I am hoping somebody could help me diagnose the cause of failing electrolytic capacitors in a circuit. The context is a charger circuit, more specifically its power supply unit:
Given a diode bridge rectifier that is connected on one set of terminals to the mains via a filter circuit and on the other side to two electrolytic capacitors connected in series. Parallel to each capacitor are two discharge-resistors. (And of course the actual consumer circuit.)
The problem is, one of the two capacitors in series, the one connected to the - terminal of the diode bridge, explodes. This fault occurs since the device was subjected to intermittent AC-supply: unstable frequency, probably over- and under-voltages.
I am wondering where to search for the problem:
- diode bridge? - I played around a bit with spice and the only way I could get reversed polarity on one of the capacitors was to assume two diodes short-circuited and the other two became isolators. This does not seem a very likely failure mode...
- circuit fault after the power supply unit? - Any hint on what to look for? The consumer circuit is non trivial...
The capacitors in question were 200Vdc, 1000uF for a device to be plugged into 230Vac. I replaced both after the first failure, plugged it in and immediately the same capacitor blew again.
Unfortunately I do not have access to an oscilloscope at the moment but only a digital multimeter.
I took a closer look at the PCB and got the following partial schematic. The main transformer is in the top left hand corner. The PWM IC chip etc. are connected to the hanging MOSFET the the bottom of the schematic.
The TRIAC is part of a STR 81145, which automatically switches to voltage doubling mode if input is 115V. (It takes it's supply from the positive rail as well.)
After removing the capacitors in question, C7 & C8, the resistance to the positive and negative rails are equal to the 50kOhm set by the voltage divider.