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Magneto grounding

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1mitchkelly(at)comcast.ne
Guest

Posted: Sun Apr 15, 2012 12:31 pm Post subject: Magneto grounding

I have Slick 4301 magnetos and ACS A-510-2K ignition / starter switch. Operation is normal, however when I measure resistance between p-lead (either mag) and ground with switch set to Both, it reads zero. This is not what I expected, what am I missing?  

 

 

  

Mitch

 [quote][b]

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john.maccallum(at)bigpond
Guest

Posted: Sun Apr 15, 2012 12:53 pm Post subject: Magneto grounding

That is correct Mitch, the Mag P leads are open circuit on both. If you put the switch to Left the Right P lead will show a short to ground

and the Left will be open circuit. Then if you reverse it and switch to the Right Mag Â the Left will be grounded.

 

Cheers

 

John MacCallum

VH-DUU

RV 10 #41016

 

From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of 1mitchkelly(at)comcast.net

Sent: Monday, 16 April 2012 6:26 AM

To: aeroelectric-list(at)matronics.com

Subject: Magneto grounding

 



I have Slick 4301 magnetos and ACS A-510-2K ignition / starter switch. Operation is normal, however when I measure resistance between p-lead (either mag) and ground with switch set to Both, it reads zero. This is not what I expected, what am I missing? 

 

Mitch



 

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  [quote][b]

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ceengland(at)bellsouth.ne
Guest

Posted: Sun Apr 15, 2012 1:13 pm Post subject: Magneto grounding

On 04/15/2012 03:25 PM, 1mitchkelly(at)comcast.net (1mitchkelly(at)comcast.net) wrote:      	  Quote: 	
		         p { margin: 0; }                

           I have Slick 4301 magnetos and ACS A-510-2K ignition / starter           switch. Operation is normal, however when I measure resistance           between p-lead (either mag) and ground with switch set to           Both, it reads zero. This is not what I expected, what am I           missing?           

            

Mitch         

         

       

     	
     What you're seeing is a normal reading. With the mag switch 'on' (so     the mag can operate), you are reading the DC resistance of the     magneto coil primary winding, which is so close to zero that a     regular ohm meter can't read it accurately. It will look like a dead     short. The *impedance* (in quick & dirty terms, the AC     resistance) is higher, but your ohm meter can't measure that. This     is the reason for 'buzz boxes', used to set the timing of magnetos.     They generate an AC signal that's fed into the mag, and can detect     the difference between reading across the coil or seeing a truly     dead short, when the points inside the mag are closed (or your mag     switch is set to 'off').

     

     Hope that helps,

     

     Charlie

      [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Apr 15, 2012 1:43 pm Post subject: Magneto grounding

At 03:48 PM 4/15/2012, you wrote:

  	  Quote: 	
		  That is correct Mitch, the Mag P leads are open circuit on both. If you put the switch to Left the Right P lead will show a short to ground

 and the Left will be open circuit. Then if you reverse it and switch to the Right Mag Â the Left will be grounded. 	


    It's a bit more complex than that.



    A magneto has a core of magnetic material which

    can be 'excited' by a rotating magnet rotated on

    a shaft. One winding has many turns of very fine

    wire in which converts the rapidly changing

    magnetic flux from the rotating core magnet

    to a high voltage pulse (4-8,000 volts). This

    pulse is routed through a distributor to the

    appropriate plug wire to cause a spark across

    the plug's gap in the cylinder.



    A second winding of very few turns of heavy wire

    is connected to the magneto's interrupter 'points'.

    The schematic of this arrangement is shown here:



 [img]cid:.0[/img]



   As long as the points are closed, a dead short

   exists across the primary winding and the rate of

   rise of magnetic flux within the core is retarded.

   As the points open, that retarding force is suddenly

   removed allowing the high voltage to be generated

   and distributed to the appropriate plug. Setting

   the position of this 'opening event' with respect

   to crankshaft and piston position is accomplished

   in a "timing procedure".



   The primary winding has VERY low dc resistance.

   So when you measure the resistance on a magneto's

   p-lead to ground, it is difficult if not impossible

   to detect when the points open as the shaft is

   rotated.



   This is why magneto "timing lights" have little

   buzzers or electronic oscillators inside to generate

   an AC excitation signal which sees a much larger

   AC impedance jump as the points open.



  http://tinyurl.com/6ttzr94



 

   Shower-of-Sparks ignition systems excite the primary

   still more strongly and repetitively during cranking

   to generate a stream of sparks when the points open.



  http://www.aeroelectric.com/articles/Shower-of-Sparks/ShowerOfSparks.pdf

 

   But unless your measurement system for resistance has

   resolution in the milliohms range . . . like this

   one . . .



  http://tinyurl.com/6o5kqub



   You won't be able to use an ohmmeter to either measure

   the primary coil resistance -OR- detect a open/close

   event on the magneto's points.



 

   Bob . . .

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