I'm trying to find a panel light rheostat

Heflin · Post by **Heflin** » Thu Oct 23, 2014 2:29 am

Looking for a rheostat assembly. Part # 0413126 for a '54. This is the one under the panel that dims the overhead red lights.

Is this a Cessna order part or can I get it somewhere else?

Thanks,
Rob

Lopez · Post by **Lopez** » Fri Oct 24, 2014 11:27 pm

I think I have and extra from my rebuild. I'll take a look next week at the hangar and make you a deal.

Heflin · Post by **Heflin** » Sat Oct 25, 2014 2:23 am

Thanks. Found one online but it's 0-15 ohm and the original is a 60 ohm. Not sure if that'd make a lot of difference.

Post by **Karl Towle** » Sat Oct 25, 2014 6:23 am

This gets into all kinds of EE theory. The original 'rheostat' was designed to dim two incandescent light bulbs, installed over-head, acting instrument floods. The simple version is the 'stat' turns electrical current into heat, limiting the current (hence brightness) of the bulbs. Very inefficient, and if other lights (post lights, nu-lights, etc) have been paralleled with this 'stat,' then it has to handle more current, which means more heat. So, if you wish to go original, you have to match the resistance, AND meet or exceed the original wattage. Ignore the wattage value, and you'll have a red-hot, momentary use heater under your panel.

The best solution is to install a dimmer/controller. It uses solid state components, and the concept that make switching power supplies possible. They handle higher wattage, and create very little heat. They can be found in one, or two channel designs (ie. one channel for floods, the other for nu-lights). Here is one that has STC approval for a long list of aircraft:
http://www.aircraftspruce.com/catalog/e ... ckkey=4042
There is one possible down-side in addition to the cost. Since it uses switching technology, it is possible to encounter interference with some audio components. I have encountered some buzzing in my head phones when operating the dimmer at anything other than full on, or full off. It is getting in through the Narco CP136 audio panel lighting circuits. Still working this issue...

Sat Oct 25, 2014 12:48 pm

ZetaByte info is pretty much right. One thing though is, at least in the '52 and earlier, all the instrument lights in the panel where one the dimmer. That is 7 bulbs in the IPC example I just counted. Then add anything else such as radio or additional instrument lights.

I must tell you I'm not an electrical engineer. However I've been fooling with electrical circuits 50 years starting with my Lionel trains which I still fool with. I've been known to get electrical theory backwards and I've burn up more than a few ICs. But I'll take a stab an explaining this.

The ohms is the number than indicates the resistance of the potentiometer (pot) and it's ability to turn electricity into heat and dim the lights. A simple statement would be a 0-15 ohm would only dim the lights 1/4 the amount the 60 ohm sample would in the same circuit because it has 1/4 the resistance capability but there is more to it which I will go into later.

As stated the most important number from a safety stand point is the wattage or the ability for the pot to handle the heat. You also have to take into consideration where the pot is mounted. In open air the installation might tolerate a hot pot. Doubling the wattage means the pot will run much cooler than one half its wattage in the same circuit and mounting situation. Now don't take my words as absolute. There are many variables to consider. I just exploring examples.

For example light is the effect of the glowing bulb element your eye sees. A 15 ohm verses a 60 ohm might actually make little difference in the light you see because 15 ohms or the first 15 ohms of the 60 ohm pot, might be enough to dim the element dark. In this case a difference you will see is the 15 ohm might need to be turned it's full travel to fully dim the light but the 60 ohm only a 1/4 travel as far as your eye is concerned. However, while the last 3/4 turn does nothing as far as light but it does quadruple the resistance and thus the heat potential.

If we search hard here at the forum one will find other discussions about the wattage and ohms of the pots used by Cessna from the factory. Which may or may not be a good choice for your current (no pun intended) installation.

BTW you want a linear pot. Non linear pots are used for audio though your not likely to find a non-linear pot in the size you require for this application.

Heflin · Post by **Heflin** » Sat Oct 25, 2014 3:24 pm

Thank you. My electrician buddy explained it much like you did (I just nodded and smiled

).

The ohms is the number than indicates the resistance of the potentiometer (pot) and it's ability to turn electricity into heat and dim the lights. A simple statement would be a 0-15 ohm would only dim the lights 1/4 the amount the 60 ohm sample would in the same circuit because it has 1/4 the resistance capability but there is more to it which I will go into later.

I found two original rheostats in my wrecked '56. I completely forgot about them. I'm going to give one a try before I buy another.

Also, the 0-15 ohm rheostat I found at Aircraft Spruce is a 25 watt. The original 60 ohm rheostat is 10 watt.

Thanks,
Rob

Post by **GAHorn** » Mon Oct 27, 2014 11:49 am

While solid-state light dimmers are more "modern"...and have certainly improved over the years, I have never forgotten the deluge of AD notes warning about their overheating and causing cockpit fires. I am happy to stay with the old steam-driven types with which these airplanes were created.

Here is a good replacement, an Ohmite RHS100E (100 Ohm, 25 watt, 1/4" shaft) : http://www.alliedelec.com/search/produc ... U=70022460

: 70022460_thumb.jpg (1.35 KiB) Viewed 16222 times

If you use the Advanced Search routine in these forums you will find several discussion threads about this. Here is one which provides most of the info:

http://cessna170.org/forums/viewtopic.p ... ite#p16213

Heflin · Post by **Heflin** » Mon Oct 27, 2014 2:12 pm

Thank you, Mr George. I searched but didn't find that discussion.

Post by **GAHorn** » Tue Oct 28, 2014 3:43 am

You're welcome. BTW, the increased wattage rating is reflective of the unit's ability to perform work and handle overloads. The Ohms ratings reflect the range of dimming capability with reference to the total load (no. of lamps) imposed.

Therefore using a 25 watt unit in lieu of a 10 watt unit is not harmful, however the reverse will likely induce failure. However, the use of a 15 ohm unit will not regulate as a dimmer very well when replacing a 60 Ohm unit, as the lamps will likely burn too brightly all the time.

Post by **Karl Towle** » Fri Nov 28, 2014 11:04 pm

A lot of good information has been presented. With regard to rheostats (two terminals in use), these function as variable resistors, and in a dimming application, increasing the resistance value simply means you can add more resistance to the circuit than original, thus making the lights dimmer than they were. However, note that some rheostats designed for dimming had insulated "landing zones" for the wiper, at the end of the variable resistance, which effectively shut the circuit off. When replacing with a simple rheostat, lacking this "off" feature, there will be a very slight amount of current flowing through your lamps any time the master switch is on. Probably not worth worrying about, but it will be there.

It is worth noting here, that increasing the resistance when the component is functioning as a potentiometer (three terminals in use) should NOT be done. In this application, the device has been designed to function as a variable voltage divider, and it's total resistance is part of the design. Change it, and you are altering the design of a circuit.

The wattage rating can always be increased with impunity, so long as the (usually) larger device will physically fit in the space allotted for it. Wattage equals heat, and the larger wattage the device, the greater amount of heat it can safely dissipate.

A post-script should be added to this discussion regarding the potential for solid state dimmers to start fires. There are several ways to design circuits using (solid state) transistors. One way is to use them as an electronically controllable variable resistor in place of a bulky mechanical rheostat. In this type of design, the transistor must dissipate heat in the same way the rheostat did; which means the transistor will have metal mass around it called a heat-sink. When the transistor is called upon to regulate/restrict the current flowing in the circuit, it gets hot, "throwing away" the extra power in the form of heat - which could possibly start a fire in a dust-filled area behind an instrument panel.

However, there is another way to use transistors - that being as solid state "switches" where they are either on, or off. In this mode of operation, the transistor doesn't get hot, and you will not observe any bulky heat-sinks installed. When a solid-state dimmer, is designed around this concept, the transistor switches the load (panel lights) on and off vary rapidly - too rapidly for the eye to notice. Then, the circuitry varies the relative amount of time the transistor is on, verses the time it is off (called duty cycle), controlling the light intensity your eye perceives. When the dimmer is all the way dim, the circuit shuts down so no current is flowing, and when the dimmer is max bright, the transistor is switched on 100 percent of the time.

Cost is a very legitimate reason to shy away from one of these newer type solid state dimmers, but the potential for fire is not. In fact, if you have avionics installed that use gas discharge displays, they will likely have switching power supplies built in, which use switching transistors in the same way just described.

Karl

wingnut · Post by **wingnut** » Fri Nov 28, 2014 11:41 pm

All that I can contribute to this discussion is, WOW, I learned a lot. Things make more sense when they are explained in "Lehmann's" terms

n2582d · Post by **n2582d** » Sun Nov 24, 2024 4:42 am

This rheostat was missing from my project. Anyone with a '52 C-170B happen to have an original knob for this rheostat? If so, does it look like the reddish Bakelite(?) one on the very original '48 Bruce took pictures of or did it have serrations like the C-170B IPC seems to show? The ‘52 Owner’s Manual has a picture of the former style knob. Here's a couple of pictures of the two different style knobs. Both rheostats have the same part number -- 0413126.