backward cap or I'm an idiot question

LittleLarrySellr

Well-known member

Donor 2019, 2024
Joined
Jun 17, 2019
Messages
579
Reaction score
736
Location
Whitehall, Pennsylvania
I always triple check myself when doing a cap kit but I finally missed one. I was doing the Williams 8784 cap kit and of all the caps to have a brain fart on when I was checking my work I got the axial one wrong (the big 18000uf one). I remember looking at it and thinking "yep that's right"... lol

I blew F4 and F5 when I turned it on and after a few seconds realized what I did and quickly turned it off. After replacing BR1, IC2 and the fuses, I got the board working again and producing clean voltages so crisis averted. I also put the old cap back in.

My question is does installing a cap backwards compromise it? Even if it was only for a few seconds? I have an ESR meter and the cap still seems to register good when I can get good contact on both legs but I wasn't sure if I should reinstall it or just use another fresh one. They aren't cheap so figured if it is still OK better to use it.
 
Installing a cap backwards just applies a negative voltage to it. If it were meant to have 5V across it and you install it backwards, well now it has -5V across it.

Is the cap still ok? Depends on the cap. Some handle that better than others. You'd need the data sheet, the exact negative voltage it got up (down) to, and how long it was held there to determine that. But then you have to consider reliability of that cap going forward. You've probably shortened its life by stressing it in unintended ways.

It might be good but I'd still toss it for less of a headache.
 
Installing a cap backwards just applies a negative voltage to it. If it were meant to have 5V across it and you install it backwards, well now it has -5V across it.

Is the cap still ok? Depends on the cap. Some handle that better than others. You'd need the data sheet, the exact negative voltage it got up (down) to, and how long it was held there to determine that. But then you have to consider reliability of that cap going forward. You've probably shortened its life by stressing it in unintended ways.

It might be good but I'd still toss it for less of a headache.
^ This is the way. Notably on one that big. You may have gassed it. It could work for a few days, and then BAM! Bad fish smell in the room, and the cap spews it's guts all over the inside of your cabinet. The electrolyte can be corrosive, and cause other damage when it blows.

So replace it.
 
Also, if it blew fuses and components, well, it's been hammered pretty good by reverse current.
 
Back
Top Bottom