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Magneto Vs. Ignition Coil

  1. Jun 3, 2008 #1
    Im trying to find out the difference. It seems that an ignition coil converts the cars battery voltage from 12V to several thousand volts; whereas, a magneto creates several thousand volts without any input from the aircrafts battery.
  2. jcsd
  3. Jun 3, 2008 #2


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    You pretty much got it. A magneto creates its voltage by passing a magnet driven by the crankshaft of the engine past a coil. Every time the magnet passes the coil it induces a sharp current in the coil. A lot of small engines, like on lawn mowers, have magnetos.
  4. Apr 22, 2009 #3

    How much voltage should a motorcycle magneto be generating when the kick start is kicked? Is it 12v or much more?
  5. Apr 22, 2009 #4


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    It should generate enough to jump the spark plug gap with ease. Going to be alot more than 12 volts.
  6. Apr 22, 2009 #5


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    Upwards of 20,000. I'm thinking that around 35,000 is more likely.
  7. Apr 22, 2009 #6
    The old-time magnetos I have seen on small engines (like lawn mowers) had a permanent magnet in a magnetic circuit with a coil around it, and a cam on the cam shaft that opened and closed the magnetic circuit, causing a dB/dt in the coil which produced the high voltage. Thus the magnetos needed no battery. The old time ignition circuits I have seen on automobiles had a 6 (or 12) volt battery, with "points" inside the distributor that were opened and closed with a cam that ran off the camshaft. The points (in parallel with a condenser (capacitor)) interrupted the current (2 to 4) amps in the primary of the ignition coil. The secondary (~20,000 - 30,000 volts) was distributed to the spark plugs.
  8. Apr 23, 2009 #7

    Ranger Mike

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    it takes 25000 volts to jump .025 inch spark plug gap unless you go to iridium spark plugs..$ 8 each
    the ignition coil converts 12 volts to 25000 volts thur differential series of wire coils internally and a n iron bar core
    the battery ignition requires a constant 12 volt source and ifin the battery drops below 9 volts the car will quit running
    magnetos were an early means to start the Internal combustion engine out on the farm in the middle of no where
    you had to hand crank the engine and set the ig advance to retard position to light the fire

    the magneto would really put out the sparks 60000 volts and up ifin the engine was twisted to 7000 rpm
    most race car sanctioning bodies don't allow magnetos as a cost savings??
    plus the battery is required to run other electrical things like gauges, tachometers cooling fans for brake ducts
    Cd ignitions are the hot setup ( capacity discharge)
    they generate multiple sparks be engine revolution at idle that help to burn gasoline/air mixture more thoroughly
    at hi rpm the CD has a real long spark time , up to 10 crankshaft degrees that helps burn the mix better
    these CD ignitions are real voltage hogs and can run down a good battery a lot quicker than the old breaker point ignitions]
    some classes of racing let you remove the alternator ( less parasitic HP drag and weight) and run a battery only..will run for 20 minutes until volts drop below 9 volts
    and requires recharge between races
    in drag racing it is not a problem
    but in round track and road racing
    could be
  9. Apr 23, 2009 #8
    I have built CD ignition circuits, and have one now. Basically what it does is use a high (audible) frequency DC-DC converter to generate a 300-400 volt charge on a capacitor, which is discharged through a standard ignition coil when electronically triggered. At low RPM, the current drain is very low, and increases as the RPM increases. At very high RPM it may exceed the current drain of a breaker point (old fashioned) automotive ignition system, depending on the choice of coil. The breaker point ignition circuit current drain depends only on the dwell time (the percentage of time the points are closed) and not on the engine RPM, so the current drain is independent of RPM. Commercially available replacement CD ignition circuits (e.g., Mark Ten Model B which I have) were designed to use the OEM breaker points as a trigger for the CD discharge.
  10. Apr 23, 2009 #9

    yeah, coming out of the ignition coil will be something like 20K volts. How much (of the magneto generated voltage) is entering the ignition coil?
  11. Apr 24, 2009 #10

    Ranger Mike

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    magnetos use same type coil as battery iginitions
    magnetos generate 12 volt primary and this gies the the coil
    iginition coils are
    iron bar core, in a sealed oil filled case to carry away heat
    primary coil windins of about 150-200
    secondary windings locvated near the iron core on order of 10,000 windings
    this is std coil of 25000 volt out put

    when 12 volt is applied to the + side of the coil no action takes place until the - or ground side is tripped by the breaker points, photo cell or what ever device
    the voltage can not go to ground so it seeks the next path which is to " jump" from the promary windings to the secondary windings to seek a ground and bingo..you got 25000 volts

    you can buy a super coil that goes to 60,000 volts out put but super battery drain
    reason no one uses magnetos is the sophistication of battery iginitions and the CD systmes perform almost as good as mags these days
    a good vertex magneto is over $ 1000 as opposed to a good aftermarket CD distributor system for a few hunderd dollars
    also the gel cell battrerys are " bullet proof' these days vs the old wet cell batteries of days gone by
    Last edited: Apr 24, 2009
  12. Sep 5, 2009 #11
    Osram Hyper White lamps 5000K 12V...60 - 55W...for Bajaj Discover 125cc DTSi???

    Respected Sir,

    I purchased Bajaj Discover 125CC DTSi bike (Self Start) in May, 2005. Last evening, I bought OSRAM Cool Blue Hyper White 5000K bulb from their Authorised Wholesalers in Mumbai.

    The specification for this bulb is as follows:
    H4 COOL BLUE Hyper White
    12V - 60/55W
    Made in Germany

    Now, since my Bajaj Discover 125CC DTSi I believe is an ES (Electrical Start) version for which Bajaj mentions in its Specification book as: 35/35 W HS1...I wonder how H4 bulbs get fitted into the front headlight connector?

    Secondly and "Most importantly”, will this Osram 60/55W H4 (Hyper White 5000K) work for my Bajaj Discover 125CC DTSi Bike (Self Start)?

    I have been reading on the internet that Electrical system needs to be changed from AC to DC...coil rewinding required b'coz the “Magento” doesn't deliver more than ~40W to a bike's headlamps?

    Is this true?

    Also, could you kindly let me know if I need to do some modification to my bike, For e.g. Electrical system?

    Kindly let me know...

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