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Question about potentiometer wiper failure in bench power supplies
ferdieCX:
Hi, there is something that has puzzled me several years.
It is very common to see "scholar book" series regulators like the one in the picture. If the wiper looses contact for a moment, the load will receive the full input voltage and, may be, get destroyed.
The strange thing is, that it seems that many commercial bench power supplies could have this kind of failure.
Even the traditional HP linear regulator gives an output voltage proportional to the resistance of a potentiometer whose wiper is connected to one of the ends.
I wonder why the designers don't care about that. :-//
After destroying several ICs about 30 years ago because such a failure, I got a Philips power supply that gives 0 V if the wiper fails.
bsfeechannel:
--- Quote from: ferdieCX on July 05, 2020, 09:06:27 pm ---It is very common to see "scholar book" series regulators like the one in the picture. If the wiper looses contact for a moment, the load will receive the full input voltage and, may be, get destroyed.
--- End quote ---
Tell me about it.
David Hess:
--- Quote from: ferdieCX on July 05, 2020, 09:06:27 pm ---It is very common to see "scholar book" series regulators like the one in the picture. If the wiper looses contact for a moment, the load will receive the full input voltage and, may be, get destroyed.
--- End quote ---
Experienced engineers avoid using potentiometers that way for the reason you identify.
--- Quote ---Even the traditional HP linear regulator gives an output voltage proportional to the resistance of a potentiometer whose wiper is connected to one of the ends.
--- End quote ---
If the wiper is connected to one end, then an open wiper cannot cause the resistance to be higher than the total resistance of the potentiometer.
--- Quote from: bsfeechannel on July 06, 2020, 05:38:49 am ---Tell me about it.
--- End quote ---
See above.
Fraser:
I had three Thurlby linear Bench power supplies in my lab. Supposedly Lab grade equipment. All suffered the same horrible failure. The track on the voltage setting potentiometer would get worn in specific areas that are most used, such as 5V, 9V and 12V. One of them was set to 9V one day and suddenly jumped to the full 15V output for a moment and destroyed the power supply circuits in an expensive Multimeter/DSO that I was working on. It was then that I investigated what had happened and found all three of the Thurlby power supplies to have ‘issues’ with the voltage setting potentiometer. Having sourced the schematic, it was clear that the design is flawed and any intermittent contact of the potentiometers wiper causes full p.d. to appear at the output terminals. All three power supplies are currently sat in a box in my garage in disgrace. I was going to fit new potentiometers but have now moved to microprocessor controlled power supplies with no potentiometer wear issues to concern me.
Fraser
David Hess:
--- Quote from: Fraser on July 06, 2020, 11:59:29 am ---One of them was set to 9V one day and suddenly jumped to the full 15V output for a moment and destroyed the power supply circuits in an expensive Multimeter/DSO that I was working on.
--- End quote ---
On the ancient Tektronix TM500 series power supplies that I have, they included instructions for changing the trip point of the output crowbar circuit to protect against accidentally setting the output voltage too high. Another feature they have is that when tracking is enabled, the individual controls can be used to limit the outputs while the tracking control is used to set the output voltage but only within those limits.
I have seen old laboratory power supplies which included a screw adjustable potentiometer for setting the trip voltage of the crowbar on the output; the Power Designs TP340 is an example that has this.
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