Electronic Ignition

[tom cloud]

>"Tony Bryant" <bryantt at psc.fp.co.nz> Wrote:
>| 
>| 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)
>| 
>| Facts, not theories, please..

        [ snip ]

>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 
>condensor stores a charge, which is discharged thru the coil to make a 
>spark.  That is CD (capacitive discharge) ignition.  Electronic ignitions 
>usually charge the coil primary during the dwell period, interrupting the 
>charge causes the field to collapse and generate a spark, no capacitor 
>needed.  HEI ignitions (from GM) are electronic, but have a condensor.  The 
>only purpose of the condensor is noise suppression for the radio, however.  
>The car will run fine without it.
>

A coil (inductor) stores magnetic energy.  When a current is passed
through it (maybe 4 amps for an auto coil) and then the exciting
voltage is removed, the collapsing magnetic field effectively tries
to keep the current at 4 amps -- impossible with no load -- so, the
voltage reverses and goes to a very high value.  That's not theory,
it's fact.

So, when points close, current begins to flow while magnetic field
expands (this is why 'dwell' is important -- need time for the field
to expand sufficiently).  When field reaches its max, the current is
also at its max.  This is all done at the applied 12 volts (approx).
The coil is an autotransformer (meaning the secondary and primary are
the same winding).  When the points open, the coil responds with a
nasty reverse voltage (maybe 300 volts on ignition coil), which is
stepped up by secondary to approx 100 times that amount -- or until
something arcs over (i.e. the plug).

Now, it's the nasty 300 volts and attempted 4 amperes that is generated
when the points open that generate a nice arc across them as they
are opening, effectively doing an arc weld job on them.  The capacitor's
job is to absorb that transient voltage / current spike for a brief time,
effectively bypassing it around your points and protecting them.

Transistor and CD systems only use the points for switching an electronic
circuit.  There is no 'real' inductance attached to the points any-
more -- therefore no real need for a cap.  In fact, the cap will
slow down the rise/fall of the signal, so it really should NOT be there.

Tom