Is it possible to destroy components with a capacitance meter? - Page 1
Forum section: electronics / projects

author: niino
author_rank: Contributor
timestamp: July 11, 2020, 05:33:16 pm
content: I tried to check the output caps of a CPU VRM. A rough total value would have been good enough, so I tried to measure in-circuit. For some reason I put the probes on reverse and held them there for several seconds - I just had my brain turned off while doing this. Is it likely this destroyed the CPU and/or other components (aside of the caps themselves)? I am especially worried about the CPU.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 11, 2020, 06:05:52 pm
content: Probably not. The test voltage and current of a capacitance meter is very low. In fact, many capacitance/ESR meters use an AC signal and are non-polarized.This would be no worst than for example using the diode check feature on a DMM in reverse bias.Very very very very unlikely to cause any kind of damage. Maybe there's some extremely sensitive components out there that might be damaged but you certainly won't find any of those on a PC motherboard.

author: niino
author_rank: Contributor
timestamp: July 11, 2020, 06:29:20 pm
content: I just had the time to measure the voltage my meter applies to the circuit when connecting it the same way as before. It was -1.1 mV DC.I found a datasheet of a CPU from the same era made using a similar process (a 90nm Pentium 4) which had an absolute minimum rating of -0.3V, so I guess I didn't do any damage. However, no such documentation is available for the CPU in question.

author: pigrew
author_rank: Frequent Contributor
timestamp: July 11, 2020, 06:39:19 pm
content: Quote from: niino on July 11, 2020, 05:33:16 pmYes. I've used LCR meters that can output 10 Vpp. I also have an LCR meter that can apply 1000 V for leakage testing....However, your meter is probably somewhat low power. What model meter are you using? Also, power rails usually have a large capacitance, so they will also limit the AC bias.The polarity matters mostly if you are using guarded measurements, or with mains-connected meters. The NEGATIVE usually is constant-voltage (sometimes a virtual ground), while the POSITIVE has large AC voltage.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 11, 2020, 06:43:33 pm
content: Digital chips often have a limit of some -0.3 V, as this is the point where substrate diodes start to conduct. The real limiting point is often the reverse current. So as long as the current is limited to a safe limit (e.g. -1 mA) the chip may see a little more than the -0.3 V, but no more than the current limit that may be at some 10 mA. Without supply voltage even more current could be tolerated, as there would be no latch up.The problem however could be static, especially if the chip is powered on.

author: niino
author_rank: Contributor
timestamp: July 11, 2020, 07:00:11 pm
content: Quote from: pigrew on July 11, 2020, 06:39:19 pmIt's a budget multimeter, Voltcraft VC185, based on the DM1106EN chip (Voltcraft is a brand of Conrad, a electronics store chain in europe). So it probably behaves completely different to dedicated capacitance meters and definitely different to LCR meters.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: July 11, 2020, 08:55:19 pm
content: I hate to be the one that says this, butwhy are you asking?Either you've still got a working system, or you've not got a working system which renders your question redundant.MarkMLl

author: niino
author_rank: Contributor
timestamp: July 12, 2020, 05:47:43 am
content: The system was defective from the beginning, and I want to know whether it's worth it investing money in replacement caps (which, as I now know, were indeed the original problem); they're not cheap after all. If I killed the CPU, it's definitely not worth it to spend money on replacement caps.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 12, 2020, 06:48:24 am
content: You likely didn't kill anything, but the real question is, how do you know it's dead due to the capacitors? Replacing them is far from guaranteed to fix it, the issue can be elsewhere, and likely is.

author: magic
author_rank: Super Contributor
timestamp: July 12, 2020, 06:59:38 am
content: Quote from: niino on July 11, 2020, 06:29:20 pmWhat about AC?Quote from: niino on July 12, 2020, 05:47:43 amIfsome current went through the CPU and damaged it, your capacitance measurement possibly wasn't accurate and you really don't know.Ifyour measurement is accurate, no current diverted to the CPU and the CPU is good.If you feel like replacing the capacitors anyway, pull one out and test out of circuit. Even removing the CPU from the socket will still not remove the VRM.Or run your capacitance meter into a scope (while measuring some actual capacitor) to see the exact waveform. If it's very low, chances are that you can use this meter in circuit without serious error and without damage to ICs.The caps may also be bad by having significant ESR but capacitance mostly in spec.

author: niino
author_rank: Contributor
timestamp: July 12, 2020, 08:33:01 am
content: Quote from: Siwastaja on July 12, 2020, 06:48:24 amI took one out and it measured way out of spec (capacitance was far too low, I don't have an ESR meter, might want to invest in one), so I suppose the other identical ones in parallel failed in a similar way. I don't own a scope, and since university is currently closed for reasons we all know, I can't use one over there. It's a pretty old device (2005-2006ish) that has been used a lot, so capacitor failure seems very well possible.According to some googling, most multimeters measure capacitance different from e.g. LCR-meters, not by applying AC, but by charging the capacitor with a defined DC current and measuring the voltage. So the resulting voltage depends on the capacitance. If that's the case with mine, I probably didn't do any damage.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 12, 2020, 08:56:36 am
content: Just replace all the electrolytic capacitors. I don't think the instrument damaged anything.Capacitors are well known to lose capacitance by electrolyte drying over time, especially those placed near heating components. Bulged capacitors are the most common failure in electronics since the switching power supplies became ubiquitous, followed closely by BGA soldering issues and failure of power transistors/diodes/ICs.