How to stop oscillation, advice needed.

[IvoS]

I have built current sink according to Dave's video. Powered by LM317, OPamp I used is LM6132. I am getting stable oscillation around 375kHz (nice sine oscillation) right from start. When I am increasing the sinking current the oscillation amplitude is also increasing. It gets up around 0.6Vrms for approx. 1A sinking current. Now, I have modified the circuit by adding 2 resistors and one cap according to video #304 ( based on sleepyrobot schematic). The behavior improved a bit. There is no oscillation up to 360mA, then, from that point, when I keep increasing the current it starts oscillate like crazy again, this time on 570kHz. What should I do to stop this oscillation on full scale current sink (around 2A).??

[tom66]

The problem is your cap across the output of the op-amp. Almost all op-amps are unstable with such a large capacitive load.

However, you are still likely to have problems with the circuit, I had the same issues when designing a constant current sink for a battery tester. So, I added an input resistor in the inverting input (220 to 330 ohms), a feedback resistor (470k) and a feedback capacitor (100n) across the resistor. This will make risetime slower but makes it very stable.

[madworm]

Also make sure the output of the LM317 is stable and doesn't have any residual ripple on there. I just recently was bitten by an unstable voltage regulator that had about 20mV ripple on it's output.

[MikeK]

The help I got here: https://www.eevblog.com/forum/beginners/non-inverting-op-amp/msg117906/#msg117906 worked for me.

[amspire]

The problem is your cap across the output of the op-amp. Almost all op-amps are unstable with such a large capacitive load.

C7 does not act as a capacitive load, so that is not the problem.

The issue is the phase margin of the opamp down near unity gain - it is too marginal. Looking at the data sheet, adding a 2K resistor from the opamp output to ground should help a lot. Also reducing the gain of the mosfet stage by reducing the shunt resistor to 0.1 ohms or less. This has the advantage of reducing the heat in the current shunt which is good, but it increases the effect of the opamp offset error.

Using a LM324 instead of the LM6132 should also make it stable.

Richard.

[IvoS]

Thanks guys. I will try it tomorrow and see.... Unfortunately, I can't (don't want to) change the shunt resistor value because I need that 1V reference for my panel meter (2V panel meter).

[LEECH666]

I plan to solve this problem by using a summing amp with gain 1 followed by an inverting amp with a gain of 10. But in your case a non inverting amp with a gain of 10 will be enough.

Just an idea.

See attached PDF file for my version of a simple dummy load.

Cheers,
Florian

[tom66]

The problem is your cap across the output of the op-amp. Almost all op-amps are unstable with such a large capacitive load.

C7 does not act as a capacitive load, so that is not the problem.

The issue is the phase margin of the opamp down near unity gain - it is too marginal. Looking at the data sheet, adding a 2K resistor from the opamp output to ground should help a lot. Also reducing the gain of the mosfet stage by reducing the shunt resistor to 0.1 ohms or less. This has the advantage of reducing the heat in the current shunt which is good, but it increases the effect of the opamp offset error.

Using a LM324 instead of the LM6132 should also make it stable.

Richard.

Are you sure? I'm no expert on this but I read many op-amps are not stable without a damping resistor when driving a large capacitor. (2.2nF in this case is very, very large for the op-amp.)

I have found the LM324 (or it's cousin the LM358) to be useless in this application; the offset error means it will sink ~50mA with zero input!

[IvoS]

Looks like the problem is solved! Many thanks to "amspire" Richard for helping with this issue. I added 2k2 resistor at the OPamp output and the oscillation disappear. Thanks again.

Ivo