Avionics Dimming Help

[lr172]

I have several devices that require input from a dimmer bus/circuit (older avionics + gear like ADI and intercom). I also want to dim my indicator lights, which all bring the circuit low (i.e. to ground)

My transponder KLN-90 says it draws .5A on the lighting bus, so I have a dimmer load of about 750 mA.

My plan for the dimmer bus was to run 12v through a 3P switch. One pole will be straight through. The other two will get a 150 and 220 ohm resistor (12 watts). I have tested this with the indicator lights and it works well. I am wondering how this will work for the avionics. I am looking to avoid a rheostat, as they are expensive in the Wattage level that I need.

Does this plan work? Please help me find the holes in this plan. I have not yet hooked up the avionics, so haven't been able to test yet.

Thanks,

Larry

[RVDan]

A 2N3055 NPN transistor mounted to a heat sink with a Pot control works very well. I think there is a diagram on Aeroelectric. If not let me know and I' ll give you one.

[humptybump]

Originally Posted by RVDan

A 2N3055 NPN transistor mounted to a heat sink with a Pot control works very well.

That's pretty much what my RV-8 had when I bought it (a simple commercial version). I swapped a fixed resistor for the pot and used an ON-ON-ON switch to give me off-low-high. It works very well for both my cockpit "eye ball" lights and the avionics scree and button light bus(es).

[lr172]

Thanks for the idea. I looked for an aeroelectric drawing, but it used an LM317. This idea seems to work better than the large resistors. Do you believe that the setup will accommodate 1A of current?

Larry

[BobTurner]

1 amp will depend on how good of a heat sink you provide (the device will automatically shut down if it gets too hot), but should work with a decent sink.

[lr172]

Originally Posted by humptybump

That's pretty much what my RV-8 had when I bought it (a simple commercial version). I swapped a fixed resistor for the pot and used an ON-ON-ON switch to give me off-low-high. It works very well for both my cockpit "eye ball" lights and the avionics scree and button light bus(es).

What wattage resistor did you use for this?

Larry

[humptybump]

Hi Larry, I have an "experimenter's kit" of 1% 1/4 watt resistors. I took an old pot and dialed until I had the desired voltage. Then a found the match in the kit. Sorry I can't be more specific.

[WingsOnWheels]

To dim my old KT-76 and KY-97 I couldn't use a PWM (I tried), so I used a 3-amp variable voltage regulator. I am not drawing anywhere near that much current, but wanted the margin. Cost was a couple of bucks. So far it is working just great. I have a separate PWM dimmers for the LED glareshield and cockpit floodlights.

The voltage reg in on the right, the black boxes are the PWMs I made.



Front panel for my dimmers: