I am using an LM324 and three TIP122 transistors to drive three 40V 40W led's. The problem I am having is the LED's don't shut of all the way (a very dim glow remains). If I use only one op amp (out of the four on the chip) and one transistor, the LED turns off properly. If I use three of the four op amps, the LED's do not turn off.
I have tried placing a resistor (20K) in line with the negative feedback to see if I was having a current bias problem, but that didn't help. I also substituted power MOSFETs for the TIP122 without luck. I also tried buffering the input using one of the op amps as a voltage follower. This actually made the problem worse. It seems there is a biasing problem, but I don't know where or how to test this.
One thought was my issue comes from the fact that the LM324 isn't rail to rail, but I am not sure about that since I can get it working with one op amp. If the input voltage is zero, I am seeing about 1.5V on the output of all three op amps.
Is there leakage between op amps on an LM324? Why would one LED work but not all three?
Any help would be greatly appreciated. Thanks.
I can't post an image of the circuit because I don't have a "reputation". However, you can find it here.