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Here is another question on the originally Czech bench power supply that I recently built, but in the version¹ of it sold by various places (e.g. eBay) very cheaply.

The circuit has two potentiometers for setting output voltage and maximum output current; these are both working fine. It also has a trimmer over one of the op-amps' offset for adjusting the 0V point of the output². Right now, when the device changes to a current regime and turns off the output, I'm seeing an output of a little more than negative 0.5V, which could be potentially damaging to an attached DUT.

Schematic as drawn by Mile Kokotov at https://www.qsl.net/z33t/dc_0-30v_0-3A_eng.html

Bill of materials as compiled by Mile Kokotov at https://www.qsl.net/z33t/dc_0-30v_0-3A_eng.html

Things I've tried:

  • measuring the trimmer's resistance while in-circuit (at various settings)
  • removing the trimmer from the PCB and measuring its resistance that way – the trimmer itself is good
  • pulling either one of the offset pins to GND during the above-described condition, to see if anything will budge
  • pulling both offset pins to GND – again, nothing
  • switching out the op-amp to another TL081
  • switching out all op-amps for TL071s
  • applying a small voltage to the offset pins from a 4.2V battery
  • playing with the trimmer at various other output voltages – it never had any effect there, either

I then remembered that while waiting for the kit to arrive I'd read something about that trimmer in the Google Translate version of a Czech electronics forum's thread. Unfortunately I can't find that anymore, but am now suspecting that something might be wrong with the circuit design itself.

What could have been the idea behind putting that trimmer there and what might be the reason(s) for it not doing anything, or at least, not doing anything apparent to me at this point?


¹ These PSUs are said to be adaptations of https://www.electronics-lab.com/project/0-30v-laboratory-power-supply/, which is an adaptation of https://www.electronics-lab.com/project/0-30-vdc-stabilized-power-supply-with-current-control-0-002-3-a/, which is an adaptation of http://paja-trb.cz/konstrukce/zdroj.html, which is an adaptation of the original work at http://paja-trb.cz/konstrukce/zdroj/zdroj_G400.pdf by (brothers?) Jindrich and Tomas Tolg.

² According to https://www.qsl.net/z33t/dc_0-30v_0-3A_eng.html ("The trimmer RV1 and the resistor R10 are used for the adjustment of the output voltages limits so that it can be reduced down to 0 V, despite any value tolerances of the other components in the circuit").

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2 Answers 2

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Right now, when the device changes to a current regime and turns off the output, I'm seeing an output of a little more than negative 0.5 V.

That seems very unlikely.

enter image description here

My reason for saying that is that the output transistor can only connect to the rectifier positive supply.

The only thing I can think of is that that Q4 is turning off completely, U2 is going negative and drawing urrent through R7, D11, D10 and R15. The output voltage would then be clamped at -VF of D11 (as R7 is only a half ohm shunt and won't drop much voltage).

Try monitoring the output of U2 under current limit conditions and see what's going on.

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  • \$\begingroup\$ "That seems very unlikely" – my three multimeters disagree, though. "Try monitoring the output of U2 under current limit conditions and see what's going on" – okay, will do tomorrow, shut everything down for the night already. Thank you! \$\endgroup\$
    – Sixtyfive
    Sep 13, 2020 at 21:34
  • \$\begingroup\$ agree: To my understanding U2 seems the top source of an neg. Voltage generated between + and - output... \$\endgroup\$
    – schnedan
    Sep 13, 2020 at 22:46
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Looks to me like the current would be limited to no more than a few mA, which is unlikely to damage anything.

A 1N5819 from the base of Q2 to ground would probably kill even that.

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