Mr RB said:
Also on my dummy loads I use quite a bit of RC integration on the drive to the power transistors, this gives massive stability and there's no need for the dummy load to be able to change current very quickly, it's adjusted by a pot!
On your circuit you could put a series resistor plus a sizable cap (10uF tantalum?) from the FETs gate-source pins, so that the FET drive is only able to change slowly. Since you have no time constants in your voltage reference or current feedback voltage, it will integrate very nicely and be as stable as the Pyramids.
This is exactly what I was first thinking too.
To me, one stops oscillation with a low-pass filter (slow things down, because the opposite of AC is of course DC.) But I understand the implications of what ronv is saying and it makes perfect sense too. However, what he says is in direct contradiction to the above idea. The more delay in a closed loop, the more it should tend toward oscillation.
I think I understand, but I'm not really sure
During simulation I can only get our FET/Op-Amps circuit to oscillate if there is some significant inductance in series the gate/output wire.
But with the resistor and capacitor integrator it hasn't produced any negative results, which is not in accordance with ronv's take on this, as this configuration does add a whole lot of delay. More than the above inductor + cap does.
It looks like the RC version has delay, but is not
phase delay, and that is making all the difference. If I had known that, I would have finished my original thought and suggested we use a RC low-pass filter on the FET gate like Mr RB has done.
Ronv: ... try putting about 100 ohms in series with the positive lead of the volt meter (value is not to important) then one of your .1 ufd caps from the meter side of the resistor to the negitive lead of the meter to see if it gets better. ...
...
jocanon: ... the noise almost complete went away ...
Good, now we are getting somewhere.
This could indicate that the circuit is in fact genuinely oscillating and needs to be worked on. But it could still also be a misbehaving power supply. We can't do anything about the power supply really, but it is fairly easy to fix oscillations.
My simulations above show that the system will oscillate if there is an inductance (the wire) in series with a capacitance (the FET gate) if there is little to no resistance in line with it. My sim also showed that if you put a resistance in the circuit, it will stop it. This is about the only thing I could see causing oscillations.