Ignition Coils.

John Dammeyer johnd at autoartisans.com
Wed Aug 25 22:16:56 GMT 1999


Hi,

I ran into the same problem when we started using the FORD Quad Coil
except I figured out that at 12V the amount of current was unreasonable
so I didn't bother destroying the coil.

You have several alternatives:  More than likely the coil is used with
some sort of Capacitive Discharge circuit or else a current limiter.

Current limiting is useful because the low resistance and high
inductance allows a very short dwell time;  the time the current is
building up and flowing in the coil.  Then when the current flow is
stopped (points open),  the magnetic field collapses and the back EMF is
often 20 times as high as the forward current was and it's this back EMF
that is stepped up by the priamary:secondary turns ratio to be the
20KV+.

You could probably work with 8 to 10 amps and not destroy the coil.  To
do this you need a small load resistor in series with the coil to
generate a voltage proportional to the current.  This is use as part of
the feed back through either a comparator or amplifier to then limit and
hold the current at a preset value.    There is a circuit in Ed
Lansinger's article in #63 October 1995 Circuit Cellar Ink.

Alternatively you can go capacitor discharge.   This is far more
complicated and there are a number of different ways to do this.  The
most common is a high frequency oscillator that chops the 12V DC and
creates a square wave that is stepped up through a transformer to
produce 200V to 400V which charges a capacitor.  The amount of current
delivered by this power supply at 400V must enough to charge the
capacitor within the time from discharging it the last time to when it
needs to be discharged again;  for a 4 cylinder engine this is 180 crank
degrees at max RPM.

Once the ignition point is reached in the engine cycle an SCR or TRIAC
or ALTERNISTER is used to discharge the capacitor into the ignition
coil.  The advantage of a CD ignition is that it usually isn't dependant
on battery voltage.

There are some other ways to generate the high voltage but in each case
the transformer/inductor design is the difficult part.

Regards,

John

>Date: Wed, 25 Aug 1999 16:27:16 +0200 (MET DST)
>From: Fredrik Skog <c95fsg at cs.umu.se>
>Subject: Igniton coils
>
>Hi!
>
>Can someone please enlighten me in how to decide how much current I can
>put into my coil. I have a coil from a Alfa Romeo 155 and it is of the
>waste-spark type. It has a primary resistance of 0.9 ohms.
>I think I have wasted one coil already due to my lack of knowledge. I
just
>pumped 12 volts into it and it sparked a few times but then went dead.
>I have one more but I don't want to kill this one too.
>Do I have to know what voltage the Alfa puts into the coil to know this
or
>can I calculate/guess what it is in some way?
>
>Any help is greatly appreciated.
>
>Thanx!





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