How can I find a short circuit on an unknown voltage rail? - Page 1
Forum section: electronics / repair

author: dicky96
author_rank: Regular Contributor
timestamp: October 23, 2017, 06:18:00 pm
content: I have a HP laptop and schematics are not available (I've asked and I've googled)The 19V is present on the motherboardHowever 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 culpritHowever in this case I don't know the correct voltageAny suggestions how to trace this further? I have the usual stuff available - Fluke DVM, bench supply, scopes, ESR meter etcI 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

author: kripton2035
author_rank: Super Contributor
timestamp: October 23, 2017, 07:18:51 pm
content: I build this :http://kripton2035.free.fr/Projects/proj-shorty.htmland 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, tryhttp://kripton2035clone.free.fr/Projects/proj-shorty.html

author: Mjolinor
author_rank: Frequent Contributor
timestamp: October 23, 2017, 07:39:44 pm
content: As well as some sort of Toneohm a thermal camera may also help.

author: mubes
author_rank: Regular Contributor
timestamp: October 23, 2017, 08:06:16 pm
content: 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!

author: Armadillo
author_rank: Super Contributor
timestamp: October 23, 2017, 08:08:00 pm
content: Quote from: kripton2035 on October 23, 2017, 07:18:51 pmWhat is the arduino for?, You mean it can speak "1 cm" distance to short?

author: kripton2035
author_rank: Super Contributor
timestamp: October 23, 2017, 09:26:53 pm
content: Quote from: Armadillo on October 23, 2017, 08:08:00 pmyou can ear a difference in the tone frequency that the arduino produces if you move the probes around 1cm away from the previous position.

author: xavier60
author_rank: Super Contributor
timestamp: October 24, 2017, 01:47:29 am
content: 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.

author: Zucca
author_rank: Unknown
timestamp: October 24, 2017, 08:19:40 am
content: Quote from: xavier60 on October 24, 2017, 01:47:29 amWhy not an milliohm meter?

author: Mjolinor
author_rank: Frequent Contributor
timestamp: October 24, 2017, 08:24:34 am
content: Presumably because he wanted to measure micro volts and not milli ohms.

author: xavier60
author_rank: Super Contributor
timestamp: October 24, 2017, 08:45:10 am
content: Quote from: zucca on October 24, 2017, 08:19:40 amI 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.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 24, 2017, 09:22:38 am
content: Quote from: dicky96 on October 23, 2017, 06:18:00 pmIf 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.

author: Armadillo
author_rank: Super Contributor
timestamp: October 24, 2017, 09:38:47 am
content: Quote from: mikeselectricstuff on October 24, 2017, 09:22:38 amMike animated gif of 9 frames is special enough...... LOLI wonder How did he does it?

author: bd139
author_rank: Super Contributor
timestamp: October 24, 2017, 09:47:41 am
content: 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 goingI 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.

author: mzacharias
author_rank: Frequent Contributor
timestamp: October 24, 2017, 09:48:13 am
content: 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.

author: Armadillo
author_rank: Super Contributor
timestamp: October 24, 2017, 09:52:00 am
content: Quote from: mzacharias on October 24, 2017, 09:48:13 amI should add, best with current limiting of say 2 or 3 amperes.

author: krivx
author_rank: Frequent Contributor
timestamp: October 24, 2017, 10:00:28 am
content: 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.

author: dicky96
author_rank: Regular Contributor
timestamp: October 24, 2017, 04:36:25 pm
content: 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 daysOther than that I will try my bench supply set to a low voltage. According to my Fluke it is a zero ohm short.CheersRich

author: kripton2035
author_rank: Super Contributor
timestamp: October 24, 2017, 05:10:31 pm
content: Quote from: dicky96 on October 24, 2017, 04:36:25 pmthe "shorty" I showed earlier is quite like a toneohm, but will cost you less than $10 to build.

author: dicky96
author_rank: Regular Contributor
timestamp: October 24, 2017, 06:10:52 pm
content: Yes the Shorty also looks like something worth building but I doubt very much it could be done for under $10?

author: kripton2035
author_rank: Super Contributor
timestamp: October 24, 2017, 07:32:47 pm
content: 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

author: bd139
author_rank: Super Contributor
timestamp: October 24, 2017, 07:51:20 pm
content: This is the sort of thing that makes mewhen 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.

author: technogeeky
author_rank: Frequent Contributor
timestamp: October 24, 2017, 09:27:41 pm
content: Quote from: bd139 on October 24, 2017, 07:51:20 pmFor 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?

author: kripton2035
author_rank: Super Contributor
timestamp: October 24, 2017, 09:55:32 pm
content: the frequency changes as you move probe on the board. the higher frequency, the lowest ohmsyes the arduino is overkill for that, but today you may find ne555 more expensive than arduinos ...

author: dicky96
author_rank: Regular Contributor
timestamp: October 25, 2017, 09:20:59 am
content: Quote from: kripton2035 on October 24, 2017, 07:32:47 pmhmmm 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 themAnyway 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 haveSo 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...... £12.50MCP6041........ £2.20 (8 pin DIL)MCP73831.......£1.98 (or I buy 20 from china for £1.50 and wait a few weeksLD-BZEN-1205 £4.92 (for a pack of six. Cant see a UK supplier sells them individuallyTrinket 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 batterySo 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 toybut it ain't $10Rich

author: kripton2035
author_rank: Super Contributor
timestamp: October 25, 2017, 09:32:31 am
content: 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 regulatorused 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...

author: dicky96
author_rank: Regular Contributor
timestamp: October 25, 2017, 09:33:30 am
content: PS I do have NE555

author: dicky96
author_rank: Regular Contributor
timestamp: October 25, 2017, 09:38:38 am
content: oOops sorry Krypton we cross postedIs the arduino nano a direct pin-for-pin compatible for the Trinket Pro? I'm sorry this is not my area of expertise

author: bd139
author_rank: Super Contributor
timestamp: October 25, 2017, 11:08:47 am
content: Finally found my copy of Troubleshooting Analogue Circuits. Was bathroom readingHere's Bob Pease's solution:(a) LM10 = £2.50 ... (b) LM331 = £2.10 ... You can sub both of them out for (a) another opamp + current source (b) 555 with voltage controlled current source.I might see how cheap I can design one of these.

author: Armadillo
author_rank: Super Contributor
timestamp: October 25, 2017, 11:11:23 am
content: Quote from: bd139 on October 25, 2017, 11:08:47 amif you can take out all the solder mask....

author: bd139
author_rank: Super Contributor
timestamp: October 25, 2017, 11:20:17 am
content: It's rarely if ever under the solder mask. It's either the components or around the pads.I think I had one board with a short after repairing about 50 of the damn things with just a manky old Thandar DMM. Most were assembly stupid.

author: Armadillo
author_rank: Super Contributor
timestamp: October 25, 2017, 11:28:51 am
content: Quote from: bd139 on October 25, 2017, 11:20:17 amObviously, but the text is kind of stupid... no such in real repair.

author: bd139
author_rank: Super Contributor
timestamp: October 25, 2017, 11:42:19 am
content: Was repairing this fecked up bodge someone did yesterday so it's not always a problem. I see a lot of boards without solder mask. I actually prefer them as they're easier to rework and last a hell of a long time (unlike the solder mask which flakes off and bubbles).

author: Armadillo
author_rank: Super Contributor
timestamp: October 25, 2017, 12:14:15 pm
content: Quote from: bd139 on October 25, 2017, 11:42:19 amOh my God, I was imagining densely populated smd circuit boards like the OP HP laptop for example, where the resistances are expected to be small because they are close by that necessitate such instrument to find the shorts. Normally for electrically devices or big chunky components widely spaced, it won't be that hard to find the shorts anyway... Well those are just my views.

author: bd139
author_rank: Super Contributor
timestamp: October 25, 2017, 12:43:54 pm
content: Fair point with the HP laptop. That's a dense multi-layer board which you're usually SOL on (unless you are Louis Rossmann).

author: kripton2035
author_rank: Super Contributor
timestamp: October 25, 2017, 12:58:23 pm
content: Quote from: dicky96 on October 25, 2017, 09:38:38 amI used the same pins names as the trinket (as I don't have one) and it worked. then I modified the software to my needs.

author: dicky96
author_rank: Regular Contributor
timestamp: October 25, 2017, 05:46:20 pm
content: Quote from: bd139 on October 25, 2017, 12:43:54 pmHmmm I've fixed similar faults before on, for example, a 1Tb SATA HDD which had a short circuit on the 12V rail on the controller board. That was a SMD cap near the motor controller chip and I used the 'bench psu with current limit and see what gets hot' technique then replaced the cap and it worked. Is the HP laptop motherboard so different?

author: eunjun
author_rank: Newbie
timestamp: September 24, 2019, 11:08:21 am
content: Good morning! I've made Shorty talking about you. However, when you close two probes, only a dull melody comes out. The parts I used are the attiny85 and mcp6041 op amps. Another is the Arduino Promini and the mcp6041 op amp. Both have the same dull melody. What op amp did you use? I want to complete this by all means. Please help.