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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 * ************************************************** *********************** |
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PA28 Light Dimming Rheostat issues
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#3
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PA28 Light Dimming Rheostat issues
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... |
#4
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PA28 Light Dimming Rheostat issues
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 * ************************************************** *********************** |
#5
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PA28 Light Dimming Rheostat issues
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 |
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PA28 Light Dimming Rheostat issues
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 * ************************************************** *********************** |
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PA28 Light Dimming Rheostat issues
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 |
#8
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PA28 Light Dimming Rheostat issues
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 |
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