Charger Shorting

[d8veh]

I have this charger that had a short in it somewhere. It kept blowing the fuse as soon as I plugged it into the wall. There are 8 components tied to the wall with plates and screws. I thought that one of the plates might be touching a component's leg as they come pretty close, but I couldn't find one touching. Everything is very tight for space, so I couldn't get my probes to the legs to check while the plares were in place. I've removed the plates now to take the photos and now the charger is working.

With the plates removed, the components are no longer tied to the case, so there's two possibilities. A component should be insulated from the case, but is making contact or one of the plates was touching something it shouldn't.

Can anyone tell me from the photos which components should be insulated. Unfortunately, it's not possible to read their designations because they're covered in white stuff. Hopefully, somebody can say from the layout what they are, but from the markings on the PCB, in the first photo theyre a bridge rectifier, transistor, transistor, then two grey plastic coated transistors. On the other side, a grey platic thing marked U70 that looks the same as the transistors on he other side, a diode an a transistor. You can click on the photos to see them in full.



Thanks for your time

 

[d8veh]

I was just playing with it again and discovered that the grey things are rubber sleeves to insulate the device. All appear to be intact, Anything else I need to check?

 

[dnmun]

there are two large npn transistors right behind the input caps that are shorting to the case.

there should be complete coverage of the tab to keep it from contacting the case because the tab of the top transistor in this push-pull arrangement reaches about 340V so the case becomes a severe risk hazard.

if you were using a two wire conductor with no ground then the case could reach fatal voltages on the surface. it is a metal case.

look at the insulators under the switching transistor and you may find a piece of metal filing embedded in the rubber or maybe the rubber has torn and the tab is touching the case that way.

it should be obvious when you look, but if not then you can buy new insulators. it does not have to be a big boot like some have, just enuff silicone to cover all of the back side of the tab. with silicone heat sink compound on both sides to help carry the heat.

if it is a boot type and you find one side is torn, then reverse it and use the other side behind the transistor.

 

[d8veh]

Thanks for that. There is an earth wire for the case, which is tied to the screw that holds the bridge rectifier. I disconnected it for the photo. I couldn't find any damage to any of the boots, but one had a chip of metal or ceramic in it. Maybe that was it. I'll put it all back together and see if it still works. I think I'll grind the tie-bars a bit narrower too to keep them clear of the legs. I don't like those tie-bars because there's one per two components with a central screw. They could easily rotate so that one end is touching the legs or pcb.

 

[dnmun]

you can hold that clamp with your finger when you tighten the screw. i find it impossible to get the screw started so i have to hold the clamp bar from above with the needle nose pliers so i can line up the hole in the bar with the screw coming over from the case.

if you had anything at all on that insulating pad then it is perforated and needs replaced. i have a bunch of them i bot on ebay i can send you.

also for anybody else who needs to have those little insulators to go under the mosfets in their controller. just pm me and i can send some.

the case has to be grounded on these chargers. or treat it with the knowledge that it could be hot. that's what i do sometimes because one of my extension cords is 2 wire. and i use it for 240V too.

 

[d8veh]

I opened up a similar shorted charger that I bought a while ago. It has the same components inside except that it has a boot on another one of the transistor, and the rectifier is insulated by a two pieces of silicone cloth on top of each other (doubled up). Maybe I should insulate everything, just to be sure!

Thanks for your kind offer of the boots. I'm in the UK, so I've ordered some here to get them quickly.

 

[dnmun]

the rectifier does not need insulating from the case. it is just the collector that high side transistor that needs to be replaced and the 12V regulator should have a silicone pad too. but none of that like the schottky would blow the fuse imo.

not sure what you are saying is on the rectifier, it could be the spacer they put under the clamp to hold the 12V regulator in place. usually it is a folded piece of plastic from my experience.