Electronic Ignition

[Mark Pitts]

Come on now guys.... The cap on points is to stop arcing at the points, =
and therefore erosion of the breaker terminals. If you take the cap off, =
it all still works, but the contact breaker pads weld to each other, =
then the distributor cam forces them apart, ripping the surface off one =
side, this is generally considered BAD!

CD ignition requires a much bigger cap, as it is the cap discharging =
into the coil that gives all the 'bang', not the collapse of the mag =
field.=20

Theory simplified:

Transformers are AC.
They induce a current in the secondary proportional to that in the =
primary (by number of turns primary to no. turns secodary).

Normal points ignition gets the coil 'saturated'. I.e. it has dc flowing =
through it. This DC comes from the points. They bit that induces the =
spark is when the points open, and the magnetic field no longer is =
supported by the DC. Something has to give. The current would continue =
to flow for a short time AFTER the points open, through plasma as the =
points arc. This is taken care of by the condenser 'buffering' this =
effect, by allowing current to flow for a short time thro' it, mean =
while the points are sufficiently open for the relatively small back EMF =
from the coil not to jump the gap, and the dielectric in the cap is also =
strong enough to hold it.=20

So now we have a coil with a magnetic field that is about to collapse. =
The back EMF caused by the collapsing field  will try and go anywhere. =
The points are now safe, they are open, with sufficient gap that arcing =
wont happen there, and the insulator (dielectric) in the cap (condenser) =
is strong enough to cope without breakdown, so the only other rout out =
for all this energy is through the secondary (which has a hell of a lot =
more turns, (if you want transformers which is what a coil is =
explained.. ask) and therefor a lot more voltage, but less current, but =
its volts that jump gaps (like spark plugs!).



Cap discharge...
charge up a big cap to hold the energy, instead of creating a stable mag =
field.
Connect this cap to a coil primary with a big mosfet or whatever you =
fancy.
Coils do not like building up magfields, they create BACK EMF to oppose =
the current trying to flow through them. This back emf is also induced =
on the secondary, but higher voltage, and this jumps the plug gap, =
thereby giving a rout for the energy to dissipate.


Before anybody kicks my ass... I know this isnt complete, and I know its =
not all strictly correct, but if ya know that... I didn't write this for =
you! This is quite good enough for most people to follow (I hope :)).

Mark.


----------
From:  SRavet at bangate.compaq.com[SMTP:SRavet at bangate.compaq.com]
Sent:  Friday, September 06, 1996 1:24 AM
To:  diy_efi at coulomb.eng.ohio-state.edu
Subject:  re: Electronic Ignition

"Tony Bryant" <bryantt at psc.fp.co.nz> Wrote:
|=20
| Does the ignition condensor/capactitor serve any function
| other than to reduce arcing across points? Do I need one
| if I switch using a transistor? (with 400V zener clamp)
|=20
| Facts, not theories, please..
|=20
| ***********************************************************
| * "Insanity is the only sane response to an insane world" *
| *                 >> bryantt at fp.co.nz <<                  *
| ***********************************************************
|=20

Are you talking about electronic ignition or point ignition?  I've never =

actually worked on a point ignition (not that old), but I'm pretty sure =
the=20
condensor stores a charge, which is discharged thru the coil to make a=20
spark.  That is CD (capacitive discharge) ignition.  Electronic =
ignitions=20
usually charge the coil primary during the dwell period, interrupting =
the=20
charge causes the field to collapse and generate a spark, no capacitor=20
needed.  HEI ignitions (from GM) are electronic, but have a condensor.  =
The=20
only purpose of the condensor is noise suppression for the radio, =
however. =20
The car will run fine without it.

--steve

Steve Ravet
sravet at bangate.compaq.com
Baby you're a genius when it comes to cooking up some chili sauce...