Dan et al ...

The "tiny fuse" you refer to is actually a single fuse mounted as close as
possible to the battery box. AIVR, you can have either 3 or 5 amps directly
from the battery (i.e. a 3 or 5 amp fuse) in a "keep alive" circuit that
doesn't go through the master relay but is wired directly to the battery.
That circuit is then wired to the clock and any other avionics that need to
draw (usually microamps) current to keep memories alive and the like.

In the Cessna, I mounted that fuse on the master relay bracket and got the
"hot" from the master relay itself on the upstream side of the battery.
Minimum wire and minimum chance for fire if the wire insulation shorts to
ground. Use a rather thin wire with an extra coat of heavy shrink sleeving;
if it DOES short to ground it will fuse rather than start a fire.

Jim

--
"It is the mark of an educated mind to be able to entertain a thought
without accepting it."
--Aristotle


wrote in message
...
On Feb 15, 9:28 am, Bryan Martin
wrote:
You are correct in that some circuits in many avionics devices will be
powered up anytime the master switch is on. The only way to be sure
these devices are completely isolated from ship's power is to pull the
fuse. But, avionics devices are not going to have any memory circuits or
clocks that require a keep alive voltage unless they have a dedicated
battery for the purpose. When the master switch in an airplane is off,
there is no power at all getting to the avionics, all circuits are
completely disconnected from the ship's power unless they have a backup
power source. About the only thing in most airplanes that might bypass
the master relay is an electric clock.

No, some avionics have a "keep-alive" feed to them via a tiny
fuse directly from the hot side of the battery contactor. Narco's MK12d
+ radios, for instance. If that isn't connected the memory is gone and
the display will default to the lowest frequencies on system shutdown.
Clocks usually have a feed through another tiny fuse. And Artex's new
ME406 ELTs also have a hot feed to the panel switch, but all it does
is feed the LED indicator so that power isn't robbed from the ELT's
battery to do it. Kannad and some others don't use a hot wire. The hot
feed is an item that adds to the installation cost, by the way. Got to
find a place for the fuse and run a wire from it to the panel. Labor,
man.

I'm wondering if Ron's problem was a low system voltage after
he installed the new regulator, until it woke up and started the
generator charging. Some of these radios will drop offline if they
aren't getting nearly the full rated voltage.

Dan