question about CDI and regular ignition

[John Dammeyer]

> 
> John
> This letter you wrote was very informative.

Thanks.

> 
> I would like to know what Voltage/ current it takes to "fire" 
> a waste spark ignition. And maybe a little more on how they 
> work and are triggered.
>

Can't help you there.  The aftermarket devices that were intended for
retrofitting into points style systems were designed for a contact
opening to do the firing.  This is when a normal coil fires too so they
were designed to be compatible.

The traditional CDI normally uses an inverter to create 300V to 400V
from 12V.  With modern toroids and switching power supply devices I
expect to see the frequency of these devices climb into the >100KHz
range.  Still though,  they have to provide enough energy during the
'off' time to deliver between 85mj and 120mj.  I've used 1.5uF and about
325V which results in 85mj as I recall.  This implies a short pulse to
fire and an off time to recharge the capacitor.

I've never been able to measure the open circuit voltage on the output
but obviously the turns ratio on the coil determines that ultimately.
For example.  If a 12V coil is designed to produce 24Kv then the ratio
is 2000:1.  Assuming 300V into that same primary doesn't damage the coil
the implication is the voltage could reach 600KV.  Of course it never
gets there as the spark starts at about 15KV to 20KV across a 0.032" gap
and at that point the voltage stops increasing.  I've seen spikes as
high as 40KV but 99% of the time 20KV seems to be as high as it gets
unless the plugs are really fouled.

To drive a black box will depend on the manufacturers specs.  If it's
opto-coupled for a 12V input you can usually expect 12V at 20ma to 40ma.
If it's a DC input it could be just straight 12V or else 5V logic
voltage.  I'd even guess that there are some units out there that take a
magnetic reluctance input from a crank trigger which is adjusted for
maximum advance (say 35 degrees) and depending on RPM delays the coil
trigger from 0 degrees to 22 degrees.  I've also heard some units have a
vacuum input to do a vacuum retard.

Best I can do here.

John Dammeyer

 
> Thanks!
> 
> Larry Koutz
> 
> -----Original Message-----
> From: John Dammeyer <johnd at autoartisans.com>
> To: diy_efi at diy-efi.org <diy_efi at diy-efi.org>
> Date: Wednesday, July 11, 2001 10:06 PM
> Subject: RE: question about CDI and regular ignition
> 
> 
> >There are essentially 3 different types of coils out there.

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