Hello all;

First off: I'm just a hobbyist so expect glaring errors!!

This is a circuit I made a little while ago. The aim was to drive the LED very brightly but protect them over a wide range of input voltages at the same time.

The arduino measures the voltage and then adjusts the PWM width to get consistent brightness and long life.

At first the circuit worked fine. The array of 12 LEDs still works great, and there is no radio noise. The array of 4 LEDs worked great in the beginning, but then stopped working. Their failure went along with some radio noise for the hour before its failure. The decoupling capacitor on the high side of the 5V regulator is a 47uf/16V alum, and the system was running at 14V at the time.

Does anybody see anything obvious that could lead to this behaviour?

Thank you

 

So there are no current limiting resistors in series with the LEDs?
In that case, when the MOSFET turns on a large, uncontrolled, current will flow. In the case of the 12 LEDs, there are three LEDs, and three voltage drops, in series in each branch but for the 4 LEDs there is only one LED voltage drop so that large current will be higher for the 4 LEDs than for the 12.

By changing the duty cycle you can control the average current but not that large current during the on time. Use resistors in series with each LED to limit the peak current to something less than the maximum specification of the LEDs.