On Feb 15, 10:11*am, Ron Wanttaja wrote:
Peter Dohm wrote:
You have been tricked and, without schematics of the Com and Transponder, I
really don't know exactly how they are supposed to work. *However, the
common problem with modern electronics is that, without an old fashoined
mechanical switch, a portion of the circutry must remain powered on when the
device is turned off.

The difficulty is the lack of definitive understanding of how my
airplane is wired. *The original builder didn't generate a schematic. *I
inherited one drawn up by the third owner:

http://www.bowersflybaby.com/tech/elect.gif

When I ran the regulator-bypass generator test, I turned on the Master
Switch, the Field Switch, and the Generator Output switch, but left off
the switches for the radio and the "Aux" (which goes to the transponder).

If the schematic is correct, then no +12v power should have been
available to the devices unless I'd turned the breaker/switches on.

However, there is the possibility the schematic is wrong...that the
switches may control connection of the devices to the GROUND, and that
the +12V is delivered to the radio and transponder whenever the master
switch is on. *Since I have a wooden airplane, there is no "ground", per
se. *Two wires go to each device, and whether the breaker/switch
connects one wire to the ground bus or the other one to +12V wouldn't
make much difference in operation.

If the breaker/switch does control connection to the ground, there is a
chance that the devices still see ground via their antenna ground plane.
* If that is the case, then an overvoltage at the +12V input *might*
still be detected and reacted to.

However....neither device will power-up if the breaker/switch is turned
off. *The ground plane for the comm radio antenna is connected to ground
by the same wire that connects the radio itself to the ground.

Curious....

Ron Wanttaja

A lot of newer avionics have DC-DC buck/boost voltage regulators built
in. This shows up on the specs as an input voltage of say, 10 - 36 V
meaning the internal regulator will deliver a constant voltage to the
circuit boards if the aircraft power bus is somewhere in that range.
It also means that the device will work on both 12v and 24V systems.

FWIW, I have a Microair 760 in my glider and find it is unusually
sensitive to input voltage. Whenever the SLA battery drops to 11V, it
goes totally bonkers.

This brings up a slight fork to this thread and that is alternator
failure. I've had three - one in hard IFR.

It seems to me that modern avionics should have a UPS just like the
computer I'm typing on. Modern avionics use far less power than they
used to and batteries are getting better. It should be possible to
have a UPS battery on the avionics bus that will power the whole panel
longer than the fuel will last.