Are some electrolytic capacitors normally domed? - Page 1
Forum section: electronics / beginners

author: cincin
author_rank: Regular Contributor
timestamp: November 15, 2021, 12:48:38 am
content: As per the title: are some electrolytic capacitors normally domed?I keep running into these large capacitors that have domed plastic tops, not metal tops with the X groove.I'm wondering if the domed shape invariably means that it's going bad and should be replaced, or if some of those larger plastic top caps are domed when new.Point in case, this Samwha 450wv 47uF:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 15, 2021, 01:16:16 am
content: What, is that just the plastic, or is the metal actually formed like that? And is it obvious how it formed (i.e. drawn that way vs. bulged)?Have seen plastic that, probably curls up under heating I'd guess -- which isn't necessarily a sign of anything, just indicates that that's happened at least once. And, again, assuming that that happens due to heating at all -- maybe it just kinda goes like that over time, plastics are weird sometimes.Electrically speaking, ESR and C measurements are a pretty strong indicator of health. Ideally checking against temperature too, as things can vary. Normally ESR rises at low temp, and is nominal at room or higher. A typical failure sign is, the curve shifts up in temp, so it's marginal or malfunctioning at RT but behaves normally once hot. That's a sign of age. And if it's doing something else entirely, it's probably very dead.Tim

author: Rerouter
author_rank: Super Contributor
timestamp: November 15, 2021, 01:17:54 am
content: Capacitors should not be that shape from the factory....

author: cincin
author_rank: Regular Contributor
timestamp: November 15, 2021, 01:25:11 am
content: Thanks for confirming!

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 15, 2021, 01:36:03 am
content: A lot of the insulated top ones have a bit of a dome to the plastic, but that one looks messed up.

author: amyk
author_rank: Super Contributor
timestamp: November 15, 2021, 05:01:26 am
content: It could be the plastic has warped from heat/age, or the vent has actually popped and only the plastic is holding back some pressure. Best way to confirm is to remove the plastic cap (carefully!)

author: cincin
author_rank: Regular Contributor
timestamp: November 15, 2021, 05:11:00 am
content: Good point, I will perform forensics once the offending cap has been extracted.

author: Capernicus
author_rank: Frequent Contributor
timestamp: November 15, 2021, 06:31:32 am
content: Looks like an overvolted cap to me.

author: bdunham7
author_rank: Super Contributor
timestamp: November 15, 2021, 06:43:34 am
content: Quote from: cincin on November 15, 2021, 12:48:38 amSometimes the plastic covers will be convex, but not that much and you an pop them up and down like a jar lid. The one in your picture looks like a roadkill racoon that is about to explode on a hot summer day.

author: ogden
author_rank: Super Contributor
timestamp: November 15, 2021, 06:52:46 am
content: Cut plastix top, look at X below. Most likely cause of bulging - outgassing. It means cap is bad.

author: CaptDon
author_rank: Super Contributor
timestamp: November 15, 2021, 07:54:52 pm
content: When the factory shrink-wraps the cylindrical sleeve it puts pressure on the plastic cap. Lots of time coupled with high heat make the cap part tend to bulge. I have seen many that just looking at them sends up a red flare and yet they are fine. If the cap pushes down flat with moderate pressure you may sleep better. Only way to know for sure is remove the cap from the capacitor and look at the aluminum body, or pull it out and measure capacitance and ESR. Cheers!!!

author: ogden
author_rank: Super Contributor
timestamp: November 15, 2021, 07:57:00 pm
content: Oh, and please do not forget to report back. Others could get valuable feedback - from you.

author: ogden
author_rank: Super Contributor
timestamp: November 15, 2021, 08:01:42 pm
content: Quote from: CaptDon on November 15, 2021, 07:54:52 pmI agree that one shall look at state of "X" of the can, rather plastics, however if can is bulged even sligtly or for god's sake X ruptured - do not even consider measuring it, just replace. Lesson here is the same as about any part which you find worn - it will inevitably fail some day. Do you want to test what happens when that part will fail? I suggest not to.

author: cincin
author_rank: Regular Contributor
timestamp: November 22, 2021, 05:55:21 pm
content: Quote from: ogden on November 15, 2021, 07:57:00 pmI'm back to report.So I took apart the amp to get to the underside of the PCB, removed the offending cap and removed the plastic cover only to find that the top of the cap itself was flat and fine. It was just the plastic bulging from the wrap... but the plastic was pretty thick and rigid and I was pretty sure the metal was right behind until it popped off.So there, sometimes it's just the plastic cap, but it can be hard to tell without removing the cap to check. I learned.And in a positive twist, replacing that cap actually cured some background noises and weird pops the amp had, so the cap must have been bad anyway hahaha.Thank you all for your help. My Electronics 101 class was pretty lame, y'all (and google) are a better resource.

author: nemail2
author_rank: Regular Contributor
timestamp: November 23, 2021, 11:47:24 pm
content: i guess this is goin to have an audio-visual alert signal when it finally fails

author: rsjsouza
author_rank: Super Contributor
timestamp: November 24, 2021, 01:53:45 am
content: Quite interesting indeed. If the capacitor waa slowly releasing gas, I could see how a plastic sealant would bulge like that. Great call removing and confirming.

author: cincin
author_rank: Regular Contributor
timestamp: November 24, 2021, 02:40:22 am
content: Quote from: rsjsouza on November 24, 2021, 01:53:45 amCheers! The plastic cap and wrap didn't seem to be air-tight, so I doubt it was gas. It seemed mechanically buckled by the cap's branding shrink wrap.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 24, 2021, 08:30:41 am
content: I wonder if the plastic deformed from an overheating cap.As the dissipation rises due to failure, the cap begins turning into a resistor. This could have been enough heat over a prolonged period to affect the plastic covering.Could explain the improvement after replacing it too.

author: cincin
author_rank: Regular Contributor
timestamp: November 24, 2021, 04:22:41 pm
content: Quote from: TERRA Operative on November 24, 2021, 08:30:41 amPerhaps it played a role, but the plastic cap immediately went back to being flat once I removed it from the shrink wrap. It looked like it was mostly held in that buckled position because of the heat shrink squeezing it.