Capacitor blowing up - Page 1
Forum section: electronics / repair

author: luudee
author_rank: Frequent Contributor
timestamp: July 07, 2025, 10:10:11 am
content: Dear Friends!This is the third time, this capacitor has "blown up". Well, actually it seems to burn inside and generate a lot of smoke, really a lot smoke ...This is from a 6KW APC UPS. Mouser has them in stock and I keep on replacing them.Is there any better capacitor brand you guys could recommend that would not blow so frequently? Extended voltage, different chemistry?This time, it engulfed a power resistor, which I also need to replace. I can see it is 0.15 Ohm and 7W. But don't know manufacturer or other requirements. Could somebody point me in the right direction?Living in Thailand, our mains are not very stable, specially during the rainy season, some larger fluctuations are to be expected.The UPS keeps on working just fine, but this thing just generates more and more smoke ...Many Thanks, guys and gals!rudi

author: Grandchuck
author_rank: Super Contributor
timestamp: July 07, 2025, 11:34:08 am
content: Are those connected in series across the ac line?Is there by any chance an MOV across the line that has gone open circuit?

author: SteveThackery
author_rank: Super Contributor
timestamp: July 07, 2025, 01:03:58 pm
content: Are you able to trace out the circuit so we can see how they are connected?

author: luudee
author_rank: Frequent Contributor
timestamp: July 08, 2025, 05:59:22 am
content: Thank you for the replies, guys !It is difficult to tell, because of a bunch of relays that seem to isolate the output and input ...But from what I can see (and measure), the resistor is connected on one side to the output of the UPS,and the capacitor across the output of the UPS (behind some relays).rudiPS, this is all on 220V AC

author: Swake
author_rank: Frequent Contributor
timestamp: July 08, 2025, 09:29:59 am
content: Eruption is an interesting failure mode. It apparently started 'cooking' for some reason.Is that capacitor subject to a lot of ambient heat or humidity?What kind of charge is on the UPS? Is it fully loaded? Is it rather inductive or capacitive?I'm thinking of that capacitor being under-dimensioned, but then you need to find the reason, and this might be also the case for the rest of the UPS.Eventually something else in the UPS is broken too, something that you don't see in normal use but causes the cap to have more work.

author: luudee
author_rank: Frequent Contributor
timestamp: July 08, 2025, 09:41:00 am
content: Quote from: Swake on July 08, 2025, 09:29:59 amYes, both !Humidity can reach well over 70% here, and heat, probably well over 40C.EDIT: I wonder, what if I cover the capacitor in conformal coating? I have some spray-on type ...Quote from: Swake on July 08, 2025, 09:29:59 amIt's usually loaded to about 50%. With two external battery packs. Total 120 x 12v 5.4AH batteriesIt uses a 192V DC total battery supply. I replace the batteries every 3 years.I'd like to replace them with Li-ion batteries, but that's for another topic!Quote from: Swake on July 08, 2025, 09:29:59 amI've had this thing for well over 15 years. Had to repair it once, lightning struck nearby (not counting the dozen times I had to replace this damn capacitor).Cheers,rudi

author: Swake
author_rank: Frequent Contributor
timestamp: July 08, 2025, 10:11:00 am
content: The cause of failure might be a combination of- Higher than average stress due to environmental factors might result in a lower that documented spec for the UPS. Sustaining a high load at 40°C is going to be at challenge I guess. Temp and humidity are probably at the limits of the design. I especially think of lowered duty cycle because of this such as how long it is capable to backup power before it needs a pause to cool down or something. This duty cycle depends on the effective load of course.- Bad power factor of the load and other specific conditions. The 50% 'perceived' load (even if it is measured by the UPS, which generally is not very precise) might in reality very well be a load that is actually over 100% of what the UPS was designed to deliver.- A cheaped out design (they manufacture to a cost rather that to long term reliability) resulting in the selection of a less capable than required capacitor at design time. Or they never really tested in operating conditions similar to your for a long enough time to notice the issue.Instead of upgrading this UPS setup with Li-something batteries (Others have done such an upgrade, very difficult actually to have a stable and safe setup due to battery management and charge curves being different from lead-acid.) you might want to evolve to an solar inverter solution with batteries. It would act as an UPS. The solar panels are actually optional, you can add them in a later stage.

author: m k
author_rank: Super Contributor
timestamp: July 08, 2025, 11:12:13 am
content: Change the capacitor to 600V version, if it still blows you can say it wasn't an AC limit.Resistor's RA007 and 7W seem to be the same.E7 15 can be something else than obvious, voltage and inductance values are still missing, possibly not so important to match.(7 rounds of 15)

author: luudee
author_rank: Frequent Contributor
timestamp: July 08, 2025, 11:37:01 am
content: I was very worried about getting the right power resistor with proper temperature characteristics.But after taking another closer look, I noticed that they are using S23P50-100D15M1 from Tamura, which is an integrated current sensor.So, maybe I can use any generic power resistor with the same values ? 0.15 ohm, 7w ?What do you guys think ?Thanks,rudi

author: conductivity
author_rank: Contributor
timestamp: July 08, 2025, 02:51:54 pm
content: Besides cost, there is no reason to hold on to such a failing product. If there is enough money available, the best solution seems to be one or more double conversion UPS products from a reliable brand like Eaton.

author: m k
author_rank: Super Contributor
timestamp: July 08, 2025, 08:03:50 pm
content: Quote from: luudee on July 08, 2025, 11:37:01 amFirst you must find out what that resistor really is.6kW means 26A and 100W over 0R15.So some current sense that resistor is, but from where exactly is another thing.Is it a constantly inverting machine?Smart UPS seems to be at least SmartConnect, Line-interactive and Double Conversion Online.Since the last one must be mentioned one can guess that it's the only real one.

author: luudee
author_rank: Frequent Contributor
timestamp: July 09, 2025, 06:55:20 am
content: Quote from: m k on July 08, 2025, 08:03:50 pmThank you for pointing out the obvious !No way this is a current-sense resistor !Quote from: m k on July 08, 2025, 08:03:50 pmThis is a APC "Smart" UPS. It has been quite reliable over the years, except for this nasty capacitor. The unit keeps on working just fine, while the cooking capacitor fills up the room with nasty smoke, lolI think if I decide to replace it, I will go for an Inverter/Charger that works with Li-Ion batteries.I am tired of replacing the sealed lead-acid batteries ...

author: Swake
author_rank: Frequent Contributor
timestamp: July 09, 2025, 07:07:48 am
content: Could it be that the capacitor is not the correct value/voltage?Because it was replaced previously with the wrong type for example.

author: luudee
author_rank: Frequent Contributor
timestamp: July 09, 2025, 07:11:42 am
content: Quote from: Swake on July 09, 2025, 07:07:48 amNo, I made it a point to get exactly the same part. Same manufacturer, same specification.Always get it from Mouser.Cheers,rudi

author: Swake
author_rank: Frequent Contributor
timestamp: July 09, 2025, 07:29:53 am
content: Tried finding pictures of a similar board but could only find a 2200VA version with 150V capacitors and a 40kVA with 600V capacitors.As suggested by m k, it will not hurt putting a higher voltage in there.

author: m k
author_rank: Super Contributor
timestamp: July 09, 2025, 02:32:06 pm
content: Quote from: luudee on July 09, 2025, 06:55:20 amDon't through the idea away just yet, there's still a battery charger.Some smaller models are sharing their guts.There battery charger is pumping a charge voltage through a FET and its sense is 5W 0R30, up to 2kW.(p. 5/7)If your model is similar it's probably lost that FET also.

author: floobydust
author_rank: Super Contributor
timestamp: July 09, 2025, 11:45:35 pm
content: Other APC UPS models, that 20uF big film cap is in series with a CT. No idea why the DC block.I would be tempted to upsize it, or just use a motor run instead cap lol.Check for the service manual or schematic atElektroTanya

author: luudee
author_rank: Frequent Contributor
timestamp: July 11, 2025, 12:44:25 pm
content: Hi Guys !Thank you all for alll suggestions !I had a replacement on order when I started this thread but have now ordered a capacitor with a larger voltage range.After reading the actual data sheet for the original 20uF 400V capacitor, I realized that the 400V is for DC only !!!If used in an AC circuit, it's rating is 250VAC. So that's right there is the reason it fails.https://www.cde.com/resources/catalogs/PHC.pdfSo now, I ordered a 20UF 600VDC/350VAC capacitor from another manufacturer ...In the meantime, I replaced the capacitor and resistor, and the UPS is working fine without generating any smokeAgain, many thanks for all replies, very much appreciated !!!Cheers,rudi

author: 1audio
author_rank: Frequent Contributor
timestamp: July 14, 2025, 01:22:43 am
content: Check the current rating on the cap and if you can check the peak current passing through it. That cap may be filtering the HF components from the switching power that emulates a sine wave. The peak currents could be high enough to heat up the internals and then it might short. If its across the AC make sure its a cap rated for that service and an X or Y cap. One shorts when it fails and the other opens. I can't tell which would be right in that application.APC is a quality vendor BUT not foolproof and they actually get the UPS's from subcontractors.A good, properly rated, surge protector upstream would be a very valid investment, especially where you have unstable power and lightning storms. Also follow the installation instructions on proper grounding to make sure the surge device will protect.

author: amyk
author_rank: Super Contributor
timestamp: July 14, 2025, 02:33:23 am
content: I agree with the others here, 240V mains is already close to 340V peak, and this type of equipment is normally rated to survive 250VAC which is ~354V peak. 400V is a bit close if you consider there may be transients, so a 600V or 630V cap is preferred.