From: Richard Prosser <rhprosser@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 3 Aug 2021 14:56:47 -0700
Subject: Re: [EE] Honda MT250 ignition waveforms
Thread-Topic: [EE] Honda MT250 ignition waveforms
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Are you sure that the points should be *closing *at the ignition mark?
Normally the points *open *at this point in order to build voltage from the
energy stored in the magneto coil.
RP

On Wed, 4 Aug 2021 at 09:16, Harold Hallikainen <harold@mai.hallikainen.org=
>
wrote:

>
> > I've had a bit more of a think about this overnight.
> > It appears that the configuration is that of a "low tension magneto", a=
s
> > used in (some) aeroplanes etc. I've not been able to get a detailed
> method
> > of operation but it looks to me as if the coil is used, more or less, a=
s
> a
> > simple transformer.
> >
> > The flywheel rotates the magnet & inducing current in the mag. coil due
> to
> > the closed contact points.
> > At close to the maximum current, the points open and this induces a hig=
h
> > voltage across the mag coil & the connected HT coil & capacitor.
> > The capacitor absorbs some of this & then rings with the HV coil used a=
s
> a
> > step-up transformer to produce spark.
> >
> > So, considering your waveform with the HV coil, capacitor & ignition
> > switch
> > disconnected, you should see a high peak voltage as the points open.
> > Probably >> 250V . Also there should be a noticeable spark at the point=
s
> > themselves. Since you're not getting this; either:-
> > The mag coil is faulty (unlikely, as it is generating low voltage).
> > The points or their connection is faulty. (Dirty / Worn / Corroded)
> > The points are opening at the incorrect time - the woodruff key is
> sheared
> > and the timing is way out. This should be fairly easy to verify.
> >
> > With the HV coil & capacitor connected, you should see ringing across t=
he
> > HV coil (& get spark).
> >
> > Hope this assists,
> > RP
>
> Thanks for the comments! The original transformer had an open secondary,
> so my brother got a replacement. I think it is not a suitable replacement=
..
> He has now ordered the exact Honda part number. In initial testing, we
> found the points corroded and not making contact at all. After cleaning,
> we are able to see them close at the correct time (resistance drops to
> zero when the mark on the flywheel lines up with the timing mark). For th=
e
> magneto coils, it seems like the only failures could be an open or short
> (shorted windings). The coils is not open. Detecting shorted windings
> would be pretty difficult. But, I think that would be an unlikely failure=
..
> Assuming the new transformer solves the issue, I'll capture some waveform=
s
> with the transformer connected and also with the magneto/points
> combination driving a resistor (maybe 100 ohms).
>
> Harold
>
>
>
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