dimmer rheostat

zero.one.victor · Post by **zero.one.victor** » Wed Dec 08, 2004 1:20 am

The dimmer rheostat in my airplane doesn't work too well, it pretty much has 3 effective lighting intensities plus totally off. I can't make out the mfr's markings, but it doesn't appear to be the original unit, it is mounted differently plus has only 2 (not 3) connections on the back. I took it apart, although the design doesn't really lean itself to doing so- if ya did it more than twice you'd probably break off the little tabs that bend over to hold everything together. Cleaning up the carbon brush and the winding it rides on improved things a little but it's far from satisfactory.
Has anyone installed the rheostat that Spruce sells? Does it work OK? It's an Ohmite, 25W 8 ohms rating, Spruce p/n 0144, $26.50. It sounds like that's the same one there's been some discussion about -- using epoxy on the windings to provide a true "off" position, etc.

Eric

dacker · Post by **dacker** » Wed Dec 08, 2004 2:43 am

Eric, I bought one from Spruce back when I was redoing my panel and it was huge! I only remember Spruce carrying one at that time. I don't have it available to look at the rating to see if it is the same or not. I'm usually a stickler for replacing aircraft parts with aircraft parts, but in this case you might just stop by Radio Shack and see if they carry anything similar (or identical).

David

DensityDog · Post by **DensityDog** » Wed Dec 08, 2004 3:04 am

Hi Eric yes I have installed one of those ceramic boat anchors in the 170 and it works fine.
Max

blueldr · Post by **blueldr** » Wed Dec 08, 2004 4:51 am

Put in a querry on that rheostat to George Horn. He seems to really be up on that stuff.-----among many other things.
Havent seen many posts of his lately. Maybe he is working , or some other terrible fate has overtaken him.

Dave Clark · Post by **Dave Clark** » Wed Dec 08, 2004 12:54 pm

I really like the electronic ones although they're a lot more expensive.

You have to be careful to size the ceramic ones correctly. To do a lot of lights it takes a huge one and there is no protection on them so they have bare windings exposed and subject to shorting on something. Also you want to modify them so that they turn fully off when at that extreme of the range. They also give off a lot of heat.

N1478D · Post by **N1478D** » Wed Dec 08, 2004 2:59 pm

blueldr wrote:Put in a querry on that rheostat to George Horn. He seems to really be up on that stuff.-----among many other things.
Havent seen many posts of his lately. Maybe he is working , or some other terrible fate has overtaken him.

George is OK! He's home in the kitchen. The computer is down the hall in the office, it will take him some time to get there, probably tomorrow he will be able to reply. He's having some green carpet put down in the hall next year to help speed things up.

zero.one.victor · Post by **zero.one.victor** » Wed Dec 08, 2004 5:39 pm

Two questions:
1) it sounds like the windings are exposed on the rheostat? The one I have now is totally encased-- takes some monkeying around to open it up for tinkering, but no electrical guts are exposed.
2) does the ohmite rheostat from Spruce have solder connections, or screw terminals? My current one has screw terminals, with 5 different wires connecting to the downstream side, so I'd have to do some fancy solder work if the new one didn't have screw terminals.
Spruce catalog sez 1-9/16" by 2-1/4" long. That sounds about right for the diameter, but a bit longer in length (depth) if that includes the adjusting shaft. If that doesn't include the shaft, it's a whole lot longer. I think I'll take a run by RadioShack Avionics or Napa Airmotive & see what they have on hand.

Eric

N2540V · Post by **N2540V** » Thu Dec 09, 2004 3:36 am

Eric,
The one I ordered through Spruce was exposed windings and solder connectors. Jim

n2582d · Post by **n2582d** » Fri Dec 10, 2004 6:27 am

Aircraft Spruce has several options for dimmer controls--some are FAA approved, others are not. See http://www.aircraftspruce.com/menus/el/ ... stems.html

zero.one.victor · Post by **zero.one.victor** » Fri Dec 10, 2004 7:31 am

Any of them other than the rheostat are more bucks, plus an approval issue. Even if the rheostat's not an exact replacement, it's close enough to avoid complications.

Eric

Post by **GAHorn** » Sat Dec 11, 2004 3:42 pm

Common electrical hardware (such as quality switches, rheostats, etc.) are not the problem with the FAA as they once were, provided they are of equal or better quality. (In other words, the FAA has relaxed it's position with regard to PMA's on common electrical hardware somewhat. This does not mean you can install Radio Shack stuff....but you can install items otherwise acceptable/common to the aviation industry as replacement parts.)
The original rheostat: The Cessna 170B IPC Fig 42A, pages 75A/75B lists the part as (no part number) an Ohmite, 60 ohm/10 watt unit. These are virtually unavailable. [Edit: but here's one that's close, a 75 ohm, 12.5 watt unit: http://www.alliedelec.com/search/produc ... U=70022458 however keep reading...because there's a better choice if you ever wish to add additional lights...]
A very similar unit now mfr'd is an Ohmite (Model J) AN3551-50-5 (50 watt 5 ohm) unit . It is an unshielded (resistance wires uncovered) ceramic unit.
My own airplane: Being a cheapskate as well as a genuine short-cut
artist, I simply replaced what was there with another Ohmite Model H, 100
Ohm /25 watt unit. It is identical to what came out (it's huge, about 1.5" dia. X 1.35" depth behind the panel, with 1/4" shaft) and works fine (although it only requires about 1/2 turn to go from full dim to full
bright. Of course, it's overkill for this simple circuit.) It cost me $1.50
from the local electronics surplus dealer, new in the box.
So, you might look around your local area.
However if unsuccessful: http://www.alliedelec.com/cart/partlookup.asp has the Ohmite RHS-100 (the one I have and which I recommend for this purpose) for $23 each and will sell only one if you want it.

EDIT: NEW LINK to rheostat: http://www.alliedelec.com/search/produc ... U=70022460

zero.one.victor · Post by **zero.one.victor** » Sat Dec 11, 2004 4:41 pm

Replacing the stock rheostat with a similar although slightly different rheostat is not a problem. I think replacing it with a fancy modern solid-state electronic wonder-gizmo would be, without supporting paperwork. At least from the FAA's point of view. That was my point.

Eric

zero.one.victor · Post by **zero.one.victor** » Wed Dec 15, 2004 2:57 am

I replaced the dimmer (rheostat) today. I used a Spruce p/n 0144 rheostat, which happens to be an Ohmite model H- 8RO. It was rigged for solder connections. Since my wiring was already set up for screw-type connectors, I didn't want to do the solder thing. The connector ears on the rheostat have a hole for sticking the wire thru when you solder it, I tapped these holes for a #4-40 machine screw. The old rheostat connectors used #8 screws, so I had to use little washers but it seemed to work OK. The ears aren't real thick so I'm a little concerned that the threaded holes might eventually strip out. Guess I'll burn that bridge when I cross it, for now they're just fine.
Thanks for the rheostat, Density Dog!

Eric

blueldr · Post by **blueldr** » Wed Dec 15, 2004 4:06 am

This is an item to be particularly cautious about. I have it on good authority that the FUZZ (Read FAA) has been training a new batch of inspectors to check instrument light dimmers for properly authorized part numbers on fifty year old airplanes.

blueldr · Post by **blueldr** » Wed Dec 15, 2004 4:17 am

I've found that Ford P/U light switch is just right. First notch turns on the nav lights and strobes, second notch brings on the landing and taxi lights, rotate the knob clockwise to brighten the instruments and all the way to the right lights the dome light! works like gangbusters!! --- and you can get 'em at Pep Boys.

The International Cessna^® 170 Association

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