I have a 120V DC Motor that I need to control the speed of. I am doing this through a High Voltage PNP transistor (MJW21193) rated at 250V and 16A with a peak of 30A as I understand it from the data sheet. The Diagram below is the circuit in question. I am using an optical coupler to supply the required 5V signal from the 3.3V logic. The circuit works. There is some heat but not enough that you can not touch the back of the transistor... on my test bench. The unit only runs for 6 seconds at a time.
However, when I place the unit in the field the electronics randomly fail. Yesterday I lost D5 and possibly the transistor. Thought it test correctly. The only real difference between the bench and the field is the length of cable run. Test bench length is about 20ft. Field length is 250FT.
The stranger part--at least to me--is that there are identical circuits on my circuit board. Both with independent rectifiers feeding the IN and with separate outputs connected to the out. At first my thought was that the motor itself had a problem. However when I change the motors around (they have twist lock plugs on my control box) both resonators will run off the good circuit.
I ran the system for about 3 weeks on the bench testing both circuits. There were no issues. Intact I ran them about about 400% duty (timed runs) to attempt to verify the longevity of the circuit and associated field unit.
The motors have an steady state current draw of 1A at 120VDC. With a inrush current of 3A at the same power. At least that is the power that I see with my meter.
One NOTE: Due to the 5V logic needing to be connected to the 120VDC logic (for the gate to fire) I do see the negative side of the power (always on) on the output side of my optical coupler. About -50V. The diodes are there to prevent this from back flowing.
I am an Aerospace Engineer by education. So this is starting to get above my head. When all else fails ask for help right? Can any one help me out on this one?