When [Tahmid Mahbub] recently reached for his ‘Lavolta’ BPS-305 bench supply, he was dismayed to find that despite it being a 30V, 5A-rated unit, the supply refused to output more than 15V. To be fair, he wasn’t sure that he had ever tried to push it beyond 15V in the years that he had owned it, but it had better live up to its specs. Ergo out came the screwdriver to open the power supply to see what had broken, and hopefully to fix it.
After some more probing around, he discovered that the unit had many more issues, including a highly unstable output voltage and output current measurement was completely wrong. Fortunately this bench power supply turns out to be very much like any number of similar 30V, 5A units, with repair videos and schematics available.
While [Tahmid] doesn’t detail his troubleshooting process, he does mention the culprits: two broken potentiometers (VR104 and VR102). VR104 is a 5 kOhm pot in the output voltage feedback circuit and VR102 (500 Ohm) sets the maximum output current. With no 500 Ohm pot at hand, a 5 kOhm one was combined with a 470 Ohm resistor to still allow for trimming. Also adjusted were the voltage and current trimpots for the front display as they were quite a bit off. Following some testing on the reassembled unit, this power supply is now back in service, for the cost of two potentiometers and a bit of time.
I have a love hate relationship with power electronics
I love what its capable of, many of my projects are power conversion related. I love getting into all the different ways to optimise the circuit.
I hate that making anything actually cool or unique requires hand winding a custom transformer with expensive copper wire, going through a few high power FETs or other components until you can get the snubber right. Its a wallet hitting subset of electronics for sure!
You can consider exploring power electronics that don’t require custom transformers. Some possibilities are point-of-load converters, non-isolated converters, (hybrid) switched-capacitor converters, and the like. You can also experiment with planar transformers and using PCBs for winding instead of wires.
Can’t argue with you about the wallet hitting, that’s for sure.
Were they bad or just dirty? One forgets that a pot never touched like a volume control on a disused radio will crackle and go open till it gets cleaned and woken up. When you can’t hear the noises is it bad? Those exposed trim pots aren’t great. Tweak a dirty pot like this and all hell breaks loose.
They appeared to be bad unfortunately. I suspect they weren’t faulty initially and just aged out and connections broke. I expect the same issue can pop up again on the replacements I added, but time will tell.
I think his troubleshooting was done well.
But I think by using a decade resistor box an optimal resistance could be found and substituted for the trimpots. There are calculators on the web to determine resistor pairing to get a desired resistance.
That’s a good idea. Now that I know what the rough values are, next time this problem pops up, I can do that!