question about CDI and regular ignition

[John Dammeyer]

There are essentially 3 different types of coils out there.

1. 8V volt coil used on older cars with a series ballast resistor which
was switched out during cranking.  This meant that during cold weather
or a flat battery there would still be sufficient spark.  Once the
starter switch is released the ballast resistor is put back in the
circuit and the coil runs on 13.8V.  

These coils worked really well with the older transistor ignition (A
power transistor was switched by the points and this lowered the current
and therefore arcing and subsequent point damage.)

These coils also worked well with the CD ignitions of the time.  Delta
made one and Radio Shack remarked it as an Archer Kit in the early 70's.
It ran a ~15KHz oscillator generating about 300V which charged up a cap.
An SCR triggered by the points dumped the 300V from the cap into the
coil.  Hence Capcitive Discharge Ignition.  It had a button on the side
which switched out the CDI an dran on the conventional points.  You
could hear the whine of the unit so you knew when it worked.

2. Standard 12V coil without ballast driven by a CD ignition similer to
the one above.  Note that SAAB (now GM) has a patent on a paraticular
method of creating the 300V to dump into the capacitor.  They even go as
far as to charge the cap during the part of the engine cycle when the
start motor has the least amount of load.  Real cool.

3. Low resistance coils.  These are the latest now that semiconductor
technology has improved.  These coils (like the FORD quad or hex) have
an extrememly low resistance and would draw more than 100 Amps before
they resembled toast.  The driver circuit limits the current to a
defined value (10A I think) just before the coil needs to fire the spark
plug.  The secondary of the these coils are connected not as an
autotransformer like standard coils but each end of the winding goes to
a spark plug.  The current goes from the center electrode of one plug to
the outer electrode and the cylinder head.  It then jumps from the outer
electrode to the center electrode of the other plug and back into the
coil.  

Called waste spark,  it is said to clean the sparkplug during the
exhaust stroke.  Yeah right... What it does is save a transitor for each
pair of cylinders and that is good thing.

Hope that helps.


Cheers,


John Dammeyer


> -----Original Message-----
> From: owner-diy_efi at diy-efi.org 
> [mailto:owner-diy_efi at diy-efi.org] On Behalf Of Jack
> Sent: Wednesday, July 11, 2001 1:12 PM
> To: diy_efi at diy-efi.org
> Subject: question about CDI and regular ignition
> 
> 
> Hi,
> 
> Got a question about regular ignition and CDI ignition. What 
> is the main difference between a regular coil and a CDI coil? 
> From a ohm meter, the CDI coil seems to have a lot less 
> resistence (hence higher current/voltage??)
> 
> Also, the driving signals seems to be a lot different. What 
> type of signal
> (voltage/current) is required to drive CDI coil?
> 
> Really appriciate if anyone can give me some insight in this subject.
> 
> Thanks,
> 
> Jack
> 
> 
> 
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