PA28 Light Dimming Rheostat issues

Hey all. Just got done with a marathon day of maintenance yesterday and figured I'd share. The
interior lights started blowing the breaker a week or two ago, so my mechanic and I repaired it
yesterday. It turns out that the breaker blowing was easy (one of the little clippy light bulbs had
unclipped, fell a bit behind the panel, and was grounding to something else). What *wasn't* easy was
that the dimming rheostat died in the process (i.e., no dimming anywhere.... just
click-off-off-off-fullon). This 1969 PA28-140/180, doesn't have the transistor circuit, and it turns out
Piper had their head up their *ss designing even this part. The rheostat is a 25 Ohm, 1.0 Amp variety.
That'd be fine if the only lights in the plane were like in the older Cherokees.... an overhead and a
compass. With the multple panel lights, it draws too much even stock. In order to put it into the
"googleable" public record, here's what I measured with a charger providing the battery system with about
13V:

Instrument lights: 1.5A
Overhead light: 0.4A
Avionics lights: 0.9A

Total: 2.8A

I figured that the problem was all the avionics that I installed drawing too much, but the system
already drew about 1.9A from the factory. Granted, the current will be a little less than that just off
full, but still... the rheostat is horribly undersized from a current perspective. The same form-factor
could be wound as an 8 Ohm, 1.8A (or 6 Ohm, 2.0A) and be much much closer to reasonable.... anything more
than 8 Ohms and the lights aren't even on. Also, between 1.5-2.0A for my whole setup seemed (in
daylight at least) to be about where it's normally set in flight at night.

Anyway, just thought I'd share.

-Cory

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* Cory Papenfuss, Ph.D. Electrical Engineering, PPSEL-IA *
* Research Associate, Vibrations and Acoustics Laboratory *
* Mechanical Engineering *
* Virginia Polytechnic Institute and State University *
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