Repair APC Back-UPS ES 750VA, model BE750G, CRISPY!

[Tim88]

I have four of this model backups, seemed to work well, just replace the batteries every few years. Two days ago, my computer just went blank, the APC beeped and I was like wth? Had put a new battery in a couple months ago. I figgered something was going wonky with the electrical innerds. So I decided to pull out my spare unit and have a look see at the board to ready myself for the one attached to my puter. To my surprise, C7(330uF 25V) cap was bulged and two SMD resistors were toasty.

 

The ground lead of the cap goes to a 1Ω resistor then to a .75Ω resistor on to ground.



Amazingly, the traces running under those resistors ohmed out good so gonna shoe in through hole replacements and see if she works.

My question is, do those resistors act like a fuse supposedly to protect the circuit and why two in series? I opened up my puter backup unit and that cap was bulged, fortunately no fire and tests 28pF . Wondering if there is a better way to design this with maybe a fuse?

Thanks!

[tkamiya]

It's impossible to say exactly without having access to schematic diagrams.  But I'd think if they wanted circuit protection, there are better ways.  My GUESS is, together, they form a time constant for something.  Also my GUESS is, cap has failed in short mode and current was dumped right into 1.75 ohm total.  It's easy enough to replace those 3 components.  I'd try that if it were mine.

[TheMG]

Resistors are likely for current sensing or something like that. Without any context as to what part of the circuit it's in and what the purpose of the circuit is, hard to say, but pretty sure those resistors aren't in there for protection/fusing purposes.

Could this be part of the battery charging circuit perhaps?