170A panel light wiring

[LBPilot82]

Bruce, when I got my plane (early A), the instrument lighting was routed through the nav light switch before it went to the rheostat. The rheostat does not have an off position but does provide enough resistance to make the bulbs appear off. A double pole nav light switch is used to isolate the panel light circuit from the nav light circuit.

[canav8]

Does anyone have a suitable replacement resource for the rehostat? thanks, mine works but it is either on or full bright. not really adjustable. Doug

[GAHorn]

I'll bet you didn't do a "search" for "rheostat".

http://www.cessna170.org/forums/viewtop ... ied#p48090

[canav8]

Oh Yes I did, oh great one. Links are broke though. LOL

[GAHorn]

Oh Yes I did, oh great one. Links are broke though. LOL

Here's the "repaired" link: http://www.alliedelec.com/Search/Search ... tk=Primary

[oz502man]

G'day all...

Bruce,
Last annual I rewired my '51 A model. I found the same thing - the rheostat is powered directly from the master (fused of course). I recall suggestions in other posts when I searched( it may have been George if I recall) to put a drop of epoxy on the 'dim' end of the rheostat to create a dead spot. IMO this is the minimum that should be done. I actually moved the rheostat up to the panel where there was a spare knob, and wired power through the spare switch (second from the rhs i think). I probably went over the top with this, but the hole was already in the panel (the spare knob was missing) and there was already a spare switch. I like the idea of the lights being on a separate independent switchable circuit. I also have a 'soderberg' (sp?) flood light on a separate circuit. The reason some aircraft inst. lights dim completely and some don't can be a function of the total circuit resistance i.e. the number and or wattage of the globes. If you have some blown globes or are using globes of lower wattage this places less load on the rheostat and therefore less voltage drop. Think about electricity as water... a trickle will run just as freely through a half inch garden hose as it will a 2" hose, but hook each one to a hydrant and the greater load will cause more resistance in the garden hose and therefore a greater pressure drop. Later model Cessna's use a small low low current rheostat to bias a transistor, which in turn drops the voltage to the lights. The load from the transistor remains constant so dimming qualities remain the same regardless of whether there is one globe or twenty provided the capacity of the transistor isn't exceeded.

Just something to think about.

Cheers

Noel

[Bruce Fenstermacher]

Noel as I discovered the rheostat is/was wired direct to the main circuit through a fuse. The on/off switch being the dead spot in the original rheostat. This original design, specially mounted under the panel, is impossible to tell visually whether it is off or not. I personally would like to see a on/off switch that could be visually checked but that is not the way it was originally done.

In my case the original rheostat was changed to a potentiometer. They work the same but are not built the same as a rheostat. Potentiometers probably won't have the OFF spot and commonly have three connections, one being the wiper and the other two each end of the resister coil. Potentiometers in my mind generally are not built to handle the load of a rheostat.

I added the OFF spot to the potentiometer someone had installed in my plane. I used Cyanoacrylate (Super Glue) to build up the spot. On the bench with little load it worked well. However in the plane it was inadvertently set at the highest resistance which created the most heat which quickly started a burning electrical smell. The circuit was quickly disabled by removal of the fuse and I plan to replace it with the correct rheostat with the off position. I would caution anyone adding the off position with any insulating material such as Cyanoacrylate or Epoxy to be aware it has to be able to handle the heat. Perhaps in a few seconds the Cyanoacrylate would have stopped letting of a (probably dangerous) smell but I didn't wait to find out.

[Kyle]

Hi Bruce,

Food for thought... This is how my NuLites looked on the back of the panel - notice the slight green tint,

[attachment=0]P1060531.JPG[/attachment]