DIY power supply PSU short to enclosure - Page 1
Forum section: electronics / beginners

author: Renegade
author_rank: Regular Contributor
timestamp: February 21, 2016, 04:23:19 pm
content: I've just finished building a benchtop power supply with a computer ATX PSU. The enclosure is solid steel. Here's a pic:I had it assembled, and it would work and then stop working. I couldn't figure it out, so I opened it up. pulled out the PSU, tested the PSU (no problems -- all voltages good), tested the disassembled unit (again), and it worked.Then... it occurred to me that I might want to short the ATX PSU case to the steel enclosure... tiny sparks. And the unit shut down.So... for some reason the ATX PSU is shorting with the case and shutting down. But I don't know why.MORE INFOIn the picture, my thumb is over a green LED that indicates that the PSU power is on.The switch in the middle is an ON switch. It connects the green ATX PSU wire to ground.The voltages of the binding posts from bottom to top are:* +3.3V* +5V* -5V* + 12V* -12V* +12V variable (see below)The 4 potentiometers are for a buck/boost CC/CV converter for the top +12V variable binding posts. They are metal, and are simply screwed in as normal. I don't think this should be an issue, but I'm not sure. (At this point, I've taken it apart and put it together so many times that I'm pretty much exhausted, and simply can't stand the thought of unscrewing it all and testing for shorts with them,so... asking here first.)For the buck/boost CC/CV converter, it has a 50V shunt wired in for the ammeter in the large display at the top. The shunt has a large plastic base, so it isn't the source of any short.The binding posts have plastic where they go through the front of the enclosure, so those aren't shorting to the enclosure.The three 3-digit displays are voltmeters and are simply hot-glued in. They are not the source of any shorts.In short, I'm not sure what's going on or why the ATX PSU is shorting to the enclosure and shutting down.Any ideas? Is there something that I'm likely to have missed? Should I be wiring the enclosure to ground in the ATX PSU?Thanks in advance for any help.

author: Ian.M
author_rank: Super Contributor
timestamp: February 21, 2016, 04:38:49 pm
content: I see you took our advice from last time to make the ATX PSU plug-in rather than hard-wired.Disconnect the connector then use your multimeter on a resitance range to check between each coloured wire and your enclosure chassis for shorts.Alternatively, with it running, simply measure the voltage between your enclosure chassis and the ATX PSU case which will also tell you which rail is shorted, but wont distinguish between +5V and +5V standby supplies.When you find the rail with the problem, start disconnecting stuff on that rail till you find the short.Look carefully at your buck/boost PCB mounting - If the PCB doesn't have isolated or grounded mounting holes and you've used metal standoffs, it could cause this.

author: djQUAN
author_rank: Frequent Contributor
timestamp: February 21, 2016, 04:41:07 pm
content: The LED displays also appear to be too far extended. The PCB it is mounted to might be shorting to the back of the case.

author: Renegade
author_rank: Regular Contributor
timestamp: February 21, 2016, 05:53:34 pm
content: Quote from: Ian.M on February 21, 2016, 04:38:49 pmHeh! Yes. It was painful to redo everything, so might as well do it right after screwing it up once.Quote from: Ian.M on February 21, 2016, 04:38:49 pmYep. This was all shorted. Continuity everywhere.Quote from: Ian.M on February 21, 2016, 04:38:49 pm+3.3V there.Quote from: djQUAN on February 21, 2016, 04:41:07 pmAnd it turns out that is exactly what the problem was for the +3.3V display.Thanks to both of you for helping there!I've now got a couple more troubleshooting skills in my pocket for the next time!