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Thread: Wiring and Toggle Switch Choices

  1. #1
    Birdseyeview's Avatar
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    Default Wiring and Toggle Switch Choices

    I have just started the wiring and its obvious to me that the original electrical schematic for my series 6 kit doesn't reflect all the newest thinking in a few areas and it doesn't reflect all of the panel switches I plan to have. My panel will be the standard steam gauges and an iPad mini and several toggle switches, along with a fuse box, so it should be a fairly basic wiring job (I hope). As I read the forum to glean info on the electrical system wiring there are so many customized versions its sometime confusing which items on each circuit are critical and which are not, and which are just plain clever. However, I do have a few basic questions to start me off:

    1) I understand the need for a back EMF diode on the starter relay to protect the start switch contacts (my start switch is a red push button) but do I also need a diode on the master relay? I see there are some on the forum who have a diode on both relays but it seems that the master relay shouldn't see the same current levels as the starter relay, or am I missing something?
    2) I notice that some on the forum have a two pole switch for controlling nav lights and strobes with one switch, with each pole on its own different size fuse (seems like a clever idea). I have an older non-LED Whelen nav light and strobe assemblies with a power supply. I'm assuming that the one two pole switch vs a separate single pole switch for each is to save panel space and eliminate one toggle switch. I'm I assuming correctly or is there some advantage to going with separate single pole switches?
    3) I currently expect to have my instruments that require DC power (turn coordinator, tach this built in hour meter and radio) all on one single pole toggle switch but may consider separate switches and/or multiple pole switch(es) to allow for different fuse sizes, or to take advantage of any other considerations that make sense. Any consensus out there on what my best options are for this one? Although I'm interested in what people have done for this one, I'm actually more interested in why they did it the way they did. So please provide a little justification if you can. Opinions are acceptable if justification is thin.

    Don't feel you need to respond to all 3 questions. I'll take input separately. Thanks in advance for any input
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  2. #2
    Senior Member jrevens's Avatar
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    Default Re: Wiring and Toggle Switch Choices

    Hi Larry,

    This will be a quick, general reply. First off, I would recommend you get a copy of The AeroElectric Connection by Bob Nuckolls. I believe it's available free online if you need info right now. I would try to study it and glean what you can from the appropriate sections. He has an excellent schematic for our Rotax powered electrical systems, IMO, that is a very good starting point, and can be modified to suit your needs of course.

    1. It's smart to use diodes on all of the circuits that control relay coils - that includes the master solenoid. The back EMF is generated in the coil circuit and is independent of the load being switched by the contacts in the relay/contactor.

    2. One reason to use separate switches is that you probably wouldn't want to always power the nav lights and strobes at the same time, for a number of reasons.

    3. Separate switches and fuses are generally preferred. The radio should definitely be on it's own fused circuit. Instruments can generally be powered by a common main fused circuit, with possibly branches to certain specific instruments with smaller wire sizes and inline fuses.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  3. #3
    Senior Member PapuaPilot's Avatar
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    Smile Re: Wiring and Toggle Switch Choices

    Item #2: I would put the nav and strobe lights on separate switches. Yes, they need their own breaker/fuse. At night you must have the nav lights on, but there are situations where you might want to turnt he strobes off (IFR in the clouds, reflections on the ground, blinding other people on the ground). All of the certified planes I know of have these on separate switches.

    Item #3:
    Most of the items you listed here just need to come on with the master switch (when you power up the "main" bus), they don't need individual switches. There isn't a logical reason to have a switch on a turn coordinator or tach, they never need to be off.

    Some airplanes have an Avionics master switch that is typically used to isolate the avionics off during the start. I didn't put one in my plane; my COM, transponder, ADSB-In, AHRS, etc. come on with the master switch. Newer avionics are less susceptible to problems with a power surge.

    Each electrical device typically needs its own wiring and circuit breaker/fuse. The breaker protects the wiring, not the device.

    There are multiple ways (and opinions) on how many, and what type of busses you need to have in a plane. Part of it depends on the complexity of your aircraft's electrical system, number of batteries, number of charging systems and whether you fly VFR/IFR. For IFR you need to keep the essential stuff working for a minimum of 30 minutes in the event of a charging system failure. For a simple day/VFR plane one battery, bus and a single charging system is usually enough. Part of it comes down to personal preference.

    In my plane I chose to use 2 busses; a main and essential. The main bus has the less important items. If my alternator fails I can isolate it to the essential bus which connects directly to the battery. This bus has everything needed to keep the Garmin G3X system alive, the COM radio, transponder, lighting (as required), boost pump (only for emergency use) and elevator trim (intermittent). My essential bus items draw 5-6 amps which means I can continue to fly for ~2 hours on my EarthX 680 battery (12.4 Ah) after an alternator failure. I like the ability to quickly eliminate all the non-essential items and get to the minimum amp draw. To do this I only have to move 2 switches: Essential power emergency switch "ON" and Master switch "OFF".
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  4. #4
    Senior Member Av8r_Sed's Avatar
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    Default Re: Wiring and Toggle Switch Choices

    Only thing I can add to the great advice from John and Phil is: 1) Many relays you might choose for a master may have a built in diode, which simplifies their use. example: https://www.steinair.com/product/relaydiode/
    -- Paul S
    Model III SN910
    582 IVO Med

  5. #5
    Birdseyeview's Avatar
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    Default Re: Wiring and Toggle Switch Choices

    Thanks guys for the good info. This is exactly why the forum is such a great resource and why I especially like the rationale provided.
    I've also been reading about the recommended orientation of the master relay but am confused by the terminology used to describe it. The attached picture is the relay I'm using.
    For my own clarity, is it recommended to be mounted as shown in the picture, with the crimped on end pointing down?
    11-03161.jpg
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  6. #6
    Senior Member Av8r_Sed's Avatar
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    Default Re: Wiring and Toggle Switch Choices

    Yes, manufacturer data sheet: Recommended mounting–– Plunger vertical with cap down
    -- Paul S
    Model III SN910
    582 IVO Med

  7. #7
    Senior Member PapuaPilot's Avatar
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    Default Re: Wiring and Toggle Switch Choices

    Quote Originally Posted by Av8r_Sed View Post
    Yes, manufacturer data sheet: Recommended mounting–– Plunger vertical with cap down
    I believe the reason for this is if the contactor is mounted the "upside down" it could open in flight under high G loadings.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  8. #8

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    Default Re: Wiring and Toggle Switch Choices

    Yes mount it crimped end down. I replaced mine last month after it wore out after 24 years service. Since my battery is located behind the passenger seat I mounted it adjacent to the battery.
    Michael Meyers
    KF IV
    Las Vegas, NV

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