The International Cessna^® 170 Association

In my owner's manual it says the overhead cockpit lights are ultraviolet. Does anyone still have any functional ultraviolet bulbs? Can any of you engineers figure a way to put ultraviolet LED bulbs in the fixtures? How about red LEDs? Also, along those lines, I am missing the light that is mounted near the pilot's wing root vent. I thing I have a replacement but when I put it where i think it goes, it blocks the vent. Can someone send me pics of theirs?

Thanks,
Greg

Ultraviolet lighting was a popular fad in the late '40's/early '50's.
Remember that back in those days, Radium was a popular thing to put on watch dials, automobile instruments, aircraft instruments, etc..
The ultraviolet lamps would excite the radium which would give off a greenish glow which was very persistent and "cool". (My 1951 Studebaker Starlight Coupe had them. The inst's would continue to glow for 20 minutes after the lights were turned off.)

Radium is now prohibited on instruments and, in fact, that's one hazard that can be experienced unexpectedly when someone purchases an old instrument on Ebay and then finds out they can't get it serviced by any instrument shop...and now they have a hazardous waste disposal fee/problem. U.S. Customs will also go wild when you try to cross the border and their Gieger counters set off alarms. (Radium was found to be the cause of mouth/throat cancers in watchmakers and watch repairers due to radiation.)

You do not want ultraviolet lamps. If you find them installed, remove/throw them away and install incandescent or LED lamps. You also do not want radium-luminescent instruments. It is illegal to ship them or to repair them unless you are certified to handle radioactive materials.

The original pilot map light was a Grimes "torpedo" unit. I altered mine when I installed LED lamps, but you'll get the idea. Here's some pics of mine, still in it 's original location, although I've substituted the shade, you'll get the idea. It doesn't really interfere with the vent, as you can see. Pics both open-vent and closed-vent.

Awesome! Thanks for the info. I have an extra torpedo light I bought on ebay. Looks like a common ground to chassis install? Only see one wire loose around there. I remember reading your post about LED's in the past, and i'll reread it soon. Did you ever try one of those large single LEDs when you were experimenting?

George, in reference to the LED lighting, can the overhead LED's be dim'd via the rheostat? Mine (old ones) just happen to have been disconnected from the rheostate at some point as they had to be as bright as possible to be of any use at all. Now that I have installed the LED's, which really put out a lot of light, I can see that they need to be on a rheostate if those bulbs can be dim'd.

LEDs can not be dimmed like an incandescent bulb. LEDs have a forward voltage that must be applied in order to get any light less than that voltage the go out. LEDs have a maximum voltage with is usually not far above the forward voltage. Adjusting the voltage through this very small range with a rheostat can produce some dimming effect but you would be lucky if it is useful and you would have to experiment to get just the right LED. An at that they will brighten and dim as your system voltage increases and decreases. LEDs can also be over driven which means more voltage than they are rated for. Overdriving an LED can extend the dimming range effect a bit of a rheostat but it will shorten the life of the LED significantly and to much will burn them out in a snap.

The LEDs I recommended in my earlier article work very nicely with the original rheostat and dim perfectly (and the supplier also stated they would behave nicely in a rheostat dimming cirucuit.)

I never tried a "single" LED lamp. The lamps I've used/tested were all multiple units.

gahorn wrote:The LEDs I recommended in my earlier article work very nicely with the original rheostat and dim perfectly (and the supplier also stated they would behave nicely in a rheostat dimming cirucuit.)

I never tried a "single" LED lamp. The lamps I've used/tested were all multiple units.

Well there you go. Actual experience shows that technically we don't always know what we are talking about. Bumble Bees can't fly either.

I am technically correct. Of course not taking into account any circuitry or the effect multiple LEDs would have on the voltage spread over several LEDs.

I'd like to remind everyone that many of these rheostats have been changed out to something with other values than stock. Am somewhere in production Cessna also added a resister to the rheostat (I think '53) when they either changed the amount of bulbs or the actual bulb called for in the system.

The point is, just because it is working fine for George, doesn't mean it will for you unless your system matches his extremely close.

Bruce Fenstermacher wrote:...The point is, just because it is working fine for George, doesn't mean it will for you unless your system matches his extremely close.

My system does not use a resistor, it's merely a rheostat in-series with the inst. lights. All a resistor does is add value to a low-capacity rheostat.
In answer to the question posed,

170C wrote:George, in reference to the LED lighting, can the overhead LED's be dim'd via the rheostat? ...

...the answer is Yes.

Bruce Fenstermacher wrote:LEDs can not be dimmed like an incandescent bulb. LEDs have a forward voltage that must be applied in order to get any light less than that voltage the go out. LEDs have a maximum voltage with is usually not far above the forward voltage. Adjusting the voltage through this very small range with a rheostat can produce some dimming effect but you would be lucky if it is useful and you would have to experiment to get just the right LED. An at that they will brighten and dim as your system voltage increases and decreases. LEDs can also be over driven which means more voltage than they are rated for. Overdriving an LED can extend the dimming range effect a bit of a rheostat but it will shorten the life of the LED significantly and to much will burn them out in a snap.

That forward voltage on the actual LED itself is only about 0.7 to 1.0 volts, and is fairly constant regardless of current through the LED. The current is actually what controls the brightness. The vast majority of LED's have built-in dropping resistors sized for the desired brightness and intended system voltage. How well the rheostat in the airplane would work would depend on the relative resistance values of the rheostat and the LED's dropping resistor, as well as how the LED brightness varies with current. I suspect it would work after a fashion, but the LED probably won't dim at the same rate as incandescent bulbs on the same rheostat.