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Question about potentiometer wiper failure in bench power supplies

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bsfeechannel:

--- Quote from: Ian.M on July 06, 2020, 12:27:07 pm ---e.g. In the LM317 circuit bsfeechannel posted:

if you put the wiper at the ground end of R2 and connect a small signal NPN transistor across R2 with its base fed by the remaining track end, you can get the output voltage to drop to approximately 2V if the wiper looses contact, instead of rising to close to the input voltage.

--- End quote ---

Neat.


--- Quote from: Ian.M on July 06, 2020, 01:09:02 pm ---@Tunk,
Yes, but that LM317 circuit is questionable in other areas.  e.g. if the regulator is fed from a rectifier with no other loads, D3 is redundant,

--- End quote ---

However, if you connect the output to some circuit whose voltage is higher than that of the rectifier your LM 317 is toast. For instance, your load has a huge filter cap that has not fully discharged from a previous operation. Your power supply is not turned on yet, therefore, the voltage at the input of the regulator is zero. I think it's advisable to leave D3 there.


--- Quote ---Also, with that decoupling capacitor on Adj, one has to consider whether the transistor will be transiently outside its SOA if wiper contact is lost.   Worst case, it could have up to 28V across it with 5.7mA of base drive, which, allowing for hFE dropping with increasing collector current, could result in a transient peak dissipation in excess of 3W.

--- End quote ---

Maybe a resistor in series with the capacitor?


--- Quote ---@GlennSprigg,
That's only effective if the circuit (and any load) can tolerate the pot going to max. resistance.  As others have stated their  experience above, if not, it can get expensive!   IMHO, in most applications, linking the wiper to one track end does little except strengthen the pot's PCB pads!

--- End quote ---

Bingo.

ferdieCX:

--- Quote from: Seekonk on July 06, 2020, 06:44:41 pm ---HP had some supplies where the series pot was connected to the output.  Any time the output transistor shorted there would be full voltage on the output.  Of course the first thing you would do was turn the pot to lower the voltage and then the pot would burn up.  Like seeing a fixed resistance on the high side with the pot pulling down the feedback. Most pot dailures are an open so voltage goes to lowest voltage.

--- End quote ---
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.

Ian.M:
@bsfeechannel,

--- Quote from: N.S. LM117/LM317 Datasheet ---Protection Diodes
When external capacitors are used with any IC regulator it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator.  Most 10 mF capacitors have low enough internal series resistance to deliver 20A spikes when shorted.  Although the surge is short, there is enough energy to damage parts of the IC.

When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge into the output of the regulator.  The discharge current depends on the value of the capacitor, the output voltage of the regulator, and the rate of decrease of VIN.  In the LM117, this discharge path is through a large junction that is able to sustain 15A surge with no problem.  This is not true of other types of positive regulators. For output capacitors of 25 mF or less, there is no need to use diodes.

--- End quote ---

So its already got an internal protection diode equivalent to D1 rated for a 15A surge.  If you reckon you still need additional protection, you should be using a beefy Schottky there, not a relatively wimpy 1N4007.

I wouldn't put a resistor in series with the Adj decoupling capacitor as that will reduce its effectiveness.  However adding a 47R emitter resistor to the protection transistor would guarantee a peak current under 600mA, for only a 0.27V increase in the output voltage with the protection circuit active.  A much lower emitter resistor, or even none at all may be practical with appropriate choice of protection transistor to have a high enough Ic surge rating and low enough hFE at high Ic, so it can operate Ib limited.

@All,
Monitoring the current through a potentiometer used as a variable resistor is another method of adding open wiper protection, that unlike my simple protection transistor suggestion,doesn't rely on the integrity of the track.  If you've got a negative bias supply available, it could be as simple as grounding the regulator's feedback network to the virtual earth at the inverting input of an OPAMP configured as an inverting current to voltage converter, and using a window comparator to check that the OPAMP output remains in the expected range for the normal feedback network current, to derive a protection signal that could e.g. be used to gate an output switch or trigger a crowbar circuit.

bsfeechannel:

--- Quote from: Ian.M on July 06, 2020, 11:27:25 pm ---@bsfeechannel,

--- Quote from: N.S. LM117/LM317 Datasheet ---Protection Diodes
When external capacitors are used with any IC regulator it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator.  Most 10 mF capacitors have low enough internal series resistance to deliver 20A spikes when shorted.  Although the surge is short, there is enough energy to damage parts of the IC.

When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge into the output of the regulator.  The discharge current depends on the value of the capacitor, the output voltage of the regulator, and the rate of decrease of VIN.  In the LM117, this discharge path is through a large junction that is able to sustain 15A surge with no problem.  This is not true of other types of positive regulators. For output capacitors of 25 mF or less, there is no need to use diodes.

--- End quote ---

So its already got an internal protection diode equivalent to D1 rated for a 15A surge.  If you reckon you still need additional protection, you should be using a beefy Schottky there, not a relatively wimpy 1N4007.

--- End quote ---

The original manual suggests a little differently, I'm a afraid.

(See attached image. This doggone image attachment plugin is b0rken).


--- Quote ---I wouldn't put a resistor in series with the Adj decoupling capacitor as that will reduce its effectiveness.  However adding a 47R emitter resistor to the protection transistor would guarantee a peak current under 600mA, for only a 0.27V increase in the output voltage with the protection circuit active.  A much lower emitter resistor, or even none at all may be practical with appropriate choice of protection transistor to have a high enough Ic surge rating and low enough hFE at high Ic, so it can operate Ib limited.

--- End quote ---

Makes sense.


--- Quote ---@All,
Monitoring the current through a potentiometer used as a variable resistor is another method of adding open wiper protection, that unlike my simple protection transistor suggestion,doesn't rely on the integrity of the track.  If you've got a negative bias supply available, it could be as simple as grounding the regulator's feedback network to the virtual earth at the inverting input of an OPAMP configured as an inverting current to voltage converter, and using a window comparator to check that the OPAMP output remains in the expected range for the normal feedback network current, to derive a protection signal that could e.g. be used to gate an output switch or trigger a crowbar circuit.

--- End quote ---

I'd like to see a schematic of that circuit.

Bassman59:

--- Quote from: David Hess on July 06, 2020, 12:13:12 pm ---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.

--- End quote ---

I have an old Power Designs 6050C which has that crowbar set-point pot. It’s easily adjusted with a fingernail.

The output adjust pot doesn’t have much friction so it can be easily disturbed. I learned early on to just set the output then set the crowbar.

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