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Question about potentiometer wiper failure in bench power supplies
David Hess:
I prefer coarse and fine adjustment potentiometers to 10-turn potentiometers unless long term stability is required.
rdl:
That LM317 adjustment set up near the bottom of the page is slick.
--- Quote from: pardo-bsso on July 07, 2020, 02:25:13 pm ---Hi, check here: http://www.imajeenyus.com/electronics/20160517_potentiometer_feedback/index.shtml
Besides going over the open wiper problem that article also explores different configurations and the response they give (voltage output vs. pot position).
--- End quote ---
bsfeechannel:
--- Quote from: Conrad Hoffman on July 07, 2020, 12:04:31 pm ---This is the same problem many audio amplifiers have with the bias pot. Pot goes open, amplifier goes to full bias and the outputs overheat and fail. Good designs don't do that. BTW, wiper current should be kept to a minimum for most pots if you want reliability.
--- End quote ---
Ah, that explains the "extra" resistor I saw on this schematic:
R3 is an emergency pull-down resistor. This ensures that if the pot wiper fails to make good contact with the rack (because it is dirty or worn out, say) the grid leaks will effectively be connected directly to the raw bias supply through R3 instead, biasing the valves safely cold. R3 can have a value of about ten times P1 so it has negligible effect on the rest of the design.
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