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Capacitive discharge ignition circuits
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bob prohaska
Sep 25, 2021, 12:03:43 AM9/25/21
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Does anybody know of links to well-informed descriptions
of how AC capacitive discharge ignition circuits work?
The case at hand is a 1986 Suzuki DT4 outboard motor. It's a
2-stroke 1-cylinder 4 HP engine, probably not much different
in ignition requirements from any other small engine. It uses
two bar-style pickup coils, one to charge the cap and one 180
degrees away to fire the plug. The CDI box is distinct from
the pickup coils, all are distinct from the ignition coil.
Parts are no longer available. There's a photo at
http://www.zefox.net/~bp/dt4/PXL_20210923_005603433.jpg
The initial diagnosis was an open ignition coil, but a
replacement produced one spark and nothing more. Maybe
the new ignition coil killed the CDI unit, or maybe we're
barking up the wrong tree entirely.
With a better idea how it worked in general, there might be
a chance at identifying some other engine with compatible
parts, or, in the best case, of identifying and fixing the
trouble. At this point it appears the two coils in the photo
are functional.
Thanks for reading, and any ideas!
bob prohaska
of how AC capacitive discharge ignition circuits work?
The case at hand is a 1986 Suzuki DT4 outboard motor. It's a
2-stroke 1-cylinder 4 HP engine, probably not much different
in ignition requirements from any other small engine. It uses
two bar-style pickup coils, one to charge the cap and one 180
degrees away to fire the plug. The CDI box is distinct from
the pickup coils, all are distinct from the ignition coil.
Parts are no longer available. There's a photo at
http://www.zefox.net/~bp/dt4/PXL_20210923_005603433.jpg
The initial diagnosis was an open ignition coil, but a
replacement produced one spark and nothing more. Maybe
the new ignition coil killed the CDI unit, or maybe we're
barking up the wrong tree entirely.
With a better idea how it worked in general, there might be
a chance at identifying some other engine with compatible
parts, or, in the best case, of identifying and fixing the
trouble. At this point it appears the two coils in the photo
are functional.
Thanks for reading, and any ideas!
bob prohaska
Phil Allison
Sep 25, 2021, 1:17:25 AM9/25/21
to
bob prohaska wrote:
================
** I used to work on CDIs back in the 70s for a kit retailer.
Those ones use a pair of 2N3055s in a self oscillating inverter to produce 300VDC.
Then an SCR to fire a charged 1uF poly cap into the vehicle's coil.
The usual points controlled SCR firing - so a bit different to a 2 stroke OB.
However, the commonest failure was the 1uF cap.
It had to be a special, high pulse current type to survive at all.
Oil filled, metal can ones were good - if you can find any.
Otherwise a " polypropylene " fluoro PF correction cap would be OK too.
...... Phil
================
Those ones use a pair of 2N3055s in a self oscillating inverter to produce 300VDC.
Then an SCR to fire a charged 1uF poly cap into the vehicle's coil.
The usual points controlled SCR firing - so a bit different to a 2 stroke OB.
However, the commonest failure was the 1uF cap.
It had to be a special, high pulse current type to survive at all.
Oil filled, metal can ones were good - if you can find any.
Otherwise a " polypropylene " fluoro PF correction cap would be OK too.
...... Phil
Mike S
Sep 25, 2021, 5:01:15 AM9/25/21
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Wond
Sep 25, 2021, 11:20:19 AM9/25/21
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