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1A current source but still not stable

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dmm2018:
Hello...

After watching Dave's videos: Precision 1A Current Source (Part 1 & Part 2), I've been wishing to build one. I have just completed one few days ago and wish to share the results.

R1, R2, R3, R4 and R5 are all 10 kohm (Vishay S102C, 5 ppm/degC, sourced from hifi-szjxic)

RM is VPR221 (0.5 ohm, 2 ppm/degC, sourced from hifi-szjxic)

RP consists of two resistors connected in series
-   RP1 = 200 ohm (TE, 15 ppm/degC, sourced from element14)
-   RPT = 50 ohm trimmer (Bourns 3296, 100 ppm/degC, sourced from element14)

Q1 is RFP12N10L (Fairchild Semiconductor, sourced from RS)

AD780AN is a voltage reference (configured to give 2.5 V, salvaged from a scrapped 15-year old DAQ board). The AD780 is powered by a +5 V, generated by LM7805, supplied by a battery.

AD708JN is an op-amp (salvaged from a scrapped 15-year old DAQ board). The AD708 is powered by dual supplies (+/- 12 V, batteries).

Two DMMs (connected in loop) were used to measure the current:
- Keysight 34470A (N) - calibrated 5 months ago;
- Keysight 34470A (L) - calibrated 54 months ago;

The DMMs were warmed up more than 1 hour.

The current source with 2 different VS, i.e. 6V and 12V, were tested. The recording started after the current source was powered up for 1 min. The current source with VS = 12V was tested first, then stopped 10 mins before continuing with the test with VS = 6V. Each record took 30 mins.

The tests were conducted in an air-conditioned room (the aircond temperature setting was 18 degC), felt a little air-flow.

With VS=12V, the current kept drifting (descreasing) in the 30-min duration. With VS = 6V, the current seems to settle faster, but still have fluctuation, probably due to the not so stable room temperature or airflow. Still thinking why with VS = 12V the current kept decreasing and not stable...

Please comment and suggest how could further improve the stability of the current source.

Thanks.







Zero999:
It's oscillating. Try looking at the output with an oscilloscope.

An additional RC circuit is required to stabilise the circuit.

https://www.electronicsweekly.com/blogs/engineer-in-wonderland/current-sink-stability-2015-10/
https://www.eevblog.com/forum/projects/compensation-methods-in-current-sink-circuits/msg735672/#msg735672

Rerouter:
Easiest way to get it more stable, Add a generic 10K resistor to the inverting leg (this one can be 5%, its just to give the feedback capacitance some room to work), and from the inverting pin to the output pin of the op amp, add something in the order of 1nF, this will decrease the response speed of the loop, but make it more stable (when its oscillating, its correcting faster than the mosfet can react)

Next up is add a generic 10K resistor from the op amp output to ground (again can be 5% cheap resistor), this keeps the output of the op amp biased on, and helps it deal with the capacitive mosfet a bit nicer.

strawberry:
https://www.analog.com/media/en/technical-documentation/application-notes/an133f.pdf
for generic OP is hard to drive capacitative(MOSFET gate) loads

dmm2018:
Thanks Zero999, Rerouter and strawberry.

When the circuit was on breadboard, we did measure the voltage across the RM (0.5 ohm) using a Tek scope and the voltage waveform looked stable. However, after the circuit was on a PCB, we have not measured the voltage waveform across the RM yet. We will check the waveform again to confirm whether there is oscillation. We will also add resistors/capacitors to further enhance the performance of the circuit.

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