Is this capacitor dead?? - Page 1
Forum section: electronics / repair

author: Bill R
author_rank: Newbie
timestamp: September 30, 2025, 11:13:22 am
content: Hi All.First off, thanks to all for allowing me to join.Much though I hate to do it, my first post is a question.So, I work at one of the museums in Central London, here in the U.K and have been running some tests/maintenance on an exhibit we have.Upon testing A capacitor in this exhibit (It's the smoothing cap on a 15kV rectifier circuit)Looking at the readings on the meter, would you agree that this cap is toast.I'm pretty sure it is but, have nothing to compare it to and, don't really want to spend the money for a new on if this actually is okay.Thank you all for your insights.Bill R

author: golden_labels
author_rank: Super Contributor
timestamp: September 30, 2025, 11:29:47 am
content: Hello.What makes you think it’s dead?It’s marked as 50 nF. Meter’s display is out of focus, but it seems to be 48.38 nF. Good luck finding a new capacitor that close to its spec!

author: wraper
author_rank: Unknown
timestamp: September 30, 2025, 11:30:33 am
content: Toast in what way? I see nothing wrong with it.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 30, 2025, 11:30:58 am
content: What is that you don't like about that capacitor?

author: chilternview
author_rank: Frequent Contributor
timestamp: September 30, 2025, 11:31:35 am
content: What test frequency are you applying to that cap?300ohms ESR may be high for a large value electrolytic, but for a 50nF cap with an impedance of about 31kohm at 100Hz, it doesn't sound that bad.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 30, 2025, 11:32:19 am
content: For comparison, i just checked the largest paper-in-oil, i found.12µF @ 4,9kV measures with DE5000 at 11,998µ and ESR = 0,02 Ohms

author: JohanH
author_rank: Frequent Contributor
timestamp: September 30, 2025, 11:32:48 am
content: I don't have experience of 15 kV capacitors, but here's a generic observation.Capacitance looks correct, .05 MFD is supposed to mean 50 nanoFarad, so that's right. But the ESR is quite high, one would expect it to be in low ohms up to ten or so for a plastic film capacitor, so it looks like it could be degraded. But it also depends on the ESR measurement frequency. If the ESR rises significantly at higher frequency, it would mean that it suffers from aging or degradation. It also depends on the application. If it's for a DC application, it could be fine, but if it's for any high-frequency, snubber or pulse application it probably means that it's degraded and won't work well.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: September 30, 2025, 11:34:32 am
content: Bill, I hope to God you know what you are doing, or you will soon be dead.That multimeter probably wouldn't take more than 1kV before blowing itself and possibly you up.There is no way I would be risking my life even THINKING about testing whether or not there might be up to 15kV on that thing, let alone measuring its capacitance.The capacitance measurement looks correct though.Blimey.

author: Bill R
author_rank: Newbie
timestamp: September 30, 2025, 11:37:49 am
content: Hi All,Thank you for all the replies thus far, and apologies for the photo being largely out of focus.My main concern was the ESR of the cap. the actual capacitance is right on the money @ 48nF.The meter is set to 100Hz frequency @ 600mVI have attached a better photo of the meter.Thanks again.

author: JohanH
author_rank: Frequent Contributor
timestamp: September 30, 2025, 11:44:13 am
content: If measured at 100 Hz, you will see high ESR values for a film cap. It will be significantly lower in 1 kHz range and higher. I would guess the cap is fine.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 30, 2025, 11:50:23 am
content: The ESR is OK.Low ESR readings sub 1 ohm or so are usually expected for electrolytic capacitors, and only for high values (from 10-100uF up). In the sub-uF range, the ESR is almost meaningless, and the value indicated depends a lot with how the instrument measures the ESR (there are more ways to measure ESR and no standardized way to measure ESR). Most of all, ESR depends at what frequency it is measured, depends with temperature, pressure, etc.Anyways, 300ohms ESR for a 0.05uF/12kV seems OK to me. At 12kV, those 300 ohms will limit the current at about 40A max. Is it expected that the circuit that capacitor is in will draw in the range of tens of A? If no, then the 300 ohms ESR is most probably not a problem.Unfortunately, the datasheet for the OF150 series capacitors doesn't specify any ESR value:https://www.plasticcapacitors.com/images/com_sellacious/products/attachments/55/OF150.pdf

author: chilternview
author_rank: Frequent Contributor
timestamp: September 30, 2025, 11:52:06 am
content: Quote from: Bill R on September 30, 2025, 11:37:49 amIn which case the ESR is just 1% of the capacitor's impedance at 100Hz.

author: golden_labels
author_rank: Super Contributor
timestamp: September 30, 2025, 11:53:53 am
content: If the capacitor was used in a low current application, that ESR wouldn’t be too much of a burden.The value being just 0.1% shy of a round E6 series value (330 Ω) prompts me to ask a question. Are you sure there is no actual resistor monted in series with it?(ignore: just noticed it was 307 in the previous reading)If there actually is any significant degradation, I would be more concerned about insulation. If it can still withstand that 15 kV.

author: Zero999
author_rank: Super Contributor
timestamp: September 30, 2025, 02:22:35 pm
content: Quote from: chilternview on September 30, 2025, 11:52:06 amThis.XC= 1/(2π*FC) = 1/(2π*100*48*10-9) = 31.8kThe meter will struggle to get an accurate reading for ESR, at such a low frequency. Set it to at least 10kHz, preferably 100kHz.

author: TimFox
author_rank: Super Contributor
timestamp: September 30, 2025, 02:43:43 pm
content: To evaluate a measured ESR for capacitor quality, compute (or measure directly) theQvalue at the measurement frequency.ESR itself is useful for circuit design and evaluation, butQis a property of the capacitor itself, relatively insensitive to the capacitance. Good data sheets give information about theQor its reciprocalD.

author: golden_labels
author_rank: Super Contributor
timestamp: September 30, 2025, 03:08:33 pm
content: Quote from: Zero999 on September 30, 2025, 02:22:35 pmBut OP said“smoothing cap on a 15kVrectifier circuit.”So it’s 100 Hz (2 · 50 Hz) that matters most.

author: gamalot
author_rank: Super Contributor
timestamp: September 30, 2025, 03:11:02 pm
content: An ESR reading of approximately 330 ohms is clearly abnorma. It indicates that either your capacitor has degraded and failed due to aging, or that your LCR meter is unable to measure accurately.Verifying this is quite simple: just test a film capacitor with a similar capacitance

author: Zero999
author_rank: Super Contributor
timestamp: September 30, 2025, 03:21:09 pm
content: Quote from: golden_labels on September 30, 2025, 03:08:33 pm1) Not necessarily.2) It's extreemly difficult to measure ESR of such a small capacitor, at such a low frequency.3) It's highly likely, it's a measurement error, rather than a bad capacitor.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 30, 2025, 03:35:58 pm
content: The capacitor / frequency is to small to get a useful ESR reading. It is just outside the range where most meters can measure ESR. Even 100 kHz may be on the low side for such a small capacitor. 50 nF is more similar to decoupling capacitors, but this one likely with quite some parasitic inductance.The ESR make sense when the overall impedance is not much higher than the ESR - this is clearly not the case here.When the capacitor is tested where it still mainly behaves like a capacitor it make more sense to use the D or Q value and thus capacitor losses this way.

author: wraper
author_rank: Unknown
timestamp: September 30, 2025, 06:05:37 pm
content: Quote from: gamalot on September 30, 2025, 03:11:02 pmIt just means you're not measuring it right. You cannot just measure 50nF at 100 Hz with a relatively cheap meter and expect a reasonable ESR reading.

author: gamalot
author_rank: Super Contributor
timestamp: October 01, 2025, 04:24:06 am
content: Quote from: wraper on September 30, 2025, 06:05:37 pmI doubt any LCR meter can achieve that at 100Hz, no matter how expensive it is. The 6 ohm reading from my LCR is clearly incorrect, it should be below 1 ohm, and for small capacitors, it should even be under 100 milliohms.

author: Psi
author_rank: Super Contributor
timestamp: October 01, 2025, 04:34:41 am
content: If it was me and I was in doubt about the DMM/LCR reading. I'd probably charge it up and see how well it holds a charge and how fast/slow the voltage falls under a known resistance (compared to what it should be given its capactance). But you'd need a high voltage source and HV meter, and there's some real dangers when messing around with charged kV caps.There's also a possibility the cap appears fine when measured but arcs across internally when the voltage reaches some point, which a DMM/LCR wont detect as a faulty cap since it never gets the voltage high enough to trigger it.But you usually know when a cap has failed like this because you hear it before removing the cap from the appliance.It's damn loud when large HV caps keep arcing over.With regard to its pules performance, i don't know, never had to check a cap for that.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 02, 2025, 12:48:09 am
content: Quote from: Bill R on September 30, 2025, 11:37:49 amMy meter states 120Hz is for testing capacitors larger than 10uF.

author: bdunham7
author_rank: Super Contributor
timestamp: October 02, 2025, 05:08:06 pm
content: Quote from: Bill R on September 30, 2025, 11:13:22 amNo, it very likely is fine and other than measuring the capacitance--which looks good--that LCR meter is useless for measuring anything relevant for that capacitor. A HV film capacitor like that is very unlikely to fail with a high ESR while the nominal capacitance value remains correct. I wouldn't even bother measuring ESR because the other parastics make it pretty difficult to measure and you don't even know what a correct value would be. Also, in reality the ESR probably could be 100X higher than it was originally and the circuit would still function correctly.If you want to test the capacitor, measuring it's capacitance and then its leakage current (at 15kV, good luck!) would be where I'd start. There's other parameters you could look at with greater difficulty, but if the nominal capacitance is correct and the leakage is low enough for your circuit then I'd be 99% sure the capacitor is fine.

author: gamalot
author_rank: Super Contributor
timestamp: October 03, 2025, 07:29:34 am
content: Go check the other post, I found the datasheet for the capacitor.https://www.eevblog.com/forum/beginners/do-i-misunderstand-esr-measurement-frequency-dependence-of-esr/