Foating B- above chassis ground is necessary & sound practice. That
said, you don't need to fear metering or otherwise called "float" resistors
between B- and chassis ground, so long as they are accompanied by a SINGLE
(or pair) of diodes facing ground (from B-)- Lunar link, e.g., uses a 1N5408
(plus resistor & cap) to ground in the PS section (& and also the same in
the RF deck ro ground -remote PS)- you just have to make the parallel
resistors = to the necessary value in parallel for the meters. 1N15408s and
single resistor from B- to ground on each side of the amp (PS & RF deck)
plus a HV cap work great. If a resistor(s) blow open, no problem, the
diodes blow the breaker or fuses. I use this configuration on all my HB
amps and 0 issues. Ommiting the diodes, however, will create an issue if
the float resistor opens- then you'll be hunting shorts like a a madman
until you realize the float resistor is open (been there, done that, in 3rd
world countries). The potential from B- to chassis ground will be so great
if the resitor(s) blow you'll get immediate shorts (or arcs anyway) from all
over until you find the float / shunt resistor is blown. Use the diodes in
parallel, keep the shunt, and you'll be fine. Just be prepared to replace
the shunt R & (maybe) the shunted diodes in case of major mishap.
73 Dave N3DB
----- Original Message -----
From: "Carl Clawson" <carlclawson@verizon.net>
To: <amps@contesting.com>
Sent: Tuesday, September 14, 2010 6:18 PM
Subject: Re: [Amps] Possible issue with B- "floating down"
> 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.
> ...
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|