Aircraft wiring with 12v ignition?

Discussion in 'Instruments / Avionics / Electrical System' started by cluttonfred, Jan 24, 2020.

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  1. Jan 24, 2020 #1

    cluttonfred

    cluttonfred

    cluttonfred

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    If you are setting up a plane with automotive 12v ignition, is there any point to having a separate master switch or just use the accessory circuit on the automotive ignition switch? You could still have an avionics switch to isolate anything sensitive, but the master switch doesn't seem to serve much purpose in this case as long as the ignition switch is either the OFF-ACC-ON-(START) or the ACC-OFF-ON-START type. Am I missing something?
     
  2. Jan 24, 2020 #2

    Dana

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    An aircraft has a separate master switch because it's totally independent from the ignition, shouldn't be necessary with auto ignition since it's dependent on the electrical system anyway. But a separate master to isolate the engine from everything else in the aircraft might not be a bad idea anyway.
     
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  3. Jan 25, 2020 #3

    TiPi

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    Consider the cases of battery or alternator failures: what switching is required to keep power to the ignition and keep that fan turning?
    If the alternator can supply power without being connected to the battery, a supply to the ignition can sustain it in case of a battery failure. In case of an alternator failure (more likely regulator or rectifier), a separate power supply to the ignition will be required (around the master switch). I would wire a 12V ignition with a separate dual power supply switch and check during run-up (ALT and BAT supply).
     
  4. Jan 25, 2020 #4

    cluttonfred

    cluttonfred

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    Thanks guys, but not sure I follow, TiPi. If alternator fails you're running on battery anyway, hopefully with a low voltage warning light. If battery is tired you can check that with IGN or ACC on the ground, you won't know it in the air anyway unless alternator dies. What does dual power switch get you? Not being snarky, I just don't get it.
     
  5. Jan 25, 2020 #5

    Wanttaja

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    My airplane was built without a master solenoid; there was an ordinary rocker switch on the panel as a master switch.

    What this meant is that "hot" power was in the main switch box all the time. If some short started from the main power lead, the only way to kill the power would be to disconnect the battery. Considering the main switch box was installed ATOP the battery, and the switch box had to be displaced to even ACCESS the battery, this had potential issues that apparently didn't occur to the original builder but bugged the bejesus out of me.

    Also bugging the bejesus out of me was the fact that the starter power cable was a DIRECT CONNECTION to the battery. It attached to a switch on the outside of the starter itself. This meant that to disconnect the power to the starter, I had to displace the switch box (which had power to it 100% of the time as well), THEN disconnect the battery.

    Now, I agree you wouldn't be THAT stupid.

    However, I did switch it over to normal aviation practice. There's a standard aircraft solenoid installed on the cabin side of the firewall. The battery +12V cable goes solely to it, and appropriate cables go from the output terminal on the solenoid to the electrical bus and the starter.

    Activation of the solenoid is a single wire to a master switch in the panel, and the solenoid activates when the control terminal is GROUNDED. This means there is no live +12V anywhere in the airplane until the master switch grounds the solenoid. With the master off, there's NO +12V power to anywhere but the two ends of the ~10 gauge cable that goes from the battery to the solenoid and the battery-charging port.

    elect_s.jpg
    Ron Wanttaja
     
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  6. Jan 25, 2020 #6

    TiPi

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    It all depends how you design your electrical system. The "normal" way is to have a master switch that allows you to turn off everything (as shown by Ron above). This works OK for ignition mags but will cut your 12V ignition.
    In the case of a battery failure, you probably want to be able to turn off everything not needed (depends on alternator type), so a power supply for the ignition taken BEFORE the ALT switch/solenoid will give you that.
    In the case of an alternator/rectifier/regulator failure, you need to be able to turn off all consumers to preserve the battery capacity for your ignition. You can either have a switch for every item in your aircraft so that you can switch off all consumers and only leave the ignition on or have the ignition power supply from BEFORE the BAT master switch/solenoid.

    I would use a 3-position switch:
    1) OFF
    2) ALT power (power supply from the laternator/rectifier/regulator output)
    3) BAT power (from the battery before the master switch/solenoid), position for normal use
    Both supplies should be appropriately fused as close to the source as possible.
    The ignition check is simply to check if that power supply is still working (fuse/connection check)

    It would be helpful if you could elaborate a bit on the overall electrical system (complexity, type of alternator, general layout).
     
    Last edited: Jan 25, 2020
  7. Jan 25, 2020 #7

    TFF

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    I would start with a master. Feeding would be three switched busses. Engine, avionics, lights. Then you can have individual switches for items under each buss. That gives you a chance to shut down systems. Under engine would be the starter, alternator field, any engine instruments. Avionics anything not engine. Lights are lights. Perfect no but simplest. Redundancy becomes its own separate setup.
     
  8. Jan 25, 2020 #8

    wsimpso1

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    Bob Nuckolls' AeroElectric Connection should be on your shopping list. The book spends many pages on electrical system design including several methods and scematics for managing an electrically dependent engine. I suspect that you will find it useful.

    Billski
     
    Last edited: Jan 25, 2020
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  9. Jan 25, 2020 #9

    cluttonfred

    cluttonfred

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    Thanks, all, for the comments.

    As usual, I have in mind something quite simple, in this case the implications of hanging a Suzuki G10 conversion on a VP-2.

    My initial KISS concept was to just wire it like a car and use the ACC circuit to feed a few small loads (some 12v instruments, LED beacon, maybe 12v outlet or USB ports for handheld radio, tablet for navigation).

    The thought was to check battery voltage during preflight and monitor during flight to trip alarm if alternator or rectifier gets wonky.

    Sounds like I need to do some more research....
     
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  10. Jan 25, 2020 #10

    Dan Thomas

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    Don't use the ACC terminal for the ignition. There should be a separate terminal on the switch for ignition, since most switches disable the ACC for start so that accessories don't get spiked when you release the starter and it's heavy windings generate that votage spike when the current is cut off. You might have fun trying to get the engine to catch during start if you use the wrong terminal.

    As Ron points out, a means of disconnecting the battery in case of electrical fire in flight is a must. FAR23 requires it for type-certified airplanes, and like most of FAR23, this was learned the hard way.

    Alternators really detest having the battery disonnected while they're operating. The battery damps load and voltage changes, and if you cut it out, that alternator regulator goes wild. You get cooked radios and stuff. Some motorbikes (choppers) used a large capacitor to act as the damping in blace of the battery.
     
  11. Jan 25, 2020 #11

    pictsidhe

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    My 2c.
    Seperate switches for ign and everything else to the battery. Ron's solenoid system being a good way. Don't try running anything from the alternator only. Alternators need a battery or very large capacitor or they can fry or spike the electronics. Spiking may not produce immediately obvious damage. IOW, preflight checks could damage vital systems, for them to fail later. This is way more likely than the battery failing at any other time than startup. A crowbar cicuit (overvoltage) and simple fuse (short) will stop an alternator failure from trashing other parts.
    Ignition only uses a few amps, so even a tiny battery is good for over an hour.
     
  12. Jan 25, 2020 #12

    cluttonfred

    cluttonfred

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    To be clear, my assumption was that the ignition would be connected to IGN terminal and other things to ACC terminal.

    My understanding of a 4-position ignition switch is that with the key at ACC only the ACC terminal is hot, key at OFF then nothing is hot, key at RUN then both IGN and ACC terminals are hot, and key at START then IGN and START starter terminals are hot.

    With the very modest loads I have in mind, I guess I am having a hard time imagining a situation where I would need to isolate the engine more than just switching off the beacon and auxiliary circuit.

    Of course, all of this makes me want to go back to a single magneto VW engine and a small 12v battery charged on the ground for the rest. ;-)
     
  13. Jan 26, 2020 #13

    TFF

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    The reason you separate is so that if you are having an unlucky day, you might be able to recover some of it. Straight full electrical failure is easy, everything is off. Deal with it. If you have a segment that is the problem, you might be able to recover. Destroying equipment or starting a fire that could at least be curtailed some. If I have arcing that’s trying to start a fire, I want to try and stop it. Stop it and still try and aviate. That’s one of the beauties of steam gauges, most work even it the power is off.
     
  14. Jan 26, 2020 #14

    cluttonfred

    cluttonfred

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    I get that TFF, it just seems less likely to be an issue in the setup I described, hard to imagine anything catastrophic that wouldn't require shutting down the whole electrical system including the engine. That said, switches are cheap. I wonder if an automotive-type battery disconnect switch would make an even simpler alternative to a master solenoid, either the push-pull kind or maybe a remote rod for turning a lightweight key-type one?
     
  15. Jan 26, 2020 #15

    pictsidhe

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    If it isolates the battery, it's suitable! If your battery is easy to reach, then a manual switch makes a lot of sense. Properly sized wiring and fuses are also a good way to prevent electrical fires. Cheap again.
     
  16. Jan 26, 2020 #16

    TFF

    TFF

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    They are all stages in hopes to prevent fires. A car you stop and get out. Airplane is minutes from getting to the ground, possibly longer if you try for an airport. Flying a small wood and fabric homebuilt with wiring right behind the fuel tank would be the worst type to have a fire in. The scenario is not about 99.9% success, it is about the .1% failure.
    Suspected fire. Shut off lights; yes or no. Shut off avionics; yes or no. Shut off charging; yes or no. And if still stuck, you shut down the engine. If it’s a battery problem, you pull the plug on it all. Switches prevent extra involvement of systems that might still help you. Ever turned the ignition off on your car while driving? Ever go too far and lock the steering? Yes there is extra stuff that could all be tied together, but then you only have all or nothing as an answer.
     
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  17. Jan 27, 2020 #17

    wsimpso1

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    Bob Nuckolls book presents his philosophy, that stuff breaks, and that when it does, it should not be dangerous. He has a series of thoroughly thought out schematics, applicable to everything from the simplest airplanes up through dual alternator dual battery systems and electrically dependant engines. Seriously, he has thought these through from an FMEA perspective... at worst you will find your best scheme validated. And at best you will find you can improve while keeping it simple.

    Billski
     
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  18. Jan 27, 2020 #18

    bmcj

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    I’ve got the perfect example of why to have a separate master switch, or at least one for avionics. I had an electrical fire in a Tripacer once. The fire was easily dealt with by switching off the master, but I still had a working engine to take me home.
     
  19. Jan 27, 2020 #19

    BJC

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    BTDT, but with a new (at the time) Piper Warrior.


    BJC
     
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  20. Jan 27, 2020 #20

    Markproa

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    While we're on electrical systems can I ask a simple question? My radio claims to be 14V; is that maximum volts? I assume it will run on a 12 volt system?

    Mark
     

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