# op amp Why does this current limiter circuit oscillate

Posted on Feb 6, 2014

Make a 0-25 mA current limiter. I want to take a control voltage of 0-5 V as input and have it control a current of 0-25 mA through a restive load (the restive load may vary between 0-200, it`s represented as R2 below). The O2 op-amp is used to make a differential amplifier with a gain of 2. Example: 25mA of current through the lo

ad R2, op-amp O2`s output should be 5V (marked CSense). The output from op-amp O2 is then the input to op-amp O1. Op-amp O1 compares the control voltage with O2`s feedback to limit the current. Or at least that`s my intention. I have triple checked my pin-out and wiring, and messed around with lots of other things, but the circuit still just oscillates. When V2 is a higher value the oscillation is slower. What I am seeing on the DSO is that mosfet T1 goes between full on and full off, and CSense is a triangle wave that varies in amplitude with the control voltage. A higher control voltage makes CSense have higher amplitude and lower frequency. The oscillation frequency varies from about 50kHz to 150kHz. Update Everyone is focusing on O1, but I think the problem is in the O2 circuit. Below is a DSO screen shot. V2 is at 1. 25 VDC. The yellow trace is the voltage over R2 @ 100. The red trace is the output of O2 (marked as CSense in my schematics). I would expect the red trace to be 2x the yellow trace, but instead it`s a completely different shape! What in the world is going on For clarity: I would expect the yellow line to be 0. 625 VDC, and the red line to be 1. 25 VDC (same as the V2 input). Also my power supply is ~8 VDC here, things blow up at 24. The V2 input is a sine wave. The output is supposed to follow the input. If you change V2 to DC, the output will also simulate as DC. On my bench, V2 is DC, but the output...

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