bosch al1820cv blowing capacitors - Page 1
Forum section: electronics / repair

author: macross
author_rank: Contributor
timestamp: March 19, 2021, 04:55:51 pm
content: So a couple of days ago a friend of mine dropped off a bosch lithium battery charger for his drill. He told me that he had accidentally shorted the outputs and it no longer charges. I have turned it on and saw the green light, but when I plugged in the battery immediately low side 35v capacitor blew its top off.After that incident, I have replaced the capacitor with the same rating as before and turned it on. Then I have measured the output voltage and it's around 8v. I'm not sure if it's correct because it's one of those smart chargers which selects voltage based on battery. So I proceeded to plug in the battery again and it showed a blinking charger light which indicates that it's charging. Then after a second, it blew the same cap again.Any ideas what could be the problem? The cap explodes ether because of over voltage or over current and my primary suspect would be the feedback loop what do you guys think?Also I can't read any of the markings of components because of heavy conformal coating. Even probing around is a challenge.

author: RayRay
author_rank: Frequent Contributor
timestamp: March 19, 2021, 08:07:19 pm
content: There's probably one or more fried diodes/transistors on there. You'd need to have em properly tested with your meter.And in regards to the cap you've replaced, do note that power supplies require low ESR caps (and not general purpose ones) so it's more than just matching the rating (not that I think it's related to it popping, but just pointing that out). Set all that aside, do note that compatible chargers can be found for around $20 on ebay/aliexpress, so this may not be worth your time.

author: bdunham7
author_rank: Super Contributor
timestamp: March 19, 2021, 09:26:51 pm
content: The cap is probably blowing because it is seeing AC instead of rectified DC, as a result of a diode (perhaps part of a bridge) being shorted.

author: macross
author_rank: Contributor
timestamp: March 20, 2021, 08:07:10 am
content: I have tested all the diodes on the board and everything seems fine. The cap blows only when you put a load on it e.g. a battery. (Also the cap I have replaced was a low esr one)

author: RayRay
author_rank: Frequent Contributor
timestamp: March 20, 2021, 05:00:31 pm
content: Well, things aren't always as they seem, so I wouldn't be too sure about those diodes! Glad to hear you've used a proper cap though. Did you test the zener ones too? Tested in both directions (polarity)? FYI, you should only get a reading in one direction (in diode mode) with the correct polarity applied, but some exceptions apply in regards to zeners (If your meter's diode mode output voltage is higher than the zener's rating, then it'd read in both directions. I.E if it outputs 3v but the diode is rated for 2.4, and typically, the reading in reverse polairty would be double, I.E 0.700 in the proper polairty, and 0.1400 in reverse) a diode that gives you a reading in both directions should be tested out of the circuit to verify its status (if a zener, look at it under a magnifier with some light, and determine its rating, because, as said above^) and if a regular diode still reads in both directions outside the circuit, then it's 100% a goner. Also, diode testing mode alone isn't completely reliable (I've had diodes thad checked out okay in diode testing mode, but failed in resistance mode!) so you should test em in resistance mode too to be on the safe side (a diode should read in resistance mode in both polarities, with the resistance in reverse being significantly higher, which some meters cannot accomodate) but as long you get a good resistance reading in the correct polarity, it's probably fine (no reading, or infinite reading til your meter goes OL means its bad!) if you have a cheap meter (no auto range) then try different ranges.Edit: While the polarity is visibly marked by the manufacturer in most circuit boards, to be on the safe side, before removing a diode from the circuit, use a marker to mark the minus position on the board.