[Amps] B- questions

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Sun, 8 Nov 2009 09:05:10 -0600, Rob Atkinson <ranchorobbo at gmail.com> wrote:

>Bill thanks for your answer.  I appreciate the lack of gobbldygook
>But I fail to understand why all this metering of B- is somehow safe,
>but doing the same thing with B+ is not.  If B- is around minus 3000
>v. it seems to me that having only a dinky resistor and/or a diode and
>meter between that and the chassis is dangerous.  There must be
>something counterintuitive about this I am missing.

REPLY:

 The issue is safety for the operator, not safety for the meter. 

The problem with B+ metering is it puts the entire B+ on the meter, which is
normally mounted where someone could touch it. Personally, I don't trust 1/16
inch of plastic to protect me from many kV. In fact, with many old meters, the
adjusting screw is metal, which places the B+ right out in the open, ready to
kill. With the meter in the B- circuit and protected by diodes, no more than one
volt or so appears on the meter. 

Regarding the "dinky" diode between B- and chassis, conventional silicon diodes
which are damaged by a massive arc always fail shorted, never open. That is what
you want for protection. The only thing you would notice with a shorted diode is
the meter(s) no longer indicate any current. That's the safest way.

73, Bill W6WRT