Advice for a Short circuit detector - Page 1
Forum section: electronics / testgear

author: zanzeoo
author_rank: Contributor
timestamp: January 05, 2023, 09:39:11 pm
content: HelloI have a trouble with a 12 layer board, Vcc is shorted somewhere with GND.In the past i used a TONEOHM 950 which is very useful in that case. But so much expensive ( 10K$ )I thinking to buy or make a Short circuit detector more simple or less expensive.Do you have any recommandationsThx

author: DL2XY
author_rank: Regular Contributor
timestamp: January 05, 2023, 09:54:01 pm
content: Thermal Imager ?!

author: zanzeoo
author_rank: Contributor
timestamp: January 05, 2023, 09:55:49 pm
content: it does not work well with small current...

author: Gyro
author_rank: Super Contributor
timestamp: January 05, 2023, 10:04:38 pm
content: You don't say whether the PCB is populated or not, but assuming not.Set a bench PSU in current current limit mode to as high a current as you sensibly can (somewhere around the expected circuit current consumption or higher) and put it across VCC and GND. Then go around with a DMM (hopefully a 4 1/2 or 5 1/2 digit) on its most sensitive mV range and work across the board, starting at the inputs and measure the voltage between VCC and GND points. The lower the mV (maybe 10s of uV) reading, the closer you are to the short. This can actually be a pretty sensitive method.If the PCB is populated, it's pretty much the same as above, but set the PSU to less than the normal VCC rail voltage before going into current limit mode.

author: zanzeoo
author_rank: Contributor
timestamp: January 05, 2023, 10:07:42 pm
content: Yes the board is populatedi found a video of a TONEOHM 950 :

author: Gyro
author_rank: Super Contributor
timestamp: January 05, 2023, 10:19:10 pm
content: Quote from: zanzeoo on January 05, 2023, 10:07:42 pmThat's easier then. Assuming the PCB was bare board tested (who wouldn't on a 12 layer), you're looking for a component, probably a decoupling cap. The current limit and measure method is easier when you have a specific number of parts to go around rather than having to look up specific test points. Turn off the PSU before clearing the fault to avoid any PSU overshoot or inductive kick issues.

author: kripton2035
author_rank: Super Contributor
timestamp: January 05, 2023, 10:23:08 pm
content: https://www.youtube.com/embed/L4V3BWReZLY

author: BillyO
author_rank: Super Contributor
timestamp: January 05, 2023, 11:31:40 pm
content: Do you have access to a meter that will resolve down to 1 milli-Ohm? Most 5.5 digit meters will. That should be plenty capable of finding shorts between voltage planes.

author: zanzeoo
author_rank: Contributor
timestamp: January 06, 2023, 06:48:54 am
content: Yes i have a milliohmeter and tried to find the lowest "resistor" between VCC and GND , but found almost the same value everywhere. I suspecting a bad via between plane ( this board came just from production )I will try the method of Gyro. But i would like to invest in a short detector for my compagny , so if you have an advice for a good equipement it's fineThx u

author: zanzeoo
author_rank: Contributor
timestamp: January 06, 2023, 06:53:12 am
content: Quote from: Gyro on January 05, 2023, 10:19:10 pmI'have already removed a lot of caps , the plane that is shorted has almost no more component on it ...just some IC maybe a small part of solder stuck under ?

author: Greybeard
author_rank: Frequent Contributor
timestamp: January 06, 2023, 09:35:32 am
content: Quote from: zanzeoo on January 05, 2023, 09:39:11 pmMaybe DIY is a way to go:https://www.pa4tim.nl/?p=5339https://www.edn.com/milliohm-squawker-great-at-finding-shorts-and-reverse-engineering-pcbs/https://www.eevblog.com/forum/repair/diy-magnetic-probe-to-find-a-short/msg1329343/#msg1329343https://web.archive.org/web/20060311135708/http://www.polarinstruments.com/manuals/service/Toneohm%20700%20and%20550%20Service%20manual.pdf

author: kripton2035
author_rank: Super Contributor
timestamp: January 06, 2023, 10:40:45 am
content: you need 0.1mΩ resolution to find a short with enough spatial precision.I assume you did but are you using a 4 wires probes ?it's strange you don't find any differences with a milliohm meter.

author: 807
author_rank: Frequent Contributor
timestamp: January 06, 2023, 01:49:13 pm
content: If you have an LM317 handy, then here's a cheap way of doing it...

author: artag
author_rank: Super Contributor
timestamp: May 04, 2023, 09:31:11 am
content: I was looking for a Toneohm. There are some on ebay but they're expensive - £3-400.But I've found a few Omnitest Hy Trak for 1/10 of that (eghttps://www.ebay.co.uk/itm/225274209822).They appear to do nominally the same thing : measure milliohms with a display and a tracking audio tone. Are they equivalent ? I've read that Toneohm have some special technique for finding groundplane shorts but that might apply to only some models, and I don't know how good it is.

author: Miti
author_rank: Super Contributor
timestamp: May 04, 2023, 02:29:59 pm
content: How about this short finder project?I have one from jdobry and the troubleshooting technicians love it.https://www.eevblog.com/forum/testgear/finding-short-on-motherboards-with-a-shorty-(with-display)/

author: artag
author_rank: Super Contributor
timestamp: May 04, 2023, 04:35:10 pm
content: Yes, there are plenty of options to build my own.But what I wanted to know was why the Toneohms are apparently so attractive, even against other reputable commercial options.