Rheostat/dimmer switch.

[Greg Bockelman]

I am looking for a dimmer switch for my panel lights that has an off position. Currently, the panel lights are on the nav light circuit. Since nav lights should pretty much be on all the time, I would rather the panel lights be turned off when not needed. My current solution is to put the panel lights on a separate circuit with its own switch. But I would rather not add an additional switch to my panel. Anyone have a solution?

 

[Bell206]

But I would rather not add an additional switch to my panel. Anyone have a solution?

E/AB or TC aircraft? Does the current switch provide dimming for the instrument lights? Schematic handy to post?

 

[Greg Bockelman]

TC. Yes, dims instrument lights. Let’s see if I can add a picture.

 

[Greg Bockelman]

 

[Greg Bockelman]

Dome light, position lights and panel lights all on the same circuit in that image.

 

[Dan Thomas]

That's a very old system. Generator and all, and the instrument light dimmer is one of those ancient rheostats that get so hot and cost a fortune now if you can find one.

If it works, just dimming the lights all the way should increase their life enormously. Do they even glow at all at the lowest setting? Check when it's really dark.

Why should position lights be on all the time? They sure don't act as anti-collision lights during the day. Far too weak for that. The landing light can help, but not the position lights.

You could put the Maxdim in it. It shuts the lights off at full low setting. It's approved for a lot of airplanes. But read the reviews on the Aircraft Spruce site first.

 

[Bell206]

Anyone have a solution?

I take it your panel is "factory"? Do you think the dimmer rheo is original? You basically have two options: change the position light switch type with some additions or change the dimmer rheo. Both I think it should stay on the minor alt side of the equation. Each option would require some fitment requirements. However all parts should be procurable from a local electronics supplier. Do you plan to sign off this or will you get someone else as this may be a hurdle to cross.

 

[Greg Bockelman]

I cannot sign anything off on my own. I have someone that I am working with. But I am still looking for a rheostat that has an off position.

 

[Jim_R]

The Maxdim doesn't have a click-detent "OFF" position, but in it's full CCW position it outputs 0V so is essentially OFF. I have one installed, not for the instrument lighting but for the avionics stack backlights, which didn't like the output of the Piper panel lighting rheostat.

The friction in the Maxdim knob is enough to keep it securely set to wherever you dial it, including full CCW.

 

[Greg Bockelman]

Sounds like I am trying to make an issue out of nothing. I will be using LED bulbs so heat and current draw shouldn’t be much of an issue.

But the one thing I forgot to consider is the backlighting on the radios. I’m going to have to figure that out.

 

[Bell206]

But I am still looking for a rheostat that has an off position.

You'll need to calculate the power ratings for your dimmer switch but there are dozens of options, to include ones with an OFF detent, from an electrical supply vender. The hardest part is to find one that will fit in the same "footprint" as your original one. If that doesn't work you can also replace the SPST position light switch with an "on-off-on" switch and split the power circuits at the switch. You'll need to add a diode or relay to reconnect both circuits when the "both" side of the switch is selected to prevent backfeed. This keeps the original dimmer in place. Or you can simply add a second SPST switch for one of the circuits but you stated that was not a preference however it is the easiest upgrade.

I will be using LED bulbs

FYI: If you're upgrading to LEDs and keep your original dimmer just make sure the dimmer will give the LEDs the power they need.

 

[Dan Thomas]

If you're using LEDs, there are two problems. First, their current draw is much smaller than incandescents, so Ohm's Law messes you up. That old rheostat varies the brightness of the incandescents by reducing their current. The rheostat does that by producing a voltage drop across itself. With the smaller current of the LEDs, the voltage drop also decreases a whole bunch, and the lights won't dim much at all.

E=IXR. Voltage drop is equal to current times resistance. Low current multiplies with the existing resistance to produce a small voltage drop.

The other problem is that many LEDs resent being dimmed, so you might need an electronic dimmer for them. Check with the folks that sell the LED lights.

 

[SoCal 182 Driver]

If you're using LEDs, then take a look at Superior Panel Technology model SPTPWMLEDS, pictured below. It has a detent for on/off, and works great. The one in the pictures below is for sale. PM me if you're interested.

 

[Ed Haywood]

Rheostat won't dim LEDs. LEDs cut out at a certain voltage, so you need a PWM. PWMs typically come with an on/off detent. Spruce sells several.

If you were sticking with incandescent, just wire a mini toggle switch in front of the rheostat and be done with it.

 

[SoCal 182 Driver]

Rheostat won't dim LEDs. LEDs cut out at a certain voltage, so you need a PWM. PWMs typically come with an on/off detent. Spruce sells several.

Note that the dimmer switch pictured above is a PWM (pulse width modulation) dimmer, and on the label it specifically says for LED and EL use.

 

[Hang 4]

Why do you want the "off" position? I'm pretty sure most of the popular legacy GA aircraft out there have the rotating rheostat with Nav lights and panel dimmer in one switch. Other than the rheostat wearing out, there seems to be no burning reason to change.

 

[mondtster]

The Piper Aztec has a dimmer with a switch built into it similar to what you’re describing. You might look at what is used there.

 

[NordicDave]

@Greg Bockelman The schematic you posted is helpful and indeed shows all lighting on one breaker, with the dome light on a switch and the instrument light on a dimmer.

Minor modification with log book entry by A/P to put a toggle switch on the instrument lights. Same with adding an appropriately rated switched dimmer, minor modification.

It would also be a minor modification with log book entry to put the instrument lights on a separate breaker. I'd feel safer with inside and outside lights on different breakers so the whole system doesn't go off-line with a fault.