Faulty capacitor?  - Page 1
Forum section: electronics / beginners

author: Sanxion
author_rank: Contributor
timestamp: July 22, 2025, 10:48:53 am
content: Hi all,I am in the process of trying to repair a non working power supply. My first step is to check the components for any obvious physical damage. The only component that looks unusual is a green capacitor that has a bulge at the top but I suspect that is normal - see attached image. It's value is 224K and my tester returns 217.2nF which seems to be correct. Would you agree?

author: madires
author_rank: Super Contributor
timestamp: July 22, 2025, 11:33:23 am
content: Yes, 22 * 10^4 pF = 220000 pF = 220 nF. The 'K' indicates the tolerance (+/- 10 %).

author: BILLPOD
author_rank: Frequent Contributor
timestamp: July 22, 2025, 01:21:44 pm
content: Good Morning Sanxion, That bulging top on that capacitor is NOT normal. Even though the capacitanceis within specs, the ESR, (Equivalent Series Resistance), may be off. You need to test the capacitor with a reliable LCR, (Inductance, Capacitance, Resistance) meter. Remove the Capacitor from the circuit, andget a reading out of circuit, since you need to replace it anyway. Good work, so far, and best of luck with your repair.

author: Fraser
author_rank: Super Contributor
timestamp: July 22, 2025, 01:33:07 pm
content: That is a polyester/mylar film capacitor and the bulge is just the epoxy resin coating and not a fault. These capacitors are dipped in resin whilst upside down so excess sits on the top causing the bulge. Very common and polyester/Mylar film capacitors are very reliable. If the green epoxy is cracked or split, I replace the capacitor even if it tests OK. Yours seems fine though. I attach a picture of a new capacitor for reference.https://static.rapidonline.com/pdf/71366.pdfFraser

author: srb1954
author_rank: Super Contributor
timestamp: July 22, 2025, 01:41:16 pm
content: Quote from: BILLPOD on July 22, 2025, 01:21:44 pmThe apparent bulging at the top is perfectly normal for that type of capacitor. This is seen even on new caps of this type so it it not a problem arising from operational use.These caps are a polyester wound-film construction and are dipped in epoxy for encapsulation. As the excess epoxy drips off it tends to form a bulge at the top of the cap before setting. This has no impact on the electrical performance of these caps.

author: Haenk
author_rank: Super Contributor
timestamp: July 22, 2025, 02:21:41 pm
content: Also these caps rarely fail (unless they experience a massive overvoltage) and pretty much last forever.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: July 23, 2025, 12:34:20 pm
content: OOOPS, I guess I should look closely at photos, before making any remarks :'(

author: MathWizard
author_rank: Super Contributor
timestamp: July 25, 2025, 01:05:50 am
content: We do it all the time, but don't take granted that new parts always work. I found a green mylar cap last year, from an Amazon or ebay source, that had some major problem and was unusable. And I've found 2-3 resistors in my 1000pcs bags of 10k and 100k's that didn't work. The 10k I found yesterday measured around 550k. But again, they were cheap ebay parts.But I'm pretty sure last year I bought some 20pcs of 10uF caps from Digikey, and 1 had a problem. I never contacted them about it, but I'm pretty sure it was from digikey.