Bob Martin wrote:
I'm working in the avionics integration test facility at Gulfstream...
one of the engineers had a question for me (being an airplane person
instead of an electrical/computer guy) that I couldn't answer... why
do airplanes use 28V DC systems (or 14V)? He says most industrial
applications use 24V DC. I tried googling on it but nowhere did it
suggest any reason why, just that it is.

Lead-Acid chemistry batteries need 13.8 to 14.4V for "charging". The
voltage regulator on the alternator is usually set to a "compromise"
setting of 14.25V, which causes the bus voltage of the aircraft to be
near this voltage all of the time the engine is running.

All of the avionics and instrumentation is running off said 14.2V for
the duration of the flight. Double all of the above voltages if the
airframe has a 12 cell battery instead of a six cell.

What happens if the alternator fails in flight? Almost immediately, the
battery voltage sags down to under 12.6V, so the avionics has to work
over a voltage range of 11.5 to 15V...

MikeM