Help with ICL power supply fault (Farnell SMPS)

Tony Duell ard at p850ug1.demon.co.uk
Thu Apr 10 16:28:20 CDT 2008


> > I think it's reasonable to asusme the chopper is working (otherwise you=
> 'd
> > get no outputs at all). Does it seem to be running continuously, or do
> > you get the 'tweet tweet tweet' of a PSU that's starting, detecting a
> > fault, shutting down, and repearing?
> 
> Scoping tbings in there would sure be informative.

The prolem is that if this is like most small SMPSUs, then almost all the 
'interesting' circuitry (oscillator, etc) is on the mains side of the 
isolation barrier. And therefore you can't just connect a 'scope to it, 
not unless you like shorting out the mains via one of the rectifier 
diodes and the earth (ground) lead of the 'scope.

The right way to do it, of course, is to use an isolating transformer, 
but not everybody has one of those in the workshop. I certainly do not 
recomend 'floating the scope;' (removing its earth lead so the 'scope 
chassis can sit at half mains voltage, or whatever), even though this is 
suggested in one service manual I have if an isolating transformer is not 
avaialble. I have this objection to large metal ohjects sitting at 
dangerous voltages.

-tony



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