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Thread: Alternator warning light on 912 Rotax

  1. #1
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    Default Alternator warning light on 912 Rotax

    First off, my least favorite thing is wiring, most likely cause it seems a bit like black magic. Or something like that...Anyway, I'm wiring up my Kitfox 4 rebuild project, and I want to just have a in panel light that goes off once the engine starts and the alternator starts charging. I have that on my Avid Mk IV with the Jabiru and it works for me.. In the Rotax installation manual, it says use a 3 watt lamp. All I can find it seems are LED lights that draw far, far less then that. Does anyone know if it will make any difference using a much smaller wattage light? Thanks, JImChuk

    PS, here is the panel so far. Leaving the upper right side open for a 8" tablet running Avare GPS when needed or wanted. Still have a couple of holes to cut. Forgot the ELT remote for one thing... :-()
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    Super Moderator Av8r3400's Avatar
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    Default Re: Alternator warning light on 912 Rotax

    I'd say you want to either find a 3 watt bulb or add a resistor to the circuit with the LED to get the resistance the same as the 3 watt bulb.
    Av8r3400
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    Default Re: Alternator warning light on 912 Rotax

    Thanks Larry, that then brings up the next question. What size resister would it take to get up to the 3 watts. I believe the LED light uses 0.2 watt. Like I said, electronics is not my strong suit... :-) JImChuk

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    Senior Member jrevens's Avatar
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    Default Re: Alternator warning light on 912 Rotax

    Jim,

    With conventional automotive alternators, something like a 3 watt load is necessary for that light to work. I don't know about the Rotax specifically, but an LED should be able to be used, as Larry said. Here is one drawing that I had, and I've attached a .pdf file with another. I hope one or the other will help you. Anyway, you may be able to use this information to fabricate a simple circuit for use with the Rotax. Here you go -

    LR3_LV_Led_1.jpg

    I like your panel. Looks like you're using my favorite simple TruTrak autopilot. I've got one just like it. I wish they still made them.
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    John Evens
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    Default Re: Alternator warning light on 912 Rotax

    Hi John, Thanks for the drawings. I sent you a private message. Thanks, JImChuk

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    Senior Member Delta Whisky's Avatar
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    Default Re: Alternator warning light on 912 Rotax

    Jim - the light is grounded by the regulator-rectifier (not powered) upon fault detection. Any 12v DC light will work just fine. It can't be more than 3W because the regulator isn't designed to handle more than that. Should you buy a 12V LED, it will have the necessary voltage drop down resistor in it. I used this (539-FL1M8SW-1-R12V) from Mouser.

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    Default Re: Alternator warning light on 912 Rotax

    Thanks everyone, I think I have it figured out now. JImChuk

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    Default Re: Alternator warning light on 912 Rotax

    A note here from an electrical engineer. The LED itself only draws maybe 20 milliamps. 20mA x 12 volts is .24 watt. In the above circuit, there is a secondary current path to dissipate through that parallel resistor. You would need a 48 ohm resistor to draw 3 watts. Now, here's the important part. 3 watts as a fair amount of heat so you want to get a resistor that can be mounted to a heat sink. I would think that the firewall should be fine.

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    Super Moderator desertdave's Avatar
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    Default Re: Alternator warning light on 912 Rotax

    Reviving this thread. I am joining the 21st century and installing EFIS. I would love to match the alternator warning light with my EarthX battery light. Will this suffice?
    https://earthxbatteries.com/product/...ult-indicator/

    12V LED Indicator Light

    $29.99
    Specifications:

    • Color: Amber
    • Mounting: Nut and Washer (included)
    • Termination style: Wire 20ga- 48″ length
    • Voltage: 12V
    • Built in resistor: 500 ohm, 1/2 Watt
    • Drill bit size: .325 or 5/16″



    12V-Red-LED.jpg
    Dave
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    Administrator DesertFox4's Avatar
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    Default Re: Alternator warning light on 912 Rotax

    Dave, I have maybe four of those L.E.D. caution lights at my hangar. You can have one or a couple if needed. I have different colors too. Green, red and yellow I believe.
    Will bring them down next week when I visit Deer Valley.


    DesertFox4
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