Now that the music festivals are getting closer this summer, I thought I would build some low-power dancefloor lights for my camp. The circuit I've found is really simple, but unfortunately I am not an expert on the field.
The driver will accept 12V as the main current for the circuit, except the control input will be 5V from a microcontroller. The FET/BJT will make the circuit always run at a specific amount of current, depending on the value of R1.
Once i'm building the shown circuit on a breadboard, it seems to work pretty okay, however when I solder it to the PCB's I have strange behaviour:
At first, the FET is open for current flowing through it, even if there's 0V on the gate. How can that even be possible? Also, the current is not limited at all!
After a while (~an hour with too much current! I suspect the FET has been very hot) it starts to work as intended, and limits the current to what I calculated. However if I put the control signal to ground, it will not be able to completely shutdown the current flow, resulting in weak but clearly visible light from the LED's.
I really cannot figure this out. Temperature (afterwards) doesn't seem to affect anything. I've built 4 of these units now, since I though i had a loose connection somewhere, but I start thinking that isn't the issue anymore.
(I'm running 25mA through each chain of LED's, i.e. 100mA in total. The voltage drop across each LED is ~3.2V (green LED's). R1 is 5R6 and R2 is 10k)