Panel lights - Rheostat Assembley

[Kyle]

Good Evening Everyone,

On a night flight last week I was adjusting the panel light dimmer and it was very hot to the touch. If I had left my finger tip their it would have been burned for sure. I'm considering replacing the rheostat (part # 0413126) as soon as possible but I recall seeing an advertisement a few years back for replacement rheostats in either the Cessna Owners Magazine or the Cessna Pilots Association magazine.

If my recall is correct they were advertised as a direct replacement that developed much less heat that the original Cessna parts .......

Problem is in looking through some back issues I can't find the ad. Does this ring a bell with anyone ? Is someone producing a better rheostat that I should consider?

Thanks in advance,

Kyle T.

[GAHorn]

Check out the following msg thread, which I found by using the "search" routine. The 100 ohm, 25 watt, Ohmite Model H I recommended in that thread is much larger capacity than the original, and therefore generates less heat.
http://www.cessna170.org/phpBB2/viewtop ... t&start=15

[Kyle]

Thanks George,

I'll check them out

Kyle T.

[gwillford]

OK, time to go public with my wild idea. I have a 53 with the panel overlay. I want to keep it original. I have the original overhead light assy also. Very poor lighting. Putting the Christmas tree away this year I got the idea of creating a string of lights out of small grain of wheat light bulbs obtained from the PMAd section of Radio Shack. I then proceeded to tape the string to the back side of the overlay and attached the leads to the rheostat. Awsome and cheap. I have no time on this setup as the plane is still sitting in my garage and under restoration. Has anyone else tried this? FYI, I have a PA-12 with from the factory lights also under overlays. That too gave me the idea.

[HA]

the "lights behind the overlay" idea pretty much duplicates some Piper setups, like Senecas etc. Piper painted the backs of the overlays silver for more reflecting. the Piper light bulbs are mounted to the actual panel, but same effect. they also have a blue lens cover, to prevent glare I think.

our old Twin Comanches had kind of a similar setup, post lights mounted to the overlays with the wiring glued to the overlay backs. wires broke a lot.

[Bruce Fenstermacher]

qwillford

I have nearly the same setup except I used NON PMA'd Red LEDs. Three per instrument wired in series and parallel with a resister to accomplish the correct voltage drop for the LEDs. There is no rheostat to adjust them and non needed. All parts from the LEDs to the wiring is hot glued to the back of the panel. The LEDs can barely be seen unless they are pointed out. This was a temporary experiment that worked so well more than 4 years ago that it has never been removed.

I have NO approval for it.

[gwillford]

Bruce,

The red LED approach sound even better to me. What type of red LED and where did you get them? Having never used them, what kind of voltage drop was required?

George

[Bruce Fenstermacher]

George

I have no records of what LEDs I used at the time. Probably what ever Radio Shack had in enough quantity, cheap.

I'm also not an electronics guy by trade but like many things I've dabbled with simple components from time to time. Actually a few grew to be complex and I'd have difficulty duplicating them today. Any way I can't site all the rules theories or laws (like OHMs law) that you would have to follow to figure it out.

The following numbers are from memory from a project I did more that 6 years ago and should not be take as correct but just an example of my thinking.

In my case I had six instruments I wanted to light. Using brand X LEDs I found I could put 3 per instrument and light the instrument nicely. I seem to remember the LEDs used 1.2 volts. 3 x1.2 =3.6 volts. If I put two instrument LED groups together ins series it would be 6 x1.2=7.2 volts. We basically have a 12 volt system so I needed a resister that would drop 4.8 volts and not be to big but not get hot. I by using OMS law I figured that out. I decided to use a single resister for each instrument so I divided the resistance in half. I used 10 watt resisters so they wouldn't get hot and with the resistance for each group of 2 instruments divided in half each would only have to drop half the voltage thus half the heat.

Anyway I wired this all up using aircraft wire soldered directly to the LED with any bare wire, anodes or cathodes covered in heat shrink tube. I hooked this to a 12 volt source and measured the over all amperage required so I was sure I wasn't overloading the wire used. I let the LED light system run on my bench for a week straight. For the first few hours I monitored the heat given off by the resistors which proved to be not detectable.

Now here is the trick. Depending on the state of your battery charge and your electrical system your aircraft voltage can vary several volts. From say 12 volts to 14.2 or more ( ) depending on your voltage regulator.

When figuring out the resistance needed you need to be conservative allowing for the higher voltage. In my case the LEDs are slightly dim at battery voltage but brighten up nicely with the generator charging. It was just luck on my part.

Another wild card is that LED made today are seldom just a LED. Lots of them also have a resistor and more built into them.

As I said in my original post I had no authorization to do any of this. And I wouldn't recommend it to anyone specially if you don't know much about electronics.

Since your post George, was centered around christmas tree lights I thought I'd through out another "wild" idea.

To be honest George if I'd had 18 grain of wheat bulbs rated at about 14 volts in a suitable color in my pile of electronics stuff, I probably wouldn't have used LEDs.

[marks]

Why not be perfectly legal and install a MaxDim unit. These are fully STC'd and can handle 12.5 AMPS, run absolutely cool and are great for LED's. The MaxPulse also has an AMOC for AD 93-24-15 which concerns cockpit fires. Just google MaxDim. The STC covers many, many airplanes so can be installed via that mechanism.
To see all the documentation on the MaxDim and MaxPulse go here
http://maxpulsemaxdim.com/latestdocs.htm

To view the AD 93-24-15
http://www.tc.gc.ca/aviation/applicatio ... -24-15.htm

To view the AMOC
http://www.seatoneng.com/docs/MaxDim/AMOC.pdf

[GAHorn]

Just to be clear, that AD does not apply to Cessna 170/170A/170B aircraft.

It is perfectly legal to replace a resistance rheostat with another. (Of course, most folks are not as content as I with the original cockpit lighting, and it's perfectly good and fun to "upgrade" such things. This replacement STC is approved for 170's, ...but it costs $165 part only, and an A&P/IA to replace a $5-$20 rheostat.)

Their other product, their Maxpulse for flashing taxi/landing lights, seems especially suited for the Del Lehmann dual light installation. It's only $195, and would be a great value, I think.

http://www.seatoneng.com/MaxPulse.htm