Reverse polarity. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 01, 2017, 10:27:29 pm
content: Bit of a mishap, for a short time I accidentally connected a circuit board with DC 20 Volts into 6 x 35 Volt Electrolytic capacitors. This vented one before breaking a trace on the circuit board. Now I've replaced the vented capacitor, but the other 5 capacitors seem ok. Is it likely they are impaired now ? I do plan to make another circuit the same in the near future, but are the capacitors that saw this reverse voltage ok for the time being.Any help appreciated.

author: ironmonkey
author_rank: Contributor
timestamp: May 01, 2017, 10:39:00 pm
content: The only one to be sure is taking apart each single capacitor and test it one by one; check also any peripheric component for possible failures.Enviado desde mi MotoG3 mediante Tapatalk

author: Zero999
author_rank: Super Contributor
timestamp: May 01, 2017, 10:55:30 pm
content: How much did the capacitors cost?What happens if they fail in the future?If this is going to be harder to replace them later, then even if remaining capacitors are fine, it makes more sense to replace them now.On the other hand, if you always have the board to hand, near your workbench and soldering iron, then you might as well leave it for now.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 01, 2017, 11:11:24 pm
content: Thanks for your replys, i guessed it might have damaged them. Its not critical to replace them, more a pain to be honest. I could kick myself, after I blew a fuse I replaced the fuse. Instead I should have stopped there to see what blew it. On installing a new fuse it vented on 4700uf 35 Volt capacitor, and burned out a track on the board. Dam, lesson learned.I wired it up reverse polarity, if only I had looked when it blew the first fuse..

author: james_s
author_rank: Super Contributor
timestamp: May 01, 2017, 11:25:33 pm
content: If it's something critical, replace the capacitors. If it's not a big deal if it malfunctions later, just patch it up and see what happens. It's unlikely that the remaining capacitors will fail catastrophically but you should keep an eye on them for a while to make sure none are leaking or bulging.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 02, 2017, 12:14:47 am
content: Quote from: james_s on May 01, 2017, 11:25:33 pmI am tempted to leave them in and see how it goes, it's not critical. There is short circuit protection and over voltage protection now. It was more wandering if there likely to let greater ripple through, or be severely degraded as filter capacitors. There's a total of 6 x 4700uf 35 Volts in parallel. But it all appears OK under load, temperatures are ok only reaching 40 - 50 ° C with an 8 Amp load. There 105 °C forgot to mention that.

author: digsys
author_rank: Unknown
timestamp: May 02, 2017, 12:23:29 am
content: Generally, the weakest member breaks over and saves the rest. As you found out, it went dead short (or close to), so the others didn't suffer the reverse voltage.It's one reason I always have at least 1 tantalum on voltage rails. In fault conditions, they fail pretty fast and savagely :-)There is no way to know if they have been "degraded" in any way, so If the device is always accessible, you shoiuld be fine.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 02, 2017, 02:02:05 am
content: Thanks again for all your reply, I'm going to see how it goes. If it all goes south I'll let you know.

author: Wilksey
author_rank: Super Contributor
timestamp: May 02, 2017, 09:19:50 am
content: QuoteDamn Bruh!Nah, we all do it, from time to time, on purpose sometimes (the smaller ones seem to go with more of a "bang" oddly.)Stick in a reverse protection diode on your next circuit.If it's just for testing, it may or may not break down in the future, if it's production, well I hope it's not production if there is no reverse polarity protection on board!

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 02, 2017, 09:41:41 am
content: Hi there, no its not a production item, just a homebrew psu. But as the other capacitors temperature are or seem ok tested with a thermocouple and circuit heavily loaded, I will keep them in and watch and see. It was a sloppy mistake, I should have looked closer into the first blown fuse, maybe it would have prevented and venting of capacitors. Peak would be 20 Volts DC at 15 Amps on the capacitors string, continuous 50% duty cycle more like 8 - 10 Amps, same voltage.

author: Wilksey
author_rank: Super Contributor
timestamp: May 02, 2017, 02:59:42 pm
content: She'll be right!

author: Zero999
author_rank: Super Contributor
timestamp: May 02, 2017, 03:08:42 pm
content: How about adding some reverse polarity protection, so this can't happen again?If a diode drop is too high then a reverse diode in parallel with a fuse or MOSFET can be used.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 04, 2017, 12:27:57 pm
content: Quote from: Hero999 on May 02, 2017, 03:08:42 pmIt was on the input from the bridge rectifier where I made the blunder. So no need for protection there. The out put has a crowbar now. It's been in service now for two days, I know it's early still, but no bulging capacitors. And at the moment all seems fine.