[4kilo]

My instrument panel lights are on a dimmer circuit which produces excellent results except for my TruTrak ADI pilot, which is much brighter than the rest of the lights. My first guess is to add a resistor in the wire going to the ADI light. Any thoughts?

Pat

 

[chrispratt]

My instrument panel lights are on a dimmer circuit which produces excellent results except for my TruTrak ADI pilot, which is much brighter than the rest of the lights. My first guess is to add a resistor in the wire going to the ADI light. Any thoughts?

Pat

Pat:

That's what I did. I'll have to pull out my logbook when I get home to remember how much I used but I think it was 15000 ohms.

Chris

 

[lucky333]

Works for me, too

An easy way to do it is to rustle up a 2K-5K potentiometer (15000 sounds a little much to me..) and wire it in series with the light, one wire on the wiper (center) and one on either of the other pins. Turn on the lights and crank the pot around until you like it.

Leave the pot where it is, turn the lights up full blast and measure the voltage between the pot's terminals. Turn off, disconnect the pot and measure the resistance. That's your resistor value in ohms.

The wattage rating is figured by (Volts*Volts)/ ohms. You should increase (double?) that to get a safe resistor power rating.

If you solder it into the harness, don't heatshrink over the body of the resistor itself unless you have very low power dissipation (LEDs)

Have fun.

 

[Harold Lanfear]

John; your'e a genius

Great idea to use a pot to get the exact brightness. Doing the ohm calculating thing certainly works, but you don't get to 'see' if its exactly what you want.

 

[Rick_A]

simple alternative

The ADI has 2 pins for the light, one is for 12V and the other for 24V. I just used the 24V input which cut the brightness. All you have to do is move one pin on the d-sub. If that doesn't do the trick then you can go with one of the other solutions previously posted.

BTW. I have a dimmer for the nu-lites that I used on my other instruments.

 

[647jc]

Dimming the ADI with a series resistor may be problematic

I don?t own a TruTrak ADI but judging from the dimming signal description for the ADI on the TruTrak web site and the fact that Chris ended up using a high value resistor to get his unit to dim, I would suspect that the dimming signal going to the ADI does not directly provide voltage or power to illuminate the device. I suspect the ADI uses an ADC (analog to digital converter) to ?sense? or measure the panel lamp voltage and then electronically adjusts its display brightness accordingly. Typically an ADC has a pretty high input impedance and to actually drop the panel lamp voltage going to the unit via a series resistance would require a pretty high resistance (like Chris empirically determined). This configuration MAY cause the ADI illumination or brightness to become sensitive to noise or induced voltage on the dimming signal line. For example, the ADI brightness may actually change when you key your mic. or perhaps turn on your aux. fuel pump. You may also see your ADI flash from full brightness to full dim when the panel lamps are off and a noise spike occurs.

In my opinion, a slightly different but better approach would be to use a 2 to 5 k ohm pot as John suggested but to ground the loose end of the pot to establish a ground reference other than through the ADI high impedance ADC circuit. Once you get the brightness adjusted to what you like you can then replace the pot with fixed value resistors as John suggested however, you will be replacing the pot with two resistors instead of the one.

 

[Randy]

Sorry this is off topic, but could the potiometer technique also be used to calibrate Van's fuel gauges? Other than re bending the wires I find no way to calibrate them and have thought about some form of variable resistor to get them reading accuratley.

Randy

 

[647jc]

Randy,

Assuming you installed the standard Vans resistive senders and assuming they were installed so that the fuel gauge reads higher than actual, that is, the tank becomes empty before the gauge reads empty, you could fudge the meter to read lower by adding series resistance to the circuit. The typical Vans resistive sender measures 30 ohms when at the top of its swing (full tank) and 250 ohms when at the bottom (empty). The lower the meter reading the higher the sender resistance therefore you can make the reading appear lower by adding more resistance. You would probably want a very low resistance pot, 50 ohms perhaps and that might be difficult to find. Also keep in mind that adding the resistance may affect your full reading. Typically Vans senders and gauges are not very accurate at the full end and they typically read full before the tank is actually full so the effect on the full reading may not be an issue. Not sure what power rating the pot should be (depends on the constant current source Vans fuel meter provides and I don’t have one to measure) but I would think 1/2 watt would be sufficient.

Warning: If for some reason the pot you added changed value (became lower resistance) in flight, vibration, temperature, humidity, etc. your fuel gauge reading could be off (read high) by up to the amount you used the pot to compensate and you could run out of fuel before the gauge read empty, just something to be aware of.

 

[chrispratt]

... the fact that Chris ended up using a high value resistor to get his unit to dim, I would suspect that the dimming signal going to the ADI does not directly provide voltage or power to illuminate the device. ... Typically an ADC has a pretty high input impedance and to actually drop the panel lamp voltage going to the unit via a series resistance would require a pretty high resistance (like Chris empirically determined).....

Joe:

Thanks for the input. I wish I understood all this electrical stuff but you've given me some things to ponder...and to monitor in regard to my ADI installation.

FWIW I have not seen any ill effects to date regarding electrical noise or interference with the operation of the unit. However, one interesting item occurs during daytime operation. I have the ADI backlight controlled by my Nav light switch, i.e. it's on when my nav lights are on. If I run my nav lights during the day, the ADI backlight intensity is so low (indicators dimmed) that it's hard to tell if the autopilot is on or not. Since I fly very little at night, I'm considering removing the resistor just so I don't inadvertently have the ADI engaged when I don't want it to be. That's a pretty unlikely scenario, I know, but it only takes one mistake to become part of aviation history.

Thanks again.

Chris

 

[4kilo]

The ADI has 2 pins for the light, one is for 12V and the other for 24V. I just used the 24V input which cut the brightness. All you have to do is move one pin on the d-sub. If that doesn't do the trick then you can go with one of the other solutions previously posted.

Rick,

I moved the pin for the light wire today, and so far it seems to have worked. At least the light intensity seems more in keeping with the rest of the instrument lights.

I am not going to pronounce it a complete success until I have actually flown the airplane at night and seen the light levels in flight.

Pat