"Secret" Overvoltage Protection?

Sorry about the top post; but I didn't want to cut your original text.

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.

I am now about ten years out of date as an electronic technician, so I
really don't know what additional parts reamain operational; but, at the
most basic, system clocks, memory of any preselected channels (or
frequencies), the on-off control circuitry, the voltage protection circuits,
and possibly any crystal heaters. It is not just an aviation problem
either, as I suspect that it may be a primary cause of failure in personal
computers and other houshold electronics; but it is easiest to see in
automotive radios--which usually have no internal batteries or capacitors to
keep the memory alive when the main battery is disconnected for even a few
seconds.

Anyway, I'm glad that the protection circuits appear to have worked without
damage.

Peter


"Ron Wanttaja" wrote in message
...
I had an unusual experience last week, and figured I'd air it here for some
opinions.

My Fly Baby had developed charging problems again. I wasn't sure if it
was the generator or regulator, so I decided to do a bit of temporary
wiring to bypass the regulator (Generator armature to +12V, ground the
Field terminal).

I knew that in this mode, the generator could produce high voltages that
could fry my radio (ICOM IC-A5) and transponder (Microair). So I left the
power switches for both off.

I started the engine and watched the voltmeter. It read 8V at idle, but
when the power came up, the voltage started to rise. I chopped the
throttle before the voltage got much past 14V on the gauge.

So, the regulator was bad. Fortunately, I'd bought a spare at a
fly-market a few years back ($5). I switched regulators, buttoned up the
cowl, and mounted up for a test flight.

Strangely enough, neither the radio nor the transponder would power-up at
first try. I cycled the power on and off, and pressed the "on" buttons on
each a couple of times.

The ICOM powered up about the third try, and worked normally. The
Microair was more stubborn, but after about 15 minutes of power-on, it too
came on when I hit the switch.

The only thing I can think of is that both units have some sort of
overvoltage protection that clears itself after a time. Neither has the
classical "On" button with mechanical contacts that make or break the
power to the unit; the "On" button undoubtedly tells some aspect of the
circuit board what to do (on the Microair, one hits the power button
multiple times to adjust the LCD lighting).

Still, the units shouldn't have seen that direct generator output...they
were turned off. So what activated the overvoltage protection?

Anybody got any ideas?

Ron Wanttaja