I have recently purchased a headlight for a car. It has two amber sets of LED’s that operate as the turn signal.

I want to separate each set so I can use them separately, one for left and one for right.
The pcb within the headlight has an output for both and measures 5.7volts. When I measure the current for one set of LED’s I get a reading of 0.4 amps. There are a total of 18 as seen in the photo.
How can I power these using a motorcycle battery which will give a voltage between 13 and 14 volts?
Thanks very much for any advice you might be able to give me.

 

A first pass is that you need to drop about 9V at 0.4 amps. That means a resistor of 9/0.4=22.5 ohms in series with your LEDs. It’ll burn off I^2*R = 3.6W so will need to be rated to 5W or more and heat sinked.

A more elegant and efficient solution would be a constant-current LED driver.

 

Thanks very much for thevreply

 

I have tried a 27 ohm resistor rated at 1 watt. It didn’t light the LED’s. Is this because it is too low ie not a 5watt resistor.
Thanks

 

I have tried a 27 ohm resistor rated at 1 watt. It didn’t light the LED’s. Is this because it is too low ie not a 5watt resistor.
Thanks

Ok well the simple approach didn’t work. We’re going to need to know a lot more about what’s going on inside that PCB. Any details you can supply such as model number, schematic, good pictures etc, would be a lot more than we have now.