8' drive on PC troubleshooting

[der Mouse]

>> 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!

But don't forget that in this use, the output ground is connected to
the output 12V of another supply, and thus is referred to earth through
that other supply.

> 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
output as a crowbar.  Or perhaps a low-current 15V zener and a big-ass
SCR, the latter being the crowbar.  Something of that ilk.

> This will cook whatever classic device you're powering from it, and
> may cook you too.  Which is more worrying depends on who you are!

:-)

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