constant current source damaged?


I just build a constant current source to drive an high power led, according to the attached scematic. When i tried to test the circuit with a 5v power supply, the led got very hot, but emitted no light at all. So i double checked everything and had to realise that the wires on my power supply were labeled wrong. Therefore i applied a reverse voltage to my circuit, that killed my led.

My question is if it’s still safe to use the parts of the current source or if it’s possible that i damaged them as well. The MOSFET didn’t get hot, but i just want to be sure before destroying other led’s.

For the npn i used a 2N5088BU and the MOSFET was a FQP50N06, I think they survived the reversed voltage treatment, but i am not very familiar with Transistors so id rather ask here just to be sure.

Thank you in advance for your awnsers

I think the low value of the current sense resistor would have prevented any significant voltage on the
BJT or rest of circuit. The current flowed through the MOSFET's body diode.

I'm surprized -5V caused any damage though, LEDs normally have a -5V reverse voltage rating.


It is possible they are damaged. But it’s easy to test. First test, when the circuit should be off, is it indeed? Add a, let’s say 1k, resistor in place of the led and measure the voltage over the resistor (aka between V+ and mosfet drain). Should be of course be 0V if off.

Next, if it passed, just replace the led with a multi meter in current mode. Of course in a mode that can handle the set current (of the circuit), 10A setting probably is the easiest. And it’s a good idea to limit the current on the power supply if you can so you don’t blow the fuse of the DMM in case of a short.

If you can’t limit the current on the supply another save option is to replace the led with a light bulb from a car. And to measure the current put the DMM in series with that bulb.

If you measure the desired current it’s still working.

or the real easy option
Just connect a new led. You weren’t very cautious (/tested it fully) the first time you powered it up. Why be it now?

or the real easy option
Just connect a new led. You weren't very cautious (/tested it fully) the first time you powered it up. Why be it now?

Once bitten, twice shy.

So i did the tests and it still works

thank you for your help

It's strange that the LED would get hot when reverse biased. Are you sure it wasn't one of the transistors warming up?