OK,
so here's the circuit:
The parts:
R1 - 100k 5%
RSense - 0.68 5% / also tried with 0.5 5%
Q1 - 600V/7.5A FQP8N60C Mosfet
DatasheetT1 - 80V/500mA MPSA06 NPN
DatasheetLED - 3.55Vf / 800mA Nichia LED
So, the problem is, for whatever reason, I am only getting 5mA of current through my LED. What gives? According to this design, I should be getting around 800mA.
The 800 mA, comes from the RSense = 0.5/I = 0.5/0.8 = 0.625 Ohm. So I chose a 0.680 Ohm. I then tried a 0.5 Ohm resistor, same deal.
For whatever reason, I am only getting 5mA to my LED. It seems as if for some reason perhaps the Mosfet is not turning on the way it's supposed to and letting current through?
I don't really understand the mechanics of the circuit that much, but I've used it successfully in the past to power 3 of these type of LEDs.
My previous schematic had, instead of the Q1 FQP8N60C, I used a IRF830PBF from Vishay (500V / 4.5A Mosfet) and a Darlington KSP13 (30V / 0.5A) from Fairchild.
I also switched out the NPN for the former Darlington, same deal. I also left the Emitter pin hung and connected to GND, same thing.
Anyone got any ideas?
Can it be that since I'm only driving one LED as opposed to 3 LEDs, the Voltage drop over the Mosfet is too big and it's shutting down?