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PICList Thread
'Electronic ignition coil driving <== Re: [AR] SV:'
2003\04\25@110421 by Alan B. Pearce

face picon face
One of the problems with doing electronic ignition is the coil type. If the
person doing the project is trying to use an ordinary ignition coil used for
a system with mechanical points then they are likely to have many problems
with semiconductors  and coils punching through. The problem is the leakage
inductance of the coil. I can only refer you to a series of articles in
(what became) Electronics Australia back in the late 1960's when they were
trying to develop such a circuit for publication. They had a photo of a
graveyard of coils and semiconductors which had not survived :))

They found that they had to have a catch network of some sort (cannot
remember the details, it may have been a high voltage, as in 150V or so
zener diode) to snub the voltage spike caused by the leakage inductance.
They could only get rid of this network when they used a toriodal
transformer. The spike was that fast they had to go in a darkened room with
the CRT brightness well up to even see something of the spike - probably
easier to see with modern high speed scopes, but still hard enough.

Now you know why mechanical points burn when the capacitor fails :))

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2003\04\26@024622 by Russell McMahon

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flavicon
face
I seem to have (mysteriously) deleted the post which mentioned assuming that
an auto ignition coild uses usual turns ratio assumptions. This is not the
case, if I correctly recall what was being said.

With a 100:1 turns ratio say the primary may rise to typically 200 volts (20
kV at secondary) prior to spark initiating.
During driver on time the primary will only be 12 volts and the ouitput
therefore 1200 volts. They could make the turns ratio whatever they wanted
to get best amp-turns magnetisation in the dwell time available as the turns
ratio plays essentially no part in the spark generation process. Sports
coils seem to have lowe rtyurns ratios which leads to higher proimary
voltages during flyback. I imagine that they do this to reduce inductance
and increase charging rate and resultant amp-turns of magnetisatuion energy
in the core.

A CDI system does use the actual turns ratio for spark initiation by dumping
typically 200 to 300 volts into the primary at firing time.

I'd appreciate it if the people who made posts in the last day on this topic
please be so kind as to send them directly to me as I think I accidentally
deleted a block of emails with them in it (I usually don't delete ANY emails
at all!) (Then of course, there's always the archive ...)

regards


       Russell McMahon

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2003\04\26@071358 by Russell McMahon

face
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face
> I seem to have (mysteriously) deleted the post which mentioned assuming
that
> an auto ignition coil uses usual turns ratio assumptions.

As you were.
Brain fade strikes.
That's what comes of copying a thread to a second list !!!
Messages were (and are) in another folder.



           Russell McMahon

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2003\04\28@034606 by hael Rigby-Jones

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> -----Original Message-----
> From: Russell McMahon [SMTP:.....apptechKILLspamspam.....PARADISE.NET.NZ]
> Sent: Saturday, April 26, 2003 7:36 AM
> To:   EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU
> Subject:      Re: Electronic ignition coil driving <== Re: [AR] SV: [AR]
> mosfet problems
>
> With a 100:1 turns ratio say the primary may rise to typically 200 volts
> (20
> kV at secondary) prior to spark initiating.
> During driver on time the primary will only be 12 volts and the ouitput
> therefore 1200 volts. They could make the turns ratio whatever they wanted
> to get best amp-turns magnetisation in the dwell time available as the
> turns
> ratio plays essentially no part in the spark generation process.
>
Like most things it's a compromise. With a lower ratio the primary voltage
would have to rise to very high values, causing problems for any
semiconductor switches, and the higher inductance would require longer dwell
times.  With a very high turns ratio, the primary current would have to be
very large, leading to heating problems.

Mike


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