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Im making a battery tester / discharger. For this i'm using a constant current circuit made with an opamp and a mosfet. I built and tested the circuit by itself with around 3-4A and it was stable. I needed higher currents so i made 2 PCB's each with 4 parallel constant current circuits. Problem is it oscillates when the current gets to high. The diode is just for polarity protection.

I am about to go crazy over this behaviour as i don't really understand what is causing the oscillations and therefore i don't know how to get rid of it.

schematic

simulate this circuit – Schematic created using CircuitLab

If you have any ideas as to how i can get rid of the oscillations, please tell me. When i tested the circuit by it self i found C1 and R2 necessary to keep it from oscillating. If you need further information, just say so and i will update the question.

In advance, thank you!

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  • \$\begingroup\$ Can you elaborate on the purpose of this circuit? maybe someone can suggest something better. \$\endgroup\$ – Norm Apr 24 '18 at 12:57
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    \$\begingroup\$ To stabilize it you can try to add a resistor (e.g. 1kOhm) from top of R1 to the inverting input and a capacitor (e.g. 50 pF) from the opamp output to the inverting input. The best values for the components are hard to guess at. \$\endgroup\$ – HKOB Apr 24 '18 at 12:57
  • \$\begingroup\$ what do you mean by parallel circuits exactly? \$\endgroup\$ – PlasmaHH Apr 24 '18 at 12:59
  • \$\begingroup\$ electronics.stackexchange.com/questions/246321/… \$\endgroup\$ – τεκ Apr 24 '18 at 13:02
  • \$\begingroup\$ electronics.stackexchange.com/questions/187563/… \$\endgroup\$ – τεκ Apr 24 '18 at 13:03
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Decouple the heavily capacitive loading on the op-amp (gate capacitance and Miller capacitance of the MOSFET) with a resistor of 100 ohms or so, and add an AC feedback path around the op-amp to stabilize it.

Op-amps will tend to oscillate with capacitive loads, anything more than tens of pF can cause problems, depending on the gain (unity gain is worst) and the op-amp design.

I don't see any point to the 10K in your circuit.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Tried your suggestion. When in my test setup it works! Also with 8 in parallel. Thank you! Got it running stable with over 30 amps. \$\endgroup\$ – Swagministeren Apr 26 '18 at 6:12

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