The International Cessna^® 170 Association

Hi guys new member here with a quick question...I have a 56 172, somewhere along the line the rheostat was moved from underneath to panel mount. The thing is it dims the panel opposite of what you think( left brighter,right dimmer) anyway I notice 3 post with 2 wires soldered on. Is it a matter of 2 wires being crossed? Any thoughts...Thanks, Pete

Yes sort of.

A Rheostat consists of a coil of resistance wire that runs in a coil around in a circle with a pin at each end. There is a wiper that makes contact with this coil and moves around it as you move the knob. Moving the knob and the wiper shortens or lengthens the resistance in play and brightens or dims the lights.

Moving the wire attached to the end pin on the rheostat to the other end pin will swap the direction the knob and wiper must be moved to effect a given change. Just swapping the wires from their current positions to the other will have no effect

And welcome to the forums. FYI answers to 172 questions cost more.

pmell22, does your 172 still have two of the rheostat's--one for the panel lighting and one for the radios? Mine only had one when I purchased it so all panel & radio/gps, etc are tied together.

PS: If, as Bruce says, "172 answers cost more than 170 answers" I am in deep dodo as many questions as I have asked (and will likely continue to ask)

Also-----Welcome

Aryana wrote:Hi Pete. Welcome to the association! I would think that logic works, but someone here will verify I'm sure.

Hi Aryana...thanks for the overlay again it worked out great. I will post some pictures when I fiqure out how to do it...

170C wrote:pmell22, does your 172 still have two of the rheostat's--one for the panel lighting and one for the radios? Mine only had one when I purchased it so all panel & radio/gps, etc are tied together.

PS: If, as Bruce says, "172 answers cost more than 170 answers" I am in deep dodo as many questions as I have asked (and will likely continue to ask)

Also-----Welcome

Thanks for the welcome...just one only tied into the panel...funny about the xtra money for a 172...

Bruce Fenstermacher wrote:Yes sort of.

A Rheostat consists of a coil of resistance wire that runs in a coil around in a circle with a pin at each end. There is a wiper that makes contact with this coil and moves around it as you move the knob. Moving the knob and the wiper shortens or lengthens the resistance in play and brightens or dims the lights.

Moving the wire attached to the end pin on the rheostat to the other end pin will swap the direction the knob and wiper must be moved to effect a given change. Just swapping the wires from their current positions to the other will have no effect

And welcome to the forums. FYI answers to 172 questions cost more.

Hi Bruce,
I noticed 3 posts on the rheostat and 2 individual wires on the last 2 individual posts...so? When you say yes sort of, is there anything I can do or does the rheostat need to be replaced?? BTW how much more cash for 172 questions...lol

In the picture below of a rheostat tyoical of what yours should look like. Power can be attached to either the center pin or one of the outside pins. You can also put power to an outside pin and the feed to the lights would then come off the center. The depending on which outside pin you connect to in either case, will determine the knob direction to turn on and increase power.

You will have only one wire from the power source. You could have two or more wires leaving the rheostat to the lights.

You can find out which wire is the feed wire by attaching a test lamp between ground and on of the pins with a wire. If the light goes full bright and the knob has no effect, you have found the power wire. If the lamp brightness can be adjusted then the power wire is the one attached to the other pin. Caution. It should not hurt to attach a test light for a few seconds. But don't leave it there turned on and walk away as the extra bulb could overload the rheostat and it will get hot.
I should also caution. I'm talking about a dimmer system found in a typical 50s airplane. Not any kind of a more modern system where the rheostat controls a dimmer circuit.

Bruce Fenstermacher wrote:In the picture below of a rheostat tyoical of what yours should look like. Power can be attached to either the center pin or one of the outside pins. You can also put power to an outside pin and the feed to the lights would then come off the center. The depending on which outside pin you connect to in either case, will determine the knob direction to turn on and increase power.

You will have only one wire from the power source. You could have two or more wires leaving the rheostat to the lights.

You can find out which wire is the feed wire by attaching a test lamp between ground and on of the pins with a wire. If the light goes full bright and the knob has no effect, you have found the power wire. If the lamp brightness can be adjusted then the power wire is the one attached to the other pin. Caution. It should not hurt to attach a test light for a few seconds. But don't leave it there turned on and walk away as the extra bulb could overload the rheostat and it will get hot.

imgres.jpg

I should also caution. I'm talking about a dimmer system found in a typical 50s airplane. Not any kind of a more modern system where the rheostat controls a dimmer circuit.

WOW thanks Bruce, now thats what I call an explanation...thanks for taking the time to make it clear...

Actually rereading it I left out a few things I intended to put in.

If the power goes to the center pin then the output comes off one of the outside pins. Which outside pin to use is determined by which way you want the knob to turn to brighten the lights. You could also have the power going to one of the outside pins and then the lead(s) to the lights would come off the middle pin. Again which outside pin is used is determined by which way you want the knob to turn to brighten the lights. In no normal circumstance would you have two power wires coming to the rheostat. In no normal circumstance would all three pins be used on the rheostat.

Bruce Fenstermacher wrote:Actually rereading it I left out a few things I intended to put in.

If the power goes to the center pin then the output comes off one of the outside pins. Which outside pin to use is determined by which way you want the knob to turn to brighten the lights. You could also have the power going to one of the outside pins and then the lead(s) to the lights would come off the middle pin. Again which outside pin is used is determined by which way you want the knob to turn to brighten the lights. In no normal circumstance would you have two power wires coming to the rheostat. In no normal circumstance would all three pins be used on the rheostat.

Thanks again!

hmmm... well this has me thinking. My EI instruments are wired as recommended by the factory so as to have the peripheral LED's on the rheostat and the center LCD light always at full bright. This setup doesn't work well because the center light is way to bright at night and it bleeds into the LED lights rendering them indistinguishable. The problem is that the outer lights dim as voltage is increased, however the center light will get brighter as voltage is increased requiring either a second rheostat or..... just use the other tab on my current one. I'll have to look into it.

Richard that would work only if the power was feed to the center pin, you probably already know that. Of course you have an exception to the normal setup. If it were me I'd still mount a separate rheostat so you could customize the amount of light from each rather than being locked into the opposite resistance of a single rheostat.

Bruce Fenstermacher wrote:...In no normal circumstance would you have two power wires coming to the rheostat. In no normal circumstance would all three pins be used on the rheostat.

Bruce, your explanation is excellent, but this last comment may not be*.
It's possible to have two wires attached to the same pin...for example, in his 172, if one of the two wires goes to the panel lights,...and the other goes to the avionics. This might be how the person who modified this airplane accomplished both circuits being controlled by one rheostat. (Per Frank's comment, there used to be two rheostats...one for each circuit.)

*(Your comment indeed uses the words "coming to the rheostat". If by that you mean the wires which supply current to the rheostat your comment is not absolutely wrong. But it's still possible, for example, if the power-lead brings power to the rheostat and then carries it onto other circuits in a series-arrangement.)

Yes George I meant to say ordinarily power would be supplied to the rheostat by one wire. As you point out there could be a second wire from that post taking power to something else not controlled by the rheostat but on the same breaker circuit.

And I forgot to mention answers covering a 172 cost twice what answers for a 170 cost.

Bruce Fenstermacher wrote:...
And I forgot to mention answers covering a 172 cost twice what answers for a 170 cost.

Maybe Frank should purchase TWO annual TIC170A Memberships!