[Elecraft] RE: Supply voltage and diode drops....

[Morrow, Michael A.]

Dave wrote:

> you may as well change a menu option to disable the automatic=20
> addition of 0.65 to the voltmeter at the same time.  If there
> was a spare input on the PIC, this could be done automatically
> with a few more parts

You'd likely still have an inaccurate reading.  The forward voltage drop =
of the polarity protection diode is not a constant.  For a particular =
diode, it depends on the current through the diode AND the diode =
junction temperature.  An increase in either increases the voltage drop.
=20
If this really bothered a person, perhaps one should bypass the diode =
and install a shunt diode at the power connection that would blow a fuse =
in the power cable if the wrong polarity is applied.  That is the scheme =
commonly used in other commercial ham rigs.  It works great as long as =
one doesn't neglect to use fused power, and as long as you don't have an =
internal battery pack that needs the reverse polarity diode in the =
external power circuit to prevent backfeed into the external power =
supply.  It also has the advantage of maximizing the applied voltage =
when external battery power is being used.

73,
Mike / KK5F