Yeah I got to thinking that too, although that would still not explain the current shunt resistor destruction, that to me still looks like a faulty component. Anyways, the pre-regulator was definitely not working last time around because the PSU makes quite different noises under load now. I guess time and some use will tell. I think if it fails one more time I might give up with it - should have my own design done by then :)
I am years late, but I am trying to repair my 3646A and came across your videos. Regarding the loud sound you heard. If during the failure it reached saturation current on the power transformer, a laminated core transformer can make a very loud bang when it ejects the field. The flash would have been a separate thing, probably the trace and gate failure at the FET.
It does amaze me just how big a visual show was created through such a small crack in the device. That is certainly a plausible scenario although I would expect the PSU linear regulator to handle that condition, although I would not be happy pulling 7A through a single Power MOSFET, it just feels wrong :) Its been reliable ever since the last fix though
I don't think I would have had the guts to try the load on it in that configuration again - good one! Check those FETs carefully - sometimes the smoke-liberating cracks are very fine. I wonder if the fault on the gate of the preregulator caused the origial problem as well.
GerryI would have had a good look at the gate drive waveform of the pre-regulator. It could be that the initial explosion caused the gate to short to the drain damaging the series gate resistor making it high in value. This is common with switchmode power supplies. I am assuming it has a series gate resistor of course.
No problem, it was bugging me that it was sitting there broken, I had to fix it! Then when I fixed it, it was bugging me even more that I could not find the source of the flash and pop so I had to find that too - now I am at rest :)
10 лет назад
Hmm, I do 10A through very similar cables when charging/discharging batteries. Sure they get lukewarm but never a sign of trouble.
This is the fault of the layout engineer and all his colleagues who allowed that preposterous trace width to connect a giant three-hole component. Reliability by design... fail.
with regret the camera was not rolling when it happened so I am afraid not. It was a big bright flash and I was expecting there to be a far bigger hole than the hairline crack I found on the device. Gerry
A graphic demonstration of just how fine a smoke-liberating crack can be! My thought was the pre-regulator failed due to the gate track problem (or was just intermittent), then the series pass regulator failed short circuit due to excessive dissipation, which cooked the shunt resistor (and probably destroyed the load as well!)
Now you know why its called magic smoke, I had a led go up in smoke on a radio and to this day I have no clue why, I just replaced it expecting the same thing to happen again it never did, go figure it? M3KQW.
