Help with ICL power supply fault (Farnell SMPS)

[Philip Pemberton]

Tony Duell wrote:
> 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.

If you don't mind losing your second channel, you can always apply one input 
to channel A and the ground reference to channel B, then set the scope to 
"A-B" mode (ChA subtract ChB). Most scopes can do this, and this is exactly 
what it's meant for.

The idea is that your ground reference remains mains earth, while one scope 
probe measures the ground level, and the other measures the signal you're 
actually interested in.


I suppose you could build an isolator of some form, but that sounds like it 
might not be as easy as it seems.

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