Gary,
The scenario begins with B+ being shorted to ground by some kind of fault.
This tries to drive B- negative because the power supply is attempting to
maintain a voltage between B- and B+. If B- is forced to stay near ground in
this circumstance, the effect is to short the supply which will then
presumably blow a fuse. But if B- is not held somewhere near ground (i.e.
left "floating") it will be driven to a negative potential equal to the full
value of the power supply.
Perhaps "float" was an inapt choice of word here, but the idea is to
indicate the lack of anything holding it at a low impedance to ground.
73, Carl WS7L
> -----Original Message-----
> From: amps-bounces@contesting.com
> [mailto:amps-bounces@contesting.com] On Behalf Of Gary Patterson
> Sent: Tuesday, September 14, 2010 1:32 PM
> To: amps@contesting.com
> Subject: Re: [Amps] Possible issue with B- "floating down"
>
>
> Could someone comment on the "floating down" of the b-.
> What is actually causing this?
>
> > From: carlclawson@verizon.net
> > To: amps@contesting.com
> > Date: Tue, 14 Sep 2010 10:28:40 -0700
> > Subject: Re: [Amps] Possible issue with B- run in wire with
> less than HV rating? (Separate HV PSU and RF decks)
> >
> > Hi Marius,
> >
> > Your scenario can happen. I recently saw B- arcing to the chassis
> > through the insulation on low voltage wiring in an LK500-ZB
> amplifier.
> > It had suffered an HV arc during which the grid meter shunt
> resistor blew open.
> > This resistor was the only thing holding B- near ground, so
> it could
> > now float downwards to a dangerous value. An unwary person
> trying to
> > probe the low voltage end of things in this situation could
> get a nasty surprise.
...
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