Electronics > Beginners
LM317 Dummy load circuit
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Ian.M:
No, by 'flat-out' I meant that it didn't have any local negative feedback.   If you add anything else with too much gain inside the negative feedback loop of an OPAMP without local negative feedback, you almost always run into trouble as there's still too much gain at the lowest frequency where the phase shift round the loop is sufficient to make the originally negative at DC feedback positive.  The gate capacitance of the classic MOSFET circuit, in combination with the OPAMP's output impedance has a massive damping effect on the loop at higher frequencies.
10K +100nF would usually be massive over-kill.  The resulting corner frequency is only 160Hz so the transient response is gonna *SUCK*.  My first response, in the absence of a proper analysis, would usually be to reach for my box of ceramic caps <1000pF, and  try one per device I need to roll-off at HF, starting with something in the 50pF to 300pF range.

As you are at work, here's the circuit as a GIF.
VEGETA:
I have tested the circuit after removing the pot stuff all together, and put a voltage instead. It seemed to work well at 3v input or more... less than that will not work. I couldn't get two 9v batteries today, I will get them tomorrow and it will be the practical test.
Ian.M:
You may want to start with the response 'slugged' for the real circuit to avoid high power HF oscillation.   Try increasing C1 to 4.7nF, and R5 to 4K7.   An LTspice .ac sim shows a very flat response up to 10Khz then within +/-3dB up to 30Khz with minimal objectionable peaking.

Sim attached.  Hint: plot Vfb - you are looking for as close as 0dB as possible with respect to the current control signal input Vset, then plenty of phase margin as it starts to roll off at its corner frequency.
VEGETA:
I put LT1013A instead of LM324 since it does not work... The result has many waves. I managed to print Vfb alone and Vfb-Vset as seen in attachment.

I notice some curves started to appear near 30KHz as you mentioned... Problem is I don't know how to analyze this AC stuff. I know a bit about poles and zeros, also about stability of op-amp according to phase margin or so. Like, looking at these graphs, what can we conclude? plus, why there are lots of lines? is it because of step function.

What is left to do in this circuit? let's put some points that we can work with.
Ian.M:
Yes, with that OPAMP and a couple of other LT 'precision' ones I tried, it doesn't seem to want to stabilise no matter what we do in the OPAMP feedback loop.  I suspect an issue with the LM317 model as it isn't always passing the minimum current it requires for regulation.   Perhaps a rethink is in order: Discard my idea of using the LM317 inside the loop, and simply let the LM317 run as a regulator, similar to your original concept.

However the LM317 model seems to implement feedback current limiting at a little over 15V across it when passing 1A, so if that's realistic you may not get the load you want with one LM317.  If you use two, Adj and In in parallel with separate 0.2R current set resistors it will probably work better.

Use the OPAMP to convert 0 to 5V from your PWM to -1.25V) to -1.15V) on the ADJ pin to vary the voltage across the 0.1R current set resistor.

You either need a precision reference for the bias voltage that offsets the OPAMP output, or two Zeners cascaded for better stabilisation as I have drawn, or it will let the current rise excessively as the + battery discharges.

I've put some attention into rolling off the HF response of the OPAMP and decoupling Adj to hopefully make it more stable in real life.

Sim attached.
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