our circuit above that wiring even two of our LED's in series would rerquire an increase in the supply voltage.)

I hope this helps with your LED backlighting designs. As usual, please call us at (440)232-8590 and talk to one of our applications specialists if you have any questions about this or any other product design issues.