How many volts is a magneto on a 5hp briggs?

Discussion in 'General Electronics Chat' started by stanman11, Sep 29, 2012.

  1. stanman11

    Thread Starter Member

    Nov 23, 2010
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    I'm trying to hook blinkers to my go kart.
    I was going to use LEDs and 9.6v bttery but the Rc batteries only last a couple days and takes 4 hours to charge.
    I'm wondering if i can splice in to the magneto.
    I cant test the motor, its not put back together im wating to get time to sandblast the frame.

    if the magneto is 12v im geussing i wont need the 555, I can just wire the LEDs or a standard 12v blinker to the magneto and the timing should make it blink?
     
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    This is only a semi-informed answer (wait for confirmation):
    The voltage generated by the magneto will depend on motor RPM and is going to be much higher than 12V. It also is not a steady output; there is a single brief pulse for every revolution.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Magnetos do not generate a DC voltage, they just generate pulses of high voltage to provide the spark. Even if you could tap off of the low voltage side of the magneto coil, any power taken would reduce the power to the spark.

    I think you would need to add a small alternator to the engine if you want to avoid having to charge the battery.
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    Forget using the mag.

    It is a dedicated and highly optimized device that is part of a dedicated and highly optimized circuit intended to do one thing, one thing only, and do it well.

    The voltage produced by a mag is dependent, as already pointed out, on a number of things. But typical mag voltages are a few tens of thousands of volts. Remember, it has to create enough voltage to initiate a spark across a gap and to do so quickly and aggressively.

    Depending on your engine, you may be able to mount a small generator to it. If it is an electric start engine, it already has one. but I'm assuming it is a rope start. You can probably rig up something to power a small generator from the drive drain. It will only output power when the gocart is moving above a certain speed, but that should be sufficient for you needs.
     
  5. stanman11

    Thread Starter Member

    Nov 23, 2010
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    Well im going to have to use my 15 minute charger for my 7.2v battery.
    my Leds are 3.2VF at 30ma. which I would use a 100ohm resistor right?
     
  6. stanman11

    Thread Starter Member

    Nov 23, 2010
    230
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    can I hardwire a motor to the shaft to use an electric start and same time use it as an alternator?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    You likely could, but motor/generators are not cheap (see these, for example).
     
  8. PackratKing

    Well-Known Member

    Jul 13, 2008
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    Your magneto, does put out a pulse, which is routed to the ignition coil to boost that pulse to adequate voltage to fire the sparkplug.
    Tap the primary coil on the same connection that enters the secondary coil, which the other end of, terminates on the sparkplug, and to ground.....Note which wire on the engine serves to ground the ignition to kill the engine.

    Make that tap a wire to a heavy duty diode for half-wave rectification - overkill it - and regulate that if necessary to maintain a couple volts over 12, let that go to maintain charge on your battery, and then go ahead and wire your turnsignal system off the battery, with a standard 12v automotive flasher, to drive bulbs or led's, whichever you choose................a small 12v sealed-lead-acid battery like those that start a fancy 21 inch rotary mower should suffice, since your signals are intermittant at best, and won't draw a significant current.
     
    Last edited: Sep 30, 2012
  9. Audioguru

    New Member

    Dec 20, 2007
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    The 7.2V battery is 9V when fully charged and is still charging.
    The LEDs are designed for 20mA.
    The absolute maximum allowed current is 30mA.

    The actual current will be (9V - 3.2V)/100 ohms= 58mA so you can see the LEDs blink once or twice brightly then you can kiss them goodbye.
     
  10. stanman11

    Thread Starter Member

    Nov 23, 2010
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    4
    Getting some good info guys thanks for all the feed back.
    But unfortunately I failed to realize these 20,000mcd LEDs arent strong enough to show up in sunlight
    Each turn signal contains 4 LEds and the brake light contains 14.

    here is a video if you want to check it out. any ideas how i can turn this in to a success?
    http://www.youtube.com/watch?v=hslM8nDUhbI&feature=youtu.be


    its a 5hp briggs and has only a magneto to the plug and is pull start.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Car manufacturers do LED lighting perfectly. Then the LEDs can be seen in sunlight but do not blind people behind when it is cloudy or at night.

    BUT Chrysler cars and Jeep trucks blind everybody in the daytime with their too bright daytime running lights (using high beams)!

    Why do people from "over there" drive at night with the parking lights on instead of the headlights on?? Even if they had good vision (most are almost blind) then they can't see anything in front of them.
     
  12. stanman11

    Thread Starter Member

    Nov 23, 2010
    230
    4
    Makes them feel like a boss
     
  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
    1,525
    The older Briggs points style magnetos had about 6 -8 volts on the primary when pulling the starter rope and around 300 - 400 volts on primary when actually running. The higher running volts are due to the collapse of the secondary and the reverse induction on the primary. Don't know what the new electronic ignitions are voltage wise.

    You could always mount another coil so it is near the flywheel magnets and use that for a low voltage coil. A lot of dirt bike do that. A magneto and lighting coil on the same flywheel.
     
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