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://forum.allaboutcircuits.com/...ble-power-supply-circuit.167704/#post-1484380
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.
I've been working on some variable power supplies and I posted a discussion trying to figure out how the circuit works here:
https://forum.allaboutcircuits.com/...ble-power-supply-circuit.167704/#post-1484380
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.
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