12 Oct
2008
12 Oct
'08
6:59 a.m.
Tony Duell wrote:
I _hate_ those wall-wart style PSUs. They're often recommended in
electronics magazines over here as being a safe way to make a mains
powered device. Sorry, even if I say so myself, Id much rather use a
quality mains transoformer, in an earthed metal case, with fuses on all
windings, and so on, wired by me. I can't put any safety logos on it, but
my homebrew PSUs have never run hot, never caught fire, and never given
even a minor shock, which is more than I can say pf wall-warts.
These are SMPSUs. I never bothered to 'scope the outputs. They were
run inside "secure" metal boxes in the lift plant room of tower blocks,
so heat buildup was often a problem. I wouldn't be surprised if some of
the caps were as dry as a... dry thing.
The bits left looked like they had once been bits of an SMPSU, yes ;-)
Gordon
OK, and how many _expalained_ PSU fires did you get
with Level One?
Heh. None, although the plugtop PSUs that came with certain
Alvarion wireless bridges had a tendency to fail and cause
weird problems like
the bridge constantly rebooting - it would display the first few
characters of its startup message, and then <BLINK BLINK> on the status
lights, and it's booting up again.
Sounds like excessive ripple on the outputs. Some of these wall-warts are
actually SMPUSs internally, and with one common-ish controller chip the
darn thing will pulse on and off at about 1Hz if one of the cpaacitors
fails. Took me a longh time to find that the first time... I tested 20 of
the "suspect" PSUs. Four of them failed, hard. Failed
hard as in "blew the case apart" failed. No, I don't know how they
managed it.
Again, if an SMPSU, a shoted chopper or similar will do this. On linear
type supplies, I've had shorted rectifiers or smoothing capacitors get
the transformer hot enough tomelt the case (no internal fusing,
apparently the transformer primary is supposed to burn out safely under
such condiitions, all I can say is that it didn't (the primary was still
continuous after I unplugged the thing and let it cool down).