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Question about potentiometer wiper failure in bench power supplies

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ferdieCX:

--- Quote from: ArthurDent on July 07, 2020, 06:52:42 pm ---A lot of the better (read 'expensive') bench power supplies have OVP, OCP, and even OPP settings to protect equipment you're testing. Here are photos of a couple. The GW Instek power supply is a modern supply with an encoder and no pot for adjustments.

--- End quote ---
Several expensive bench power supplies that were built for many years, have this issue with the pot wiper failure.
I suppose that microporocessor controlled power supplies could also have their own problems, for example firmware issues.

David Hess:

--- Quote from: ferdieCX on July 06, 2020, 10:18:52 pm ---In several HP supplies, the output voltage is a linear function of potentiometer resistance. If the pot fails open, the voltage goes to the maximum.
--- End quote ---

That may be deliberate so that the power supply can support remote programming.

duak:
Based on a sample space of five or so lab supplies with hazardous problems I've encountered, only one had a bad output voltage pot.  It's one I'd like to fix and I'll get back to it in a bit.

The most common fault was a shorted pass transistor.  The hp 6002A had a shorted pass transistor and someones misguided replacement of an op-amp.  The crowbar circuit was tripped at anything other than zero output so I got the unit for free.

A Kikusui auto ranging supply had a bad output bypass cap that could break into oscillation at certain load currents.  Having a supply set to 12 V to test an electronic speedometer assembly and have it superimpose a 30 Vp-p AC signal on it was exciting.  I thought I fried the speedo but it's a good thing that it had good filtering and clipping circuits on the power input.

I suppose one way to make the supply fail-safe is to use a dual ganged pot and some sort of wired-OR circuit to select the lower voltage.  Or the second section could set a variable limit that would disable the output or trip a crowbar if there was a discrepancy.

In an hp type floating supply the regulator sources a reference current that is converted to a variable command voltage by the pot.  It should be possible to disable the regulator or trip the crowbar if the reference current drops below a threshold, as when the pot opens or goes over a rough patch.  However, because these supplies can be put into various series and parallel configurations, this will likely cause trouble in some cases.

To get back to the supply with the bad pot, I have a 40 V Lambda unit with a twitchy output. On these supplies, the output capacitor and circuit is discharged by the pot when it is turned down.  The pot  is fried, cleaning no longer helps and there are no new replacements.  It's a dual coaxial wirewound with values something like 8K+500R.  I don't like 10 turn pots because it takes so long to wind from one end of the range to the other.  The hp supply above has a 10 turn pot but I prefer the Lambda for many tasks.  At one time, it was easier to get coaxial pots so I've been casting about for solutions.  Some of them are:

1.) just get a 10 K WW pot and put a resistor across it to restore the original voltage limit,  Years ago, I ran across a two speed planetary drive where small movements were at a 3:1 ratio but larger movements (> 90 degrees) were 1:1.
2.) look for a coaxial switch and pot combo.  Use the switch to select fixed resistors that are then trimmed by the pot.  At one time things like this were easier to get - I'm sort thinking about an old Tek or hp scope attenuator or timebase switch assembly.  I've even thought of making something with gears, but there isn't much room.
3.) design an electronic replacement with an encoder and either a DAC or EPOT.  The circuit would not have to simulate a resistance but only have to clamp the programming voltage into the floating regulator to an arbitary voltage determined by the encoder setting.  A coaxial push-pull switch could control the resolution or some sort of speed sensitive ballistic action.

Any thoughts?

phil from seattle:
with the encoder approach you could implement a proportional response - slow means very small increments, fast means much larger. Seems like a big win. I have a cheapo Chinese PS that works surprisingly well but the last few mV or mA of adjust is twichy. Would love proportional adjust.

Vovk_Z:
Described in first post problem is why I like to use a multy-turn potentiometers in power supplys to control the voltage - they "jump" too, but usually they don't totally loose a contact but only jump for about one turn (so it isn't very high in voltage).

If I not used them - I'll loose to much devices too.

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