Author Topic: Find the short on my perfboard  (Read 808 times)

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Offline TheUnnamedNewbie

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Find the short on my perfboard
« on: April 19, 2017, 07:37:01 PM »
Hi guys,
I'm working on a project using a perf board. I just powered it up, I discovered that my 5V rail wasn't up. Quick poke around the board and I found that my 5V rail is shorted to ground, somewhere.

I only have a simple, mediumquality 3.5 digit DMM, and the smallest resistance range is 200 Ohm. So no matter which two points on my ground and 5V rail I pick, I measure 0.7 Ohm.

Do you guys have any tips on where to get started? I am gonna give it a second visual inspection after cleaning it like a madman to get all the flux off so I'm sure I don't miss something. I've also been tempted to find one of my old PC supplies and try just forcing a few amps through and see what smokes.

I could also try and make some kind of makeshift low resistance measurement thing (constant current source and a basic amplifier into my multimeter?)
RF design flow: 10% Literature study, 20% Simulation, 10% Black Magic, 60% Hunting digikey for an inducter that actually behaves itself at your operating frequency
 

Offline Buriedcode

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Re: Find the short on my perfboard
« Reply #1 on: April 19, 2017, 09:00:09 PM »
A simple continuity test (your <200ohm setting on your multimeter) should be enough.  I have never needed anything more than that and a good eye.

You could also suspect any semiconductors across the rails - often they tend to fail short-circuit.

For stripboard (perfboard with tracks) you can run the back of a craft knife down between the tracks to break any small solder bridges and also break up flux.
 

Offline TimFox

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Re: Find the short on my perfboard
« Reply #2 on: April 20, 2017, 02:21:15 AM »
Also, look for polarized (aluminum or tantalum electrolytic) capacitors installed reversed across a rail.
 

Offline Siwastaja

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Re: Find the short on my perfboard
« Reply #3 on: April 20, 2017, 03:29:29 AM »
I typically apply a few amps of short-circuit current from a current limited lab supply, then use the smallest 0.1mV range of a multimeter to "poke around" the traces to see the voltage drops. This way, you can measure the miniscule resistance and find the short in no time.
 

Offline Pitrsek

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Re: Find the short on my perfboard
« Reply #4 on: April 20, 2017, 03:42:25 AM »
 
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Offline sahko123

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Re: Find the short on my perfboard
« Reply #5 on: April 20, 2017, 04:17:08 AM »
Another issue might be a faulty microchip which has shorted internally. If the chips are socketed try removing each individual chip one by one(if there are multiple). If the board doesn't have any chips try taking out the individual components. might take a while but worth a try.
 

Offline danmcb

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Re: Find the short on my perfboard
« Reply #6 on: April 20, 2017, 05:05:17 AM »
take a watchmakers screwdriver or something sharp and run between the tracks. There can be tiny flecks of solder that are hard to see.

check thoroughly for wiring errors

if you cut holes to break the track, check they are really cut

use an eyeglass and good light - sometimes you can't see without. x4 is a good magnification I find.
 

Online KL27x

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Re: Find the short on my perfboard
« Reply #7 on: April 20, 2017, 05:13:23 AM »
Run a half an amp and feel for warmth? Spray bottle of alcohol and see where it dries up?

When all else fails, cut your power rail in half. Test both halves for a short. On the half with the short, repeat this process until you pin down the location. Just mark your cuts, clearly, so you can fix them, after the fact.
 

Offline james_s

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Re: Find the short on my perfboard
« Reply #8 on: April 20, 2017, 05:24:25 AM »
Connect it to a high current power supply and see what blows up :)

Or just follow your ground and power rails and look closely for bridges or other wiring errors.
 

Offline danmcb

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Re: Find the short on my perfboard
« Reply #9 on: April 20, 2017, 05:27:01 AM »
Connect it to a high current power supply and see what blows up :)

works pretty well as a last resort on PCB's with hairline shorts! but these days that doesn't seem to happen if you go to a decent supplier.
 

Offline james_s

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Re: Find the short on my perfboard
« Reply #10 on: April 20, 2017, 05:29:59 AM »
I have actually used a similar technique from time to time, but it isn't something I recommend unless one is really desperate. A slightly more gentle approach of feeding it a reasonable limited current and feeling around for warm parts is a bit safer.
 

Offline danmcb

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Re: Find the short on my perfboard
« Reply #11 on: April 20, 2017, 05:33:46 AM »
aw  come on. we  all need our Swedish chef moments now and again. It's part of the fun!
 

Offline kripton2035

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Re: Find the short on my perfboard
« Reply #12 on: April 20, 2017, 05:41:13 AM »
build something like this on another perfboard, and check for milliohms !
http://kripton2035.free.fr/Continuity%20Meters/continuity-short.html

Offline rob77

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Re: Find the short on my perfboard
« Reply #13 on: April 20, 2017, 07:08:53 AM »
Connect it to a high current power supply and see what blows up :)

but definitely wear eye protection ! ICs can send out nasty shrapnel when blowing up ;)
 

Offline james_s

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Re: Find the short on my perfboard
« Reply #14 on: April 20, 2017, 08:27:10 AM »
aw  come on. we  all need our Swedish chef moments now and again. It's part of the fun!

Bork Bork Bork!!!  :-DD
 

Offline mubes

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Re: Find the short on my perfboard
« Reply #15 on: April 21, 2017, 02:32:59 AM »
Pump a couple of hundred mA into the board and spray with enough freezer spray to form ice. Where there ain't no ice, go looking.

...And wear goggles. Nasty things can happen to pretty much any component under short conditions.

Dave
 

Offline bson

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Re: Find the short on my perfboard
« Reply #16 on: April 21, 2017, 11:20:54 AM »
If your DMM is sensitive enough you can put one probe on +5V and the other on GND, then simply move them one at a time in the direction of lowest resistance.
<This space intentionally left blank>
 

Online Luminax

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Re: Find the short on my perfboard
« Reply #17 on: April 21, 2017, 05:15:26 PM »
If you're going with the kaboom approach... make sure you have a video camera ready so you can submit to EEVSmoke  :-DD
All joking aside, can we get some screenshot top and bottom ? maybe we can spot something
Jack of all trade - Master of some... I hope...
 

Offline cvanc

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Re: Find the short on my perfboard
« Reply #18 on: April 21, 2017, 10:32:37 PM »
^^^What Luminax said.  Post some nice clear photos here on the forum and get more eyes on the problem.
 

Online NANDBlog

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Re: Find the short on my perfboard
« Reply #19 on: April 21, 2017, 11:31:54 PM »
It is surprisingly easy. You increase the current limit on your power supply, until the short disappears or its starts smoking. First case, the problem is already solved, second it is easy to detect.
 

Offline SouthPark

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Re: Find the short on my perfboard
« Reply #20 on: April 22, 2017, 12:48:07 AM »
Check to see if you didn't inadvertently leave certain tracks connected that aren't meant to be connected.
 


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