Electronic – I’m trying to use 3v LEDs in a 12V circuit, in the car


I'm trying to make a circuit that will run 1-3 LED in a parallel, powered by a 12V power source. I used a voltage divider calculator and it said to use a 100ohm and 33ohm resistor. when I wire them in correctly, the 100ohm resistor overheats and eventually fails. How would I change this circuit to keep this resistor from overheating??
Both resistors are 1/2 Watt.
LEDs are 3.6v max.

Best Answer

By decoding your question, this is what I arrive at:

You are using the two resistors as a voltage divider, with the LEDs wired in parallel connected to the junction of these two resistors.


simulate this circuit – Schematic created using CircuitLab

This is not a valid approach, the voltage divider will not provide the 1:4 (approximately) voltage division your calculations indicate.

Firstly, either apply the three LEDs in series, with one resistor in series, or if you must have them in parallel, each will need a series resistor.


simulate this circuit

The above indicative values of resistors are assuming 20 mA per LED as optimal. Do your own calculations for your specific LEDs.

For a ready to use online calculator, use an "LED resistance calculator" instead of a voltage divider one. This one even provides the correct schematic or wiring diagram as you prefer, to wire the LEDs up.