Ray Andraka wrote:

Err, that transistor is there so that the rheostat doesn't double as a space
heater. It is there for a very good reason, although today there are better
ways to dim rather than a pass transistor. For that price, you could have
installed one of the pulse width modulated dimmers and done away with the old
fashioned piper circuit. Would have required a 337, but then it would be the
last time you'd ever have to deal with it. You might have even been able to
get separate dimmer circuits for instrument and overhead lights out of the
deal.

Be a bit carefull with PWM dimmers., if they get placed near the avionics
you might have problems with RFI .

Have a look at the circuitry for the alternator-fail annunciator
on the Piper Arrow. It wastes 4 watts just to keep a globe off.
(I'd always wondered why the comm-panel switches were always hot)