Wall wart gone boom - Page 1
Forum section: electronics / repair

author: capt bullshot
author_rank: Super Contributor
timestamp: October 18, 2017, 05:30:54 am
content: So I've found this litte innocent 5V wall wart adaptor in the scrap box. Nosy as I am, I picked it and checked - nope, dead as a dodo.Took it apart, et voila - there's been some arcing inside, you can see some blasted components and a clear trace of arcing from the gap underneath the transformer to some other trace. Distance bridged is 5mm. I wonder if some plasma from the other blasted components ignited this arc?

author: Zucca
author_rank: Unknown
timestamp: October 18, 2017, 02:09:32 pm
content: walking plasma wave? I mean fist arc, then the sencod and then the third until the complete board is toasted?Ah, you don't want to repair that board, it makes no sense to me.

author: wraper
author_rank: Unknown
timestamp: October 18, 2017, 02:22:07 pm
content: It think it's just magic smoke which escaped from resistor and deposited on PCB. No actual arching. Or if it was, then there should've been some water on the PCB when this happened.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 18, 2017, 02:42:24 pm
content: Quote from: wraper on October 18, 2017, 02:22:07 pmThere must have been an arc from the resistor (R26 or maybe 28) to the trace near the gap. You can see the burn marks at the end of the track. Some of the evidence looks like copper or tin deposited, yes that's true.And no, I don't want to repair this thing.

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 18, 2017, 03:27:53 pm
content: It is indeed a wide distance.My theory would be that there was ionic contamination left over on the board (due to improper washing) and a dendrite or two which caused the flashover.That theory would have to be validated though, with surface contamination testing.Anyways, it appears that it arced between the primary (hot) and secondary. I'm glad that this accident did not happen to you while holding the phone, or whatever device was being powered up at the time.

author: wraper
author_rank: Unknown
timestamp: October 18, 2017, 07:44:05 pm
content: Quote from: schmitt trigger on October 18, 2017, 03:27:53 pmI don't see any trace of damage on secondary side.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 19, 2017, 06:48:25 am
content: I've taken a close-up of the damages and a view of a good one (that is broken in some other way :-)From the good one, R26 is the current sense resistor in the emitter line of the main switching transistor. The transistor is shorted B-C-E, so my guess is:Some kind of unknown event took out the main transistor, this let the magic smoke out of R26 and did some more collateral damage (the controller IC lost a leg and vented through a pinhole, some other resistors are blast and one more transistor lifted its hat).IMO the R26 event started the arc to the 5mm distant PCB trace.No, the secondary side was not hit by the arc, there's no visible evidence.

author: jahonen
author_rank: Super Contributor
timestamp: October 19, 2017, 08:41:42 am
content: My guess is that some line transient/surge event ignited the arc (to R28?) and caused the destruction of the rest of the circuit, as there is evidence of the arc going from line input to the internals of the circuit. I don't see how it otherwise could have happened.Regards,Janne

author: Cerebus
author_rank: Super Contributor
timestamp: October 19, 2017, 04:21:15 pm
content: Quote from: zucca on October 18, 2017, 02:09:32 pmI realize that's just a typo, but the image of a "fist arc" punching components to death is irresistible.