finding shorted components - Page 1
Forum section: electronics / testgear

author: bsco
author_rank: Newbie
timestamp: February 12, 2014, 01:32:55 pm
content: Hi there. Just joined this forum.....Have seen lots of Dave's videos an youtube...great stuff...I did a search for this but came up empty so here goes.....I want to build an in-expensive tool for detecting shorted components by following copper traces on the board to lead me to the defective component....example ..ic chip....or chip transistor, etc...Can this be done with a Hall sensor.. or.something that can detect the amount of current flow in a copper trace...I used to use a voltmeter set on millivolts which was connected at the power supply source and then probe along the trace with the other lead at all the solder connections to try and find the component that had the largest voltage drop...just wondering if there was a better way which could be built.....instead of purchased....the above technique does work but is not really effective..neither is it overly accurate...And you always can't depend on the component being overheated to the point so that it can be found by touch...Cheers,Bernie

author: nctnico
author_rank: Super Contributor
timestamp: February 12, 2014, 01:53:19 pm
content: Many decades ago I have used a current detector probe during an internship. In the end the trick with the power supply and the voltmeter works much better. Nowadays a thermal imaging camera may do the trick even better.

author: rstoer
author_rank: Regular Contributor
timestamp: February 12, 2014, 02:01:52 pm
content: When I was a technician we did what you're describing with just an ohmmeter. We'd ground one lead and move the other to different points along the shorted trace. The resistance was measurably lower as you moved toward the shorted component. Like most techniques it didn't always work but it often did.

author: bsco
author_rank: Newbie
timestamp: February 12, 2014, 02:53:03 pm
content: Thanks for your input guys...I guess this technique I was using might be the best way to do this...I never used the ohmmeter test I used the millivolt test.....by the sounds of it, they serve the same purpose.....Cheers,BernieP.S. If I find the time I might toy around with using a 50uA meter movement....the magic phase here is "find the time"....I repair all the music equipment for our local music store and they keep me very busy...I have 2 keyboards, a powered PA cabinet and three guitar amps to bring back this week....and a Line 6 Guitar processor.......plus all the musicians that have all kinds of gear as well....vacuum tube, solid state and micro based.....this keeps me very busy and I get to work from home.....on another note...how do you go about getting notices sent to your email when somebody replies to your post.......

author: Fraser
author_rank: Super Contributor
timestamp: February 12, 2014, 02:57:48 pm
content: Research 'POLAR Toneohm'The user manuals contain the schematic and I found them on the POLAR web site usingg teh wayback machine. The manuals are also to be found elsewhere on the net.Model numbers: 500, 550, 700, 800, 850.I have the 550, 700 and 850 units and they all work well. They have a built in milliohm meter and a signal injector with inductive signal tracer probe. They are not that complex and youd build your own using the schematics

author: tszaboo
author_rank: Super Contributor
timestamp: February 12, 2014, 03:25:33 pm
content: My index finger has a built in temperature sensor, which was a great help to find shorted components in the past.

author: kripton2035
author_rank: Super Contributor
timestamp: February 12, 2014, 03:40:07 pm
content: Quote from: Aurora on February 12, 2014, 02:57:48 pmdo you have the schematics of these??

author: kripton2035
author_rank: Super Contributor
timestamp: February 12, 2014, 03:41:09 pm
content: Quote from: bsco on February 12, 2014, 01:32:55 pmsome milli ohm meters to build here :http://kripton2035.free.fr/continuity-repos.html

author: kripton2035
author_rank: Super Contributor
timestamp: February 12, 2014, 03:45:03 pm
content: Quote from: kripton2035 on February 12, 2014, 03:40:07 pmfound them - thanks.

author: Shock
author_rank: Super Contributor
timestamp: February 12, 2014, 04:03:36 pm
content: How about using an ESR meter that has both a suitable resolution and a low test voltage that doesn't turn on semiconductor junctions?I've not used one for this purpose but interested in any reasons why it wouldn't work.

author: kripton2035
author_rank: Super Contributor
timestamp: February 12, 2014, 04:43:08 pm
content: Quote from: Shock on February 12, 2014, 04:03:36 pmmost of the time esr meters have a max 0.01 ohm resolution (and accuracy ?)for short circuit locator, you need the milliohm or better IMHO

author: PaulAm
author_rank: Frequent Contributor
timestamp: February 12, 2014, 04:50:18 pm
content: The HP 547 current probe with a pulser is useful to track down short circuits. Any good milliohm meter will work as well. The current probe can be used on multilayer boards since it's non-contact. You can find the manual with schematic for that online. Fabrication of the sensor head would be the biggest problem, but the basic principle is clear enough.There are other current sensors available now but I haven't seen them incorporated into a short finder.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 12, 2014, 04:54:42 pm
content: Quote from: kripton2035 on February 12, 2014, 04:43:08 pmThe DE 5000 has 1 milliohm resolution (and from experience, also accuracy), and also kelvin connections. You can actually use it e.g. to determine cable lengths quite precisely. I can't see why you couldn't use it to check PCBs for shorts. Just build yourself a Kelvin probe using pogo pins or something like that.

author: JoeyP
author_rank: Frequent Contributor
timestamp: February 12, 2014, 05:12:45 pm
content: Quote from: NANDBlog on February 12, 2014, 03:25:33 pmI've often used that method too. Another one is to use a power supply with lots of current available (>20 amps - voltage limited of course). Sometimes, the shorted component will actually send you smoke signals to signal its location

author: ConKbot
author_rank: Super Contributor
timestamp: February 12, 2014, 05:44:06 pm
content: Quote from: JoeyP on February 12, 2014, 05:12:45 pm+1 for self identification!

author: Fraser
author_rank: Super Contributor
timestamp: February 12, 2014, 06:00:17 pm
content: ">20A voltage limited"Kiss goodbye to those fine pcb traces buried inside the multi layer board ! Then Kiss goodbye to the PCB if you can'r repair the damage done.Brute force has its place, but it can be fatal if used without serious amounts of care. Not recommended for newbies.

author: kripton2035
author_rank: Super Contributor
timestamp: February 12, 2014, 06:00:25 pm
content: Quote from: mos6502 on February 12, 2014, 04:54:42 pmthe de 5000 is a lcr meter, with the milliohm accuracy. most of the diy esr meters doesnt have 4 wires probes nor milliohm display.

author: pmcouto
author_rank: Unknown
timestamp: February 12, 2014, 07:00:57 pm
content: Quote from: NANDBlog on February 12, 2014, 03:25:33 pmThis is a genuine digital probe!But be careful – I’ve already caused damage to my digital probes using that method…

author: JoeyP
author_rank: Frequent Contributor
timestamp: February 12, 2014, 08:17:10 pm
content: Quote from: Aurora on February 12, 2014, 06:00:17 pmNope! I've done it many times over the years and never once damaged a board. Those traces can take a lot more current than you think. A while back I was considering using a trace as a crude fuse in a product. I tested a 6 mil wide trace (1oz finished copper), and was surprised to find that it barely got warm at 20a. And normally, we're talking about a short on a power supply net, and those traces would virtually never be that narrow anyway.

author: Fraser
author_rank: Super Contributor
timestamp: February 12, 2014, 08:38:08 pm
content: @JoeyP,I stand corrected. Its just that 20A sounds an awful lot of current to pass though such traces. But you have proven them to stand up to that so no worriesThe Thermal Camera would certainly show up the current carrying traces at 20AI shall have to do some tests on a scrap PC motherboard.

author: JoeyP
author_rank: Frequent Contributor
timestamp: February 12, 2014, 09:02:13 pm
content: @Aurora: Ya, I was *really* surprised to find that out myself. Intuitively, I was sure I was just going to see a flash when I applied 20a to a 6 mil trace, but I sat there in amazement as it just simmered for several minutes without even heating up enough to discolor the soldermask! I agree about the thermal camera though. I have fantasized about having one for that purpose for a long time. Getting closer and closer to buying one.

author: Dave Turner
author_rank: Frequent Contributor
timestamp: February 13, 2014, 12:49:29 am
content: I have used the knuckle of my index finger. If there is an unexpected current present the "frog's leg" reaction automatically disconnects one from the source.No kidding; this has saved me from a serious jolt on more than one occasion.

author: Shock
author_rank: Super Contributor
timestamp: February 13, 2014, 09:35:56 am
content: Want one!

author: mcinque
author_rank: Unknown
timestamp: February 13, 2014, 10:12:45 am
content: Added to my wishlist

author: dave_k
author_rank: Frequent Contributor
timestamp: February 13, 2014, 11:14:34 am
content: Sometimes you get lucky and the shorted part identifies itself