Electronics > Beginners

OpAmp oscillation

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Dubbie:
I needed a low power, relatively precise electronic load for some small battery testing I am doing, so I thought it would be good practice to design one.

Because this is just a one off, I wanted to keep things simple.

I have attached the design I came up with. I simmed it and everything seemed to be fine.
Problem is, now that I have made it on a 3M breadboard, It oscillates like crazy at about 400Khz
besides the oscillation it does actually work... I get the right average current. I'd just rather have it without the crazy oscillation.

I have decoupled the LMC6482 opamp well.

Where do I start to calculate a way to kill the oscillation?

floobydust:
Good practice would be a capacitor to GND on the setpoint (+) input to prevent noise on 5V from getting in, and a comp cap from op-amp output to its (-) input- but after adding an additional 1k-10k to the sense-resistor. The 10R resistor is pretty low impedance there.

The sense resistor 10R at 70mA = 0.7V, plus the mosfet VGS losses likely a couple volts. What mosfet are you using? You might not be able to maintain constant current below several volts, which is not the greatest for battery work.

Dubbie:
I had tried a 10nF in there Blueskull, and it didn't really do anything. However I put a 100nF in there and it worked perfectly.

Everything seems good now!

I am using a 30N06L mosfet which is working fine down to 2.4V which is below where I need.

Thanks Floobydust for the tip regarding the cap to decouple the 5V rail noise. Hadn't occurred to me.

Thanks for the advice guys!

R

AndersJ:
To me,
the cap is a quick and dirty fix to another primary problem.
What is the reasoning behind the cap?
What problem does it fix?

Alex Nikitin:
The LMC6482 is not very tolerant to capacitive loads, something like 1nF (which is roughly the input capacitance of the 30N06L) on the output could easily make it unstable (and it is well documented in the datasheet for LMC6482). So, RTFM!

Cheers

Alex

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