+ and - power supply short on a board - Page 1
Forum section: electronics / repair

author: jfphp
author_rank: Regular Contributor
timestamp: November 01, 2016, 12:01:02 pm
content: Hello to all,I am facing a puzzling problem : on a board full of Fet, op amp (and TTL IC), of course without sockets, I have a short between + and - 15V with no short from either supply to the ground. What do you suggest : a big double power supply delivering several amp and waiting for smoke ? Thank you !

author: DTJ
author_rank: Super Contributor
timestamp: November 01, 2016, 12:24:10 pm
content: Is it a newly built board? One that failed in service?If its new, go over it with a magnifier / microscope really really well.Set PSU for constant current of say 200mA, let it run and feel for warm spots. Finger tips first, back of hand, sniff and then use your nose to sense warm spots (only half joking).With it powered up use DMM on mV range and try to track current path.Good luck!

author: CJay
author_rank: Super Contributor
timestamp: November 01, 2016, 12:34:19 pm
content: If the +/- 15V is powering the op-amps why would you expect a short to ground?Sounds pretty simple to me, you've got a stuffed op-amp or op-amps.Tracing it, hmm, if the power supply doesn't go into shutdown then as DTJ says, you may well be able to work out which op-amp by the heat but to save your nose from the possibility of an exploding epoxy impact, spray the chips with a little alcohol and see which one(s) dry up fastest.

author: DTJ
author_rank: Super Contributor
timestamp: November 01, 2016, 01:07:12 pm
content: Quote from: CJay on November 01, 2016, 12:34:19 pmThat's a great idea. I'll store that one away.

author: grifftech
author_rank: Frequent Contributor
timestamp: November 01, 2016, 02:19:13 pm
content: Quote from: CJay on November 01, 2016, 12:34:19 pmfire hazard

author: CJay
author_rank: Super Contributor
timestamp: November 01, 2016, 02:31:16 pm
content: Quote from: grifftech on November 01, 2016, 02:19:13 pmPotentially, if you're stupid.However I'd assume that anyone who is capable of using a little critical thought might have the common sense to realise 'a little' wouldn't pour a gallon of alcohol on the board. it just takes enough to 'wet' the top of the chip., not make a puddle.

author: Monkeh
author_rank: Super Contributor
timestamp: November 01, 2016, 02:53:41 pm
content: Quote from: grifftech on November 01, 2016, 02:19:13 pmIPA evaporates very, very fast, it's not easy to ignite it without a flame.

author: Someone
author_rank: Super Contributor
timestamp: November 01, 2016, 10:17:54 pm
content: Quote from: Monkeh on November 01, 2016, 02:53:41 pmPlunge a soldering iron into alcohol and watch all the interns start screaming. Autoignition temperature of ethanol: 363 °C (685 °F)https://en.wikipedia.org/wiki/Autoignition_temperatureYet its still quite hard to ignite even with the standard metal cartridges which should be well above that temperature. And when alcohol does burn its easy to control and not a great risk.

author: julian1
author_rank: Frequent Contributor
timestamp: November 01, 2016, 10:22:15 pm
content: If possible, hook it up to a lab psu, and slowly increase the voltage/current - while trying identify the source of the heat (indicating the short).