I'm currently trying to regulate the voltage on an inductive load. The idea is to use an opamp to divide the actual voltage across the coil down, compare it to a set voltage and drive a FET so that the desired voltage is reached.
This is my current design. The first try was without the push-pull buffer but adding it (because the OP can't deliver much current) didn't really improve things. I know that I'm actually measuring the voltage across the transistor, not the coil, but since they're in series, it should have the same effect.
Current through the coil will be between 1 and 1.5A, the coils DC Resistance is 10.5 Ohms.
The circuit works in simulation but is heavily oscillating in reality. When I turn the potentiometer, the "duty cycle" of the oscillation changes, but it only ever stops oscillating if the control voltage is either 0 or 5V.
It looks like this on the scope, where the pink trace is the control voltage on the - input of the OP, yellow is the divided down voltage across the FET on the + input of the OP and blue is the output of the OP:
I would be very greatful if someone could point me in the right direction. I don't know, what else I could try.
Thanks in advance
Edit: I kind of improved the situation by leaving out the push-pull stage and conencting the opamp output via a ~200kOhm series resistor to the FET gate. The circuit looks like this now: The first stage calculated the differential voltage across the coil and divides it by approximately 7 so it can fit in the 5V rails of the opamps.
It does what it's supposed to, now, at least somewhat reasonably. Any hints on how to reduce the ringing on the step response would be much appreciated though. (Blue trace is a step on the control voltage, yellow is the response)