8' drive on PC troubleshooting

[Tony Duell]

> 
> >> But you probably could do fine, I'd guess, by just cutting the mains
> >> ground connection for one or the other supply, as appropriate.
> > No, don;'t do that.  FOr one thing most PSUs have a mains filter
> > including 3 delta-connected capacitors.  If you take off the ground
> > wire, the chassis wil try and float at half mains voltage (!).
> 
> Well, only to the extent that the capcitors pass current.  It doesn't
> much matter if it floats at half mains voltage open-circuit if it has
> multiple megohms impedance/resistance to either mains rail!


My experience is that most mains filters will pass enough current to the 
(open-circuit)_ ground connectiong to give you quite a tingle and to zap 
some semiconductor devices.

> > And for another thing, if you have a component failure, you could end
> > up with mains on the case or output side, and the fuse won't fail.
> 
> Well, it depends on what happens when the other supply sees mains
> voltage between its output 12V line and its ground line.  But yes, this
> may be a concern; you may want to find something like a nice hefty 15V
> zener - one that can sink substantial current - and install it on the

I would have to nbe able to pass the short-circuit current of the mains 
IMHO. That's a fair few kA. A really big zener (or zener + SCR).

I'd be much happier working with a supply desigend to give 24V. Heck, I'd 
be happier trying to rewind the transformer in a PC SMPSU to give 24V.

-tony