Magneto Igniter ? What's in side?

[CalG]

Sort of a general technical question on how these little components do the job of replacing a set of points and condensor.

I have a walk behind mower with a Kaw FC400v engine that failed to produce spark yesterday. It's been "difficult" to start for years, but always would take off eventually. Yesterday is was a couple of bangs as it fired at the wrong time, then nothing even with a new plug.

So the little "Made in Australia, Power Start" module sold by Power Lawnmower Parts out of Rochester NY. is now here at my desk, The plastic case appears weather cracked so I ordered out a Mega Fire universal unit described as a replacement for the OEM 21119-2161. ($16 vs $56 for the Kawasaki unit on line)

But while I wait, a question..... How do those devices work? What determines just when the spark is formed? (timing?) I can appreciate that the FW magnet and the coil position can act as a rough timing trigger, but have not yet caught the jist of how that little igniter does it's job to replace the timing aspect of mechanical points. How does it know? Or perhaps it doesn't matter.

I've seen Briggs electro-magic coils with an additional magnetic sensor, but the item I'm speaking of is screw mounted to a light bracket on the side of the crankcase. There is no mechanical connection to the flywheel position.


I'm OK with electronics and components, so fire away!
TIA

Cal

 

[CalG]

What's inside?

Cut the case open to have a peak. there is a 8440 N channel MOSFET and a BC337 general purpose NPN transistor. That and a pair of wires .

The solder joints have all corroded and falling apart. The two components pulled off the tiny connector board without any tugging. So did one of the two wires.

I guess it is 30 years old...

Still don't get how the "timing" is controlled....

Cheers

 

[jaotguy]

as the pole pieces on the flywheel wizz past the coil , the polarity of the charge is reversed , causing the transistor to turn on and trigger the mosfet to dump the charge stored in the condenser ( from the last charge cycle) ... with the flywheel timed via the key way , the magnets are at the proper position , every thing is in sync ..... .. as the engine spins faster , the timing advances because the induced power also increases ...

if no condenser , the mosfet just dumps the power stored in the coil ...

 

[CalG]

as the pole pieces on the flywheel wizz past the coil , the polarity of the charge is reversed , causing the transistor to turn on and trigger the mosfet to dump the charge stored in the condenser ( from the last charge cycle) ... with the flywheel timed via the key way , the magnets are at the proper position , every thing is in sync ..... .. as the engine spins faster , the timing advances because the induced power also increases ...

if no condenser , the mosfet just dumps the power stored in the coil ...

\
Thanks for the reply!

No condenser on this one, And I did find an explanation of the reverse field polarity with an hours time on the internet., which makes perfect sense!

Best of all, I went to the local hardware store that also is the go to shop for small engine repair and they had one Oregon unit in a back drawer, even though they "quite selling those" some time ago. The price was .. "tell me what you think is right". so I bought it, same as the internet price. The engine started first pull, so I mowed the lawn ;-)

$2 worth of components in a $10 wrapper ;-)