Low ohms load tracing (not short circuit) - Page 1
Forum section: electronics / testgear

author: dicky96
author_rank: Regular Contributor
timestamp: November 14, 2019, 10:18:37 am
content: Hi guysI am trying to solve a problem. A good example would be fault finding on multiphase VRM power supplies - eg Vcore on desktop motherboardsIf I have a short circuit on Vcore (with the CPU removed) then I've found the best way to trace the short is to use my ESR meter. On my Fluke multimeter I just see short everywhere on the Vcore rail because the Buck inductors just effectively connect every phase together from a DC point of viewBut using my ESR meter the inductors act as.... well.... an inductance, so it's easy to see for example 0.5ohm on the short circuit side of an inductor and 8 ohms on the good side. That makes it really easy to trace the fault. The same technique works well on any buck regulator circuit and saves all the bother of removing the inductor to see if the short is on the supply or load side.So far so good. The problem I am trying to trace at the moment is on VCore supply with 8 phases. With the CPU removed there is a resistance of 27 ohms from Vcore to ground and I don't know where it is because again all points in the circuit phases are effectively DC connected. The problem here is that my ESR meter just sees ~30 ohms as open circuit. And removing the through hole inductors in turn is a pain in the ass due to the very high thermal mass of the PCB in this areaSo what I would really like is something like an ESR meter that can read higher resistances. I am sure with that I would see something like 27 ohms on the bad side of the buck inductor(s) and 35 ohms on the good sideIs there something like that available?Also it occurred to me (though I haven't tried this) that an ESR meter may be able to see the difference between a very low resistance 1-2ohm GPU load, and a short circuit GPU, if the ESR meter output was low enough level not to cause the semiconductors in the GPU to conduct?Any thoughts on that?Rich

author: Zucca
author_rank: Unknown
timestamp: November 14, 2019, 10:23:23 am
content: I am so happy to have my IR camera!try to rent one FLIR E4 or something like that. Normally in 10 Minutes you will find where the problem is, one day rent should not cost too much.does it make sense?

author: nctnico
author_rank: Super Contributor
timestamp: November 14, 2019, 10:35:53 am
content: Actually the much cheaper Flir C2 works well and doesn't cost as much. One of the things the C2 can do is fix the readout range. I have not managed to do this on a much more expensive Flir camera so this is a feature to check for.What works well is to push current into a short from a power supply. With a setting of 1V and 0.5A you shouldn't do any damage. Then either measure where the voltage is the lowest using a DMM or use a thermal camera. The thermal camera will be quicker.

author: Berni
author_rank: Super Contributor
timestamp: November 14, 2019, 11:00:59 am
content: Forcing lots of current into the problematic rail using a lab PSU is indeed a good way to trace shorts.Using a thermal camera is also nice since the short will get hot or you can see the traces going towards the short becoming warm. But if you don't have a thermal camera you can just crank the current higher until its warm enough to feel with your finger.The trick is to always set the PSU voltage below the rated voltage for the rail. That way you can't harm anything, but if the short is really low resistance it could require many amps of current (even >3A) to become nice and warm enough to see where it is. I seen cases where this even fixes a board because enough current can burn out tin whiskers or very slight solder bridges, making the short go away.

author: dicky96
author_rank: Regular Contributor
timestamp: November 14, 2019, 11:47:42 am
content: Guys, I know how to trace short circuits.What I want to be able to do is trace the cause of a low resistance (say 20 ohms) on a voltage rail which should be high impedance because the load (CPU) is not present. If it is Vcore which is approx 1V I can't send a lot of current into a 20 ohm load can IIt's like those replies were made without reading the question

author: Berni
author_rank: Super Contributor
timestamp: November 14, 2019, 11:55:40 am
content: Its still possible its a semiconductor related short and those don't give sensible readings in resistance. But they might start conducting lots of current when enough voltage appears.

author: Shock
author_rank: Super Contributor
timestamp: November 14, 2019, 12:30:21 pm
content: You could try using the mV voltage drop method for deducing the current path, for this you need a multimeter with a decent mV range. It's explained in this document below. This is like performing a 4 wire measurement.Note that you aren't limited to where you start the measurement and you want to always use an appropriate voltage level and current suitable to your circuit. Or if the circuit is not blowing fuses, being damaged or overloading the supply, you can use your discretion in using the circuits original supply.You can also measure directly across components in parallel on the same rail, the lowest voltage drop indicates the most current.http://www.kennethkuhn.com/electronics/short_circuit_tracing.pdf

author: nctnico
author_rank: Super Contributor
timestamp: November 14, 2019, 12:41:29 pm
content: Quote from: dicky96 on November 14, 2019, 11:47:42 am20 Ohms doesn't sound like a short to me. At 1V that is just 50mA so you can probably ignore that. The 20 Ohm could in fact be a resistor to make sure the power drops quick enough in order to meet power sequencing timing. Recently I created a design which has such resistors for that purpose. With a thermal camera you should be able to find out where the current is flowing. 50mW is enough to show up on a thermal camera.

author: Kean
author_rank: Unknown
timestamp: November 14, 2019, 12:51:53 pm
content: Quote from: dicky96 on November 14, 2019, 10:18:37 amNot sure if it would help, but you could add a low-ish value resistor in parallel with the test leads to bring the readings into the range of the ESR meter, and then back caculate the real values.

author: dicky96
author_rank: Regular Contributor
timestamp: November 14, 2019, 01:16:52 pm
content: Quote from: nctnico on November 14, 2019, 12:41:29 pmI never thought to buy a thermal imaging camera before. Are there any reasonable priced ones that are well suited to this type of fault finding?

author: nctnico
author_rank: Super Contributor
timestamp: November 14, 2019, 01:22:34 pm
content: The C2 is the cheapest from Flir and it serves me well although a higher resolution would be nice.I used it for find a shorted capacitor in a complex board from a spectrum analyser:https://www.eevblog.com/forum/testgear/advantest-r3477-9khz-to-13ghz-spectrum-analyzer/msg1446566/#msg1446566However, this was a hard short.I have also managed to spot resistors with the wrong value on a board using the thermal camera.

author: Zucca
author_rank: Unknown
timestamp: November 14, 2019, 11:04:06 pm
content: YesnctnicoandBerniare right.Little example:Where is the low ohms load?Hope you understand now why I pointed you in the IR Camera direction.