Variable supply has become faulty (LM317 no ref. Voltage).

[Yamin]

Hi guys,
I've been working on some variable power supplies and I posted a discussion trying to figure out how the circuit works here:
https://www.eevblog.com/forum/projects/trying-to-understand-this-variable-power-supply-circuit-(lm317)/new/?topicseen#new

I must admit I am not 100% sure with some parts of the circuit. Anyway now 'shit has hit the fan', I have got a couple of power supplies fail on me. Its used at the local college for teaching purposes. The variable output should go from 0V - 30V but what happens with the faulty units are the range is from around 6V - 11V. I have changed the LM317 regulator. The regulator I soldered were pre checked in a working circuit and it works fine. I changed the JFET as well. But no luck! There are no change to the result. Voltage measurements against a good working unit agrees with the faulty units. eg. across the JFET and some resistors. I have checked all the diodes everything checks out ok. One important thing I noticed was that across LM317's Adjust pin and output pin it does not read 1.25V its around 8V!.
I haven't de-soldered the regulators to check whether they are damaged yet. If something in circuit was damaging the new regulators what could it be.

I was hoping that you guys could shed some light into what could be the issue and what to check for. Thanks so much.

p.s I should add that these units were working well for years before they started to fail. Some of the damage might have been caused by the students, for instance shorting out the outputs.

[xavier60]

What voltage is VSS?
The symptom could be because something is open or high resistance in the path between the the Adjust and Output pins.
What is the 8V dropped across mainly?

[Yamin]

VSS is -15V. I would have to check where the voltage drop happens mainly. But I couldn't find anything out of place. Everything seems to check out :S.

Thanks so much

[duak]

Some things to check:

What are the individual voltages across R7, R91 and R90?

Have you observed the output voltage with an oscilloscope?  One of the capacitors may have failed.

The LM317 can be damaged by reverse currents from the ouput to the input. eg., connecting this power supply to an energized circuit will send current back through the regulator to charge the capacitors on its input.  The solution is to add a 1 amp diode from the LM317 output pin to the input pin with the cathode of the diode on the input pin.

The LM317 has a maximum voltage rating of 37 to 40 V.  Could the input voltage ever get this high?

[Yamin]

Its true the input voltage is quite high! The functioning units also measured above 40V at the input of the regulator. The baffling thing is that these units have been working for years with no trouble what so ever apart from maybe a fuse blowing.

[duak]

The manufacturer of the LM317 says that it is expected to operate up to the maximum voltage but it doesn't guarantee that it will be damaged above that voltage.   The manufacturer may use a process that allows 50 V but cannot guarantee it under all conditions.

Black humor warning: it is generally accepted that smoking can cause cancer but it is not guaranteed to do so.

By the way, there is or was an LM317HV that is rated for a higher voltage.  It should work in this circuit.  It may be that the manufacturer doesn't use a different process and simply that some parts survive a higher voltage during testing and become HV parts.

The transformer may be saturating and thus protecting the following circuitry.  Are you able to change the transformer input or primary taps to reduce the voltage?  Is there room in the unit for any additional circuitry to reduce the voltage?

Good luck with finding the problem and a good solution to it.

[xavier60]

I wonder why the Polyswitch is there. Polyswitches can go weak, tripping at a much lower current but it wouldn't be seeing much current anyways.

[aqibi2000]

-15V for VSS for a positive voltage regulator?

[floobydust]

I wonder why the Polyswitch is there. Polyswitches can go weak, tripping at a much lower current but it wouldn't be seeing much current anyways.

When an LM317 fails short-circuit, it smokes the potentiometer. I see the polyfuse protect the potentiometer, I've always used a small fuse to protect multi-turn pot from damage there.

edit: OP's power supply actually has the polyfuse in the wrong spot - it should be right in series with the potentiometer. Failed LM317 usually ADJ to VIN shorted which smokes the potentiometer when you try dial down the voltage.

[Yamin]

@floobydust - Which potentiometer do you mean? The main one or the small 'tuning' potentiometer? I was able to contact manufacture and they also suggested to change the small tuning potentiometer even though it test ok.

Thanks everyone for the help and hope everyone is safe!

[floobydust]

I was talking about the main output adjust potentiometer R4, but tuning pot R7 is also vulnerable.

Think of the LM317 shorted while you turn down the output pot R4.
High current will flow through PTC R90, R91, R7 trimmer, R4 pot, V5 diode. The weakest element will trip, hopefully the PTC fuse but instead it could be R7 or R4 that smoke.

If the LM317 ADJ pin is shorted to Vin, while you turn down the output pot R4 high current will flow through R4, V8 V34 diodes. The weakest element will fail.

Use an ohmmeter to check resistance of R7 string and R4, and see if the diodes survived. Check for -2V bias.
This power supply is missing the Vin-Vout protection diode. It could not handle battery charging or loads with high capacitance, that could damage the LM317.

[tautech]

Some things to check:

What are the individual voltages across R7, R91 and R90?

Have you observed the output voltage with an oscilloscope?  One of the capacitors may have failed.

The LM317 can be damaged by reverse currents from the ouput to the input. eg., connecting this power supply to an energized circuit will send current back through the regulator to charge the capacitors on its input.  The solution is to add a 1 amp diode from the LM317 output pin to the input pin with the cathode of the diode on the input pin.

The LM317 has a maximum voltage rating of 37 to 40 V.  Could the input voltage ever get this high?

This ! ^^^
Any design using these LM*** regulators NOT using reverse current protection is doomed to fail.

Consult datasheets for correct implementation of protection diodes.