Electronics > Projects, Designs, and Technical Stuff

Troubleshooting a constant current sink

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StillTrying:
"In the process of troubleshooting I think I've managed to kill the ADC, the MCU and the digital pot"

That must have been difficult with only 5V on the board, you must be an expert. :)

"Then I also had a look at the output of the op amp and there are some bizarre oscillations on 2 of the ranges:"

You usually give just not enough info, I've found very low frequency oscillations are often on the supply lines, and much higher frequencies usually through the signal path.

magic:
Oscillation is normal and expected in circuits involving opamps with gain devices in their feedback loop.
The transistor adds both delay and gain, making it easier to hit the "unity gain and 180° delay" condition.

A simple way to stabilize such BJT circuit is with a base resistor, which requires the opamp to swing a lot more before an appreciable change happens in BJT output current and therefore in the voltage across the shunt resistor which is fed back to the opa.

OM222O:
I'm happy to announce that it was only the micro controller (which was salvaged from another failed project, so maybe it had enough heat cycles to die XD ) and that everything works flawlessly now. I just need to finish up the code and create a small attachment which allows this device to act as a voltmeter and ammeter as well :D need to add a few buttons for those functionalities but the major part would be the programming of it. also bought different op amps and fets to see if they stop the oscillations or if I need to add a RC filter on the output of the op amp.

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