Magneto kill terminal...

[Externet]

How does it work ?
Grounding it kills spark to shut down an engine like in a lawn mower. What is the potential on that wire when running ?
Or, is there many flavors ?

 

[BobK]

How does it work? You can't have a spark when the spark plug gap is shorted. No spark and the engine stops.

Bob

 

[shrtrnd]

I never researched it, I just know my experience as a kid mowing lawns.
Some mowers wouldn't shut down because of switching problems and I just used a (insulated handle) needle nose pliers to short the spark plug wire to the engine housing to kill it.
I would imagine that the voltage present would be whatever it took to fire the spark plug.
Worst feeling of electrical shock I remember, accidentally touching the spark plug wire while the mower was running.

 

[Externet]

Hi Bob.
That cannot be. Would put high tension to the keyswitch while running in order to be grounded when keyswitch turns off.

It is about a wire coming out of the magneto which is not high tension, grounded by a lever if operator releases grip of a push mower or turns key off on a riding mower.

 

[kellys_eye]

Works the other way around doessn't it? i.e. the connection is 'grounded' to allow the spark to be generated and 'open' to shut the spark off.

 

[Alec_t]

On simple magneto-generators, as used e.g. on motorbikes, I think the kill switch merely short-circuits the (low voltage) generator coil that powers the ignition circuit.

 

[BobK]

That is how it works on lawnmowers too. The switch shorts across the magneto coil and the spark plug which are in parallel.

Bob

 

[debe]

This is whats on the Kill conection on a Wipac ignition coil, about 110V pulse on the primary windings.

 

[debe]

This is a Ryobi whipersniper motor with a solidstate driven coil & the wave form on the kill switch conector.

 

[Externet]

Thanks for your elaborated and dedicated response, debe.
OK; about 110V peak unloaded.

These schematics shows that when the primary is grounded/shorted by the switch, the engine turns off, as the passing flywheel magnet 1 does not produce spark in that condition. Primary and secondary are not in parallel.

That primary kill terminal is this spade:

Now, with that kill terminal open circuited there is no current in the primary. But the engine runs.
Is then, the magnet induction picked only by the secondary coil ? There is no transformer action from primary to secondary ?

With that kill terminal grounded, there is current in the primary when the magnet passes by. Then, if there is current, there should be transformation from primary to secondary for spark to be created. But the engine stops.

Some light, please ?

 

[duke37]

A transformer has a primary winding which generates a magnetic field in the iron core and this is coupled to the the secondary. The output voltage is proportional to the ratio of the turns.

In this case, the magnetic field is produced by the passing magnet. The voltage is generated in the two windings in proportion to the number of turns. Shorting one winding forces zero volts on the other winding - engine stopped.

 

[debe]

The bottom 2 pictures are of a Briggs & Stratton ignition system. Theres a primary & a secondary winding in that coil with an electronic module built in. The Kill switch again shorts out the primary winding to stop the engine.

 

[Externet]

...Shorting one winding forces zero volts on the other winding...

OK. That is the key !

 

[duke37]

There will have to be a method of opening the winding at the critical time to get the spark. In the early car days, the contacts were opened in the cylinder and an arc was drawn at low voltage. Later a set of points were opened in the primary of a transformer to induce a large voltage on the plug in the cylinder.

In this case @debe says that there is an electronic module in a B&S engine to do this. The energy in the inductance is then diverted to the spark.

 

[Externet]

I believe the B&S magneto has no active components; just the coil windings. The flywheel magnet passing past the coil collapses the magnetic field kicking the spark generation.

 

[debe]

Heres 2 Briggs Magnetron Ign coils, one has the module built in the other is added to the side of the coil. I did dismantle one to see what type of transistor it had in it.

 

[dandy1]

Sorry to dig about in this post as it is a bit off topic. I am looking to sense the primary side with resistor/transistor setup. I've had some success using arduino but the signal seems to be messy. Going on the values captured by bebe with the scope, is there any advice for a circuit i could implement to capture the signal cleanly. I had tried implementing ceramic caps (various values 10n -100n) but it only seemed to clean up on lower rpm but as soon as it hit over 1500 rpm it got messy again and missed out pulses.

A

 

[twister]

After going thru 3 coils on a 2.5 hp honda clone engine, I now idle the engine down and kill the engine with the choke. Been a few years now and I haven't had to buy another coil.