Help with ICL power supply fault (Farnell SMPS)

Roy J. Tellason rtellason at
Thu Apr 10 17:16:21 CDT 2008

On Thursday 10 April 2008 17:28, Tony Duell wrote:
> > > 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.

This is true,  though at the time I wrote that I was thinking about scoping 
the outputs.

> 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.

Yes indeed.

One way to achieve isolation is to get a couple of filament transformers that 
are of similar voltage ratings and use them both,  tying both low-voltage 
windings together and using one primary as the output.  I'd think that they 
wouldn't need to be particularly hefty if you're not going to load the supply 

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M Dakin

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