First, I'll agree that this is potentially very dangerous. Think also of
the next person to use to scope...when it develops a fault.....

If you have a dual trace oscilloscope, remember that often you have an

'A-B'

Button. Use this to subtract one reading from another. You can then put
the A probe tip at the supply side input, and then the B probe some
voltage lower. The 'scope will then read the difference, I.e. -B volts
with respect to A. Often there is an invert feature too, so you can
switch this back to +B volts if you need to.


Aside from this, be extremely careful when working on mains voltage. If
possible use an isolating transformer between the kit you are measuring
and real mains. Use RCDs. Take extreme care. Remember that if you have
230V mains (as we do here in the UK) the peak voltage is 325V,
nominally, but the mains spec allows the RMS level to rise to 252V, so
the peak value will become 356V.

Regards, Kevin

> >
> > NO !  NO  ! NO !   THIS IS NOT THE RIGHT WAY TO DO THIS !

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