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

--

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

wrote:
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



Cory,

Piper is not alone. All of the 50s to 70s 100 Series Cessnas have the
same problem. The worst case dissipation in the rheostat is 7V * about
2A, about 14W. While you can buy rheostats with up to 20W dissipation
rating, they have a separate max spec which is exceeded: the maximum
current rating of the wiper contact is typically ~1.2A. The rheostats
tend to burn up the contact wiper, or the last couple of turns of the
resistance wire.

I converted mine to transistorized dimmers, where the power dissipation
takes place in the transistor rather than in the rheostat...

Or even better, a well-filtered buck chopper which dissipates nearly zero
power at any rotation of the control.

Jim



I converted mine to transistorized dimmers, where the power dissipation
takes place in the transistor rather than in the rheostat...

RST Engineering wrote:
: Or even better, a well-filtered buck chopper which dissipates nearly zero
: power at any rotation of the control.

: Jim


:
: I converted mine to transistorized dimmers, where the power dissipation
: takes place in the transistor rather than in the rheostat...

Of course a switching converter is better for power (gotta be careful about RFI), and a linear
transistor is better in that it actual *regulates* as opposed to fudges it with a resistor. IIRC though,
Piper's transistorized dimmer is a brain-dead design that will blow the transistor if there's a fault to
ground.

What got me is not that there was too much current in the circuit, but that there was too much
from the factory! It's one thing if you install enough avionics to overload it, but it was improperly
sized in the first place. Like I said, even just a change in rheostat change (I/R tradeoff... same power
capability and form-factor) would have been more acceptable. Probably had a surplus of 25Ohm/1A
rheostats in 1969...

-Cory


--

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

MikeMl wrote:


I converted mine to transistorized dimmers, where the power dissipation
takes place in the transistor rather than in the rheostat...

Did you buy one of the off the shelf or "make it youself"? I have had to
replace the radio dimmer twice already. The last couple of windings burn
though. The panel flood light has been fine. But both get rather warm.
--

Regards, Ross
C-172F 180HP
KSWI

Of course a switching converter is better for power (gotta be careful
about RFI),

I thought I said "well-filtered", no?



and a linear
transistor is better in that it actual *regulates* as opposed to fudges it
with a resistor. IIRC though,
Piper's transistorized dimmer is a brain-dead design that will blow the
transistor if there's a fault to
ground.

A chopper regulates better than a linear regulator of the same complexity
and it has a short-circuit current limit resistor that lets you set where
the device simply refuses to give any more current, thus protecting the
output device.

Kitplanes article, anybody?

Jim



What got me is not that there was too much current in the circuit, but
that there was too much
from the factory! It's one thing if you install enough avionics to
overload it, but it was improperly
sized in the first place. Like I said, even just a change in rheostat
change (I/R tradeoff... same power
capability and form-factor) would have been more acceptable. Probably had
a surplus of 25Ohm/1A
rheostats in 1969...

-Cory


--

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

On Apr 28, 5:10 pm, "RST Engineering" wrote:

A chopper regulates better than a linear regulator of the same complexity
and it has a short-circuit current limit resistor that lets you set where
the device simply refuses to give any more current, thus protecting the
output device.

Kitplanes article, anybody?


Good idea. Isn't that what a cordless drill uses these days? I
can hear it sing at low draws. It would need some filtration, though,
I'd think, to keep noise out of the airplane's radios and intercoms.

Dan

Well, yes and no. I've said from my first reply that filtering would be
necessary. However, by choosing the switching frequency appropriately, you
can place the switching frequency much higher than what the ear can hear
while at the same time reducing the size of the filter capacitors needed to
keep the crap (that's a technical term, you'll get used to it) out of the RF
sections of the radios. And away from any harmonic that falls directly on
the Loran frequency 100 kHz.. I've used switching frequencies in the 40
kHz. range quite commonly and haven't found any adverse effect on the
radios...with the exception of ADF beacons on 240, 280, 320, 360...kHz.,
where you could hear a very weak whistle if you listened closely. Didn't
seem to affect the needle to amount to a hill of beans.

I've used frequencies MUCH higher (in the fractional MHz. range) but the
core losses of the inductor start to heat up the chassis.

Jim

--
"If you think you can, or think you can't, you're right."
--Henry Ford





Good idea. Isn't that what a cordless drill uses these days? I
can hear it sing at low draws. It would need some filtration, though,
I'd think, to keep noise out of the airplane's radios and intercoms.

Dan