UC3842 Power Adapter Blowing Fuse
Good day folks......wow this new layout is weird. It's like I woke up one day and I'm no longer on Planet Earth or something 
Anyway....got this here power brick from a USB to SATA adapter which keeps blowing the fuse after I almost repaired it. The reason I even bother to fix a power brick (wall wart, whatever) in the first place is because this thing was really convenient to use: I'd just plug it and that's it. It's a dual output supply, for both 12v and 5v, which although I COULD easily obtain with a DC-DC converter from a standard 12v adapter, I thought I should crack open the original adapter to have a look inside, just in case it's something really easy to fix.
It seemed simple at first, but ended up being a rabbit hole: what happened is I shorted the output for a few seconds by accident on a metal surface. I noticed the LED was not coming on and I began investigating, but by the time I noticed what the cause really was, I heard a small "poof" and that was it. Cracking the case revealed 2 (or 3, not sure, since one may have been completely pulverized) destroyed current sense resistors on the source of the FET. Although they were charred, they were still decipherable - 2R2, so 2.2 ohms each. The FET and IC were also dead. I didn't have the exact transistor on hand so I used something equivalent out of my junk bin. Same goes for the resistors: didn't have them, much less so in SMD form, so I botch-soldered a single 0.82 ohm one on there hoping it'd be close enough. Output diodes were also shot - replaced those too....this was getting out of hand at this point, but I pushed on nonetheless.
Where I stand now: running it in series with a 25w bulb on the mains side, as usual, produces the correct output voltages (12v and 5v), but the bulb glows too much, hinting it's drawing too much current. I tried it without the bulb just to confirm and it indeed blew the fuse. Thankfully, it didn't kill any parts, so it wasn't a violent current draw, but not functional either.
Having replaced virtually everything that could be bad (even the main cap), my money is left on the transformer itself having a shorted winding inside, which is a lost cause at this point, since I'm clearly not going to find a direct replacement for it.
Things I'm not entirely sure about: I'm not sure whether there were 2 or 3 current sense resistors. I assumed there were 2 and went with that 0.82ohm resistor. I tried it with a 0.5ohm resistor and the same behavior occurred.
Second, is there a difference between a UC3842AL (original) and the UC3824B that I used ? I couldn't find any difference in the datasheets myself, so I doubt it.
Third, can bad output diodes cause this ? "Leaky" ones, since shorted ones can definitely can. I didn't have new diodes and they are also not the same type: the originals, if I remember correctly, are SR540 and I put in a pair of SR515 which I concluded sure should work after going over the datasheet.
Any ideas ? It's probably heading towards the trash bin (after I recover all of the useful components that I spent so much time soldering in)....
Anyway....got this here power brick from a USB to SATA adapter which keeps blowing the fuse after I almost repaired it. The reason I even bother to fix a power brick (wall wart, whatever) in the first place is because this thing was really convenient to use: I'd just plug it and that's it. It's a dual output supply, for both 12v and 5v, which although I COULD easily obtain with a DC-DC converter from a standard 12v adapter, I thought I should crack open the original adapter to have a look inside, just in case it's something really easy to fix.
It seemed simple at first, but ended up being a rabbit hole: what happened is I shorted the output for a few seconds by accident on a metal surface. I noticed the LED was not coming on and I began investigating, but by the time I noticed what the cause really was, I heard a small "poof" and that was it. Cracking the case revealed 2 (or 3, not sure, since one may have been completely pulverized) destroyed current sense resistors on the source of the FET. Although they were charred, they were still decipherable - 2R2, so 2.2 ohms each. The FET and IC were also dead. I didn't have the exact transistor on hand so I used something equivalent out of my junk bin. Same goes for the resistors: didn't have them, much less so in SMD form, so I botch-soldered a single 0.82 ohm one on there hoping it'd be close enough. Output diodes were also shot - replaced those too....this was getting out of hand at this point, but I pushed on nonetheless.
Where I stand now: running it in series with a 25w bulb on the mains side, as usual, produces the correct output voltages (12v and 5v), but the bulb glows too much, hinting it's drawing too much current. I tried it without the bulb just to confirm and it indeed blew the fuse. Thankfully, it didn't kill any parts, so it wasn't a violent current draw, but not functional either.
Having replaced virtually everything that could be bad (even the main cap), my money is left on the transformer itself having a shorted winding inside, which is a lost cause at this point, since I'm clearly not going to find a direct replacement for it.
Things I'm not entirely sure about: I'm not sure whether there were 2 or 3 current sense resistors. I assumed there were 2 and went with that 0.82ohm resistor. I tried it with a 0.5ohm resistor and the same behavior occurred.
Second, is there a difference between a UC3842AL (original) and the UC3824B that I used ? I couldn't find any difference in the datasheets myself, so I doubt it.
Third, can bad output diodes cause this ? "Leaky" ones, since shorted ones can definitely can. I didn't have new diodes and they are also not the same type: the originals, if I remember correctly, are SR540 and I put in a pair of SR515 which I concluded sure should work after going over the datasheet.
Any ideas ? It's probably heading towards the trash bin (after I recover all of the useful components that I spent so much time soldering in)
....
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