Author Topic: How can I find a short circuit on an unknown voltage rail?  (Read 4837 times)

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Offline dicky96

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How can I find a short circuit on an unknown voltage rail?
« on: October 23, 2017, 06:18:00 pm »
I have a HP laptop and schematics are not available (I've asked and I've googled)

The 19V is present on the motherboard

However I have a short circuit on at least one derived voltage rail. 

Normally i would connect my bench supply to the rail, set to the correct voltage and gradually bring the current limit up until something gets hot or otherwise identifies itself as the culprit

However in this case I don't know the correct voltage

Any suggestions how to trace this further?  I have the usual stuff available - Fluke DVM, bench supply, scopes, ESR meter etc

I have a zero ohm short from +ve of PC78 to ground.  Also short to ground both sides of PQ12, PL14 and PC242 (or is it PCZ42?)   see pic

 

Online kripton2035

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #1 on: October 23, 2017, 07:18:51 pm »
I build this : http://kripton2035.free.fr/Projects/proj-shorty.html
and it really helps when you need to find a short circuit on a pcb. you can ear one cm when moving the probe to the short or not.

if you can't reach this url, try http://kripton2035clone.free.fr/Projects/proj-shorty.html


Offline Mjolinor

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #2 on: October 23, 2017, 07:39:44 pm »

As well as some sort of Toneohm a thermal camera may also help.
 

Offline mubes

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #3 on: October 23, 2017, 08:06:16 pm »
Cool it using freezer spray or inverted air duster and watch where the ice melts first....but remember turn it off and wait for the condensate to evaporate before you apply power again!
 

Offline Armadillo

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #4 on: October 23, 2017, 08:08:00 pm »
I build this : http://kripton2035.free.fr/Projects/proj-shorty.html

What is the arduino for?, You mean it can speak "1 cm" distance to short?
 

Online kripton2035

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #5 on: October 23, 2017, 09:26:53 pm »
I build this : http://kripton2035.free.fr/Projects/proj-shorty.html

What is the arduino for?, You mean it can speak "1 cm" distance to short?
you can ear a difference in the tone frequency that the arduino produces if you move the probes around 1cm away from the previous position.

Online xavier60

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #6 on: October 24, 2017, 01:47:29 am »
If the applied voltage is no more than 0.5v, it's unlikely that any extra damage will be caused. The method I use is to supply a low current to the shorted rail. I used to use 500ma, now I only need 200ma because I recently purchased a DMM with 1uv resolution, an AN8008.
I find where the current is flowing to ground by measuring voltage drops across the ground plane at various points. I can find shorted capacitors and shorted low side transistors in a short time.
Btw, not all low side transistors are MOSFETs. I was recently misled into thinking that I had found shorted low side MOSFETs on an Asus P4P800s-x MB. They were in fact JFETs, normally on. 
« Last Edit: October 24, 2017, 05:07:01 am by xavier60 »
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline Zucca

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #7 on: October 24, 2017, 08:19:40 am »
I recently purchased a DMM with 1uv resolution, an AN8008.

Why not an milliohm meter?
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Offline Mjolinor

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #8 on: October 24, 2017, 08:24:34 am »

Presumably because he wanted to measure micro volts and not milli ohms. :)
 

Online xavier60

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #9 on: October 24, 2017, 08:45:10 am »
I recently purchased a DMM with 1uv resolution, an AN8008.

Why not an milliohm meter?
I don't know for certain. I have never used a  milliohm meter. I'd imagine though that it would be slow at taking stable readings because of varying probe tip contact resistance. And it would involve being able to follow the routing of the power rail trace. The ground plane is much easier to find. When I do find the suspect shorted component, say a surface mount ceramic capacitor, I'm able to initially confirm it by observing differing uv readings on various points on its ground end terminal.
 Also, the An8008 cost $24.00.
« Last Edit: October 24, 2017, 08:53:51 am by xavier60 »
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline mikeselectricstuff

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #10 on: October 24, 2017, 09:22:38 am »

Normally i would connect my bench supply to the rail, set to the correct voltage and gradually bring the current limit up until something gets hot or otherwise identifies itself as the culprit

If it's a dead short, just use a low voltage like 0.5v and use a thermal camera. Or just crank up the current til something smokes.
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Offline Armadillo

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #11 on: October 24, 2017, 09:38:47 am »

Normally i would connect my bench supply to the rail, set to the correct voltage and gradually bring the current limit up until something gets hot or otherwise identifies itself as the culprit

If it's a dead short, just use a low voltage like 0.5v and use a thermal camera. Or just crank up the current til something smokes.

Mike animated gif of 9 frames is special enough...... LOL   :-DD
I wonder How did he does it?
« Last Edit: October 24, 2017, 10:42:26 am by Armadillo »
 

Online bd139

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #12 on: October 24, 2017, 09:47:41 am »
Getting pretty adept at finding these what with tektronix shorted tants.

Any old 4.5 digit meter with relative function and a decent-ish low ohms mode should do the job. Touch the probes and poke the relative button then work your way around the board until you see the lowest resistance, remove that part and re-check the pads. If the resistance is much higher then problem solved. If not, it wasn't. Keep going :)

I have used Uni-T UT61E and GW GDM-8341 for this.

I can't recommend what people have said here which involves dancing with the magic smoke. If you burn a trace off or the fault goes slightly higher resistance and starts dissipating power (tantalum capacitors like to do this0 then you're SOL. Woosh, boom, poof etc. It's the same mentality as "the engine is making a clunking sound, let's drive it and see what falls out". Next thing you know there's a timing chain, half the sump and a pool of oil in the middle of the road.
« Last Edit: October 24, 2017, 09:53:30 am by bd139 »
 

Offline mzacharias

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #13 on: October 24, 2017, 09:48:13 am »
So far as feeding with external current limited power supply:

If it's a short, the current-limited voltage at the board side will stay quite low UNLESS the short burns open. I would say set the voltage to 1.2 volts to start with, and probably not go over 3.3 volts.
 

Offline Armadillo

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #14 on: October 24, 2017, 09:52:00 am »
So far as feeding with external current limited power supply:

If it's a short, the current-limited voltage at the board side will stay quite low UNLESS the short burns open. I would say set the voltage to 1.2 volts to start with, and probably not go over 3.3 volts.

I should add, best with current limiting of say 2 or 3 amperes.
 

Offline krivx

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #15 on: October 24, 2017, 10:00:28 am »
I bought a cheap Toneohm 550 for this kind of thing. I recently found a shorted power rail inside an FPGA by finding a minimum resistance on a decoupling cap directly behind the FPGA, on the backside of the board. It works, and was fairly quick.
 

Offline dicky96

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #16 on: October 24, 2017, 04:36:25 pm »
Thanks guys for all the interesting suggestions.

I will get my hands on an AN8008 as they are very cheap - I've not used this method before but it sounds quite interesting. 

I've used a ToneOhm for short tracking back in the 80s when I used to work as a component level electronics repair engineer for a large computer manufacturer.  They seem quite expensive/difficult to get your hands on these days

Other than that I will try my bench supply set to a low voltage.  According to my Fluke it is a zero ohm short.

Cheers
Rich

 

Online kripton2035

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #17 on: October 24, 2017, 05:10:31 pm »
I've used a ToneOhm for short tracking back in the 80s when I used to work as a component level electronics repair engineer for a large computer manufacturer.  They seem quite expensive/difficult to get your hands on these days
the "shorty" I showed earlier is quite like a toneohm, but will cost you less than $10 to build.

Offline dicky96

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #18 on: October 24, 2017, 06:10:52 pm »
Yes the Shorty also looks like something worth building but I doubt very much it could be done for under $10?
 

Online kripton2035

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #19 on: October 24, 2017, 07:32:47 pm »
the arduino nano is around $3, all the other components are drawer junks. even if you buy one opamp, some resistors, one speaker I doubt it will cost a lot more than $10. the box is optionnal and should cost me some $4

Online bd139

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #20 on: October 24, 2017, 07:51:20 pm »
This is the sort of thing that makes me  :palm: when I see an Arduino in it. You can achieve the same thing with an op amp / 555 or something. There is a circuit in Troubleshooting Analog Circuits that does the job.
 

Offline technogeeky

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #21 on: October 24, 2017, 09:27:41 pm »
This is the sort of thing that makes me  :palm: when I see an Arduino in it. You can achieve the same thing with an op amp / 555 or something. There is a circuit in Troubleshooting Analog Circuits that does the job.

For someone like me, on any given day it's more likely that I have a handful of STMs or adruinos handy rather than a opamp or a 555.  Well, I have a handful of opamps but I don't know where my 555s went.

In any case, what are the advantages / disadvantages of that circuit? And what would you be probing exactly, and what would you be listening for?

Presumably you put one probe on the power rail or ground, and the other probe goes to various points of the opposite rail (power rail or ground). But what would you learn then? Is the beeping going to get louder or quieter? Or is the frequency going to change?


 

Online kripton2035

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #22 on: October 24, 2017, 09:55:32 pm »
the frequency changes as you move probe on the board. the higher frequency, the lowest ohms

yes the arduino is overkill for that, but today you may find ne555 more expensive than arduinos ...

Offline dicky96

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #23 on: October 25, 2017, 09:20:59 am »
the arduino nano is around $3, all the other components are drawer junks. even if you buy one opamp, some resistors, one speaker I doubt it will cost a lot more than $10. the box is optionnal and should cost me some $4

hmmm I don't know a lot about arduino though I have done some projects with Arduino Uno and a 32bit 3.3V Arduino I don't quite recall the model so I learned some stuff about programming them

Anyway the schematic says Trinket Pro which I don't have. I don't have the buzzer, the MCP6041 or the MCP73831.  All the other components I have

So just looking at ebay for current UK prices inc postage (I buy my components when I briefly return to the UK every 6-8 weeks)

Trinket Pro...... £12.50
MCP6041........ £2.20 (8 pin DIL)
MCP73831.......£1.98 (or I buy 20 from china for £1.50 and wait a few weeks
LD-BZEN-1205 £4.92 (for a pack of six.  Cant see a UK supplier sells them individually

Trinket Li/Lion backpack is £7.50 and then I don't need the MCP73831 so add £5 to do it that way.  And I still need to get the battery

So that's about £20 to build (approx $26) or £25 using the backpack - and I get some spare Buzzers and maybe Op amps to go in my component stock.  I'm not saying that's expensive for s useful toy  :P but it ain't $10

Rich
« Last Edit: October 25, 2017, 09:23:50 am by dicky96 »
 

Online kripton2035

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Re: How can I find a short circuit on an unknown voltage rail?
« Reply #24 on: October 25, 2017, 09:32:31 am »
I used an arduino nano (clone ?) to make mine, I didnt use the battery charger circuit, just wired a 9v li-ion and a 78l05 regulator
used some spare aop I had in a drawer (with low voltage supply, a 741 will NOT work) and voilà.
but yes the complete original shorty is more expensive to build.

I also modified the software, so that pushing the button memorises the probes resistance in the eeprom, and not like it is originaly to wake up the trinked only. added a real on-off switch as i don't like auto off systems that always drain batteries in the end...


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