So why dis Piper wire it that way? Cessna has a separate circuit, the keep
alive circuit for what ever piece of equipment that needs it, that is wired
to the hot side of the master contactor. There is a fuse right next to the
master contactor for this circuit.



"Mark Mallory" wrote in message
...

Jay Honeck wrote:

Also, why is there a 7-second delay after turning the master switch on?

The 7-second delay is probably in your clock. That's how long the clock
continues to run after it's power is removed.


Here's how it works:

1. The clock is connected to the "hot" side of the master.

The "hot" side of the master does not go to the battery; rather, it goes
to the
COIL(-) of the master contactor. The battery goes to the *other* side
[coil(+)]

Your clock is thus not connected directly to the battery. The COIL of the
contactor is between the battery and the clock.


2. It works fine with master "off"

Yes, because it's receiving voltage *thru* the COIL (a resistance of
several
tens of ohms or so.) It works just fine.


3. Turn Master Switch "on" -- works fine for seven seconds.

The master switch connects the coil(-) to GROUND. The voltage at the
"hot" side
of the master (and the clock) goes to ZERO. Current flows from the
battery thru
the coil and switch to ground; causing the master contactor to close
(you'll
hear the "clunk" if you listen) and the airplane's electrical system is
energized.


4. At seven seconds, the clock goes out.

Because that's how long the clock runs after power is removed.


5. Turn Master Switch "off" and the clock comes back on.

Yes, the COIL is un-grounded, the contactor opens ("clunk") and the
electrical
system is de-energized. The voltage at the "hot" side of the master
returns,
and the clock runs again (unless it's wiped out by the inductive kick-back
from
the coil


What's going on? Why is this circuit set up to do this?

Ask the idiot (Certified Mechanic?) who wired in your clock that way!