Hi Barney,
As an electronics tech and field engineer with 35 years in the gun and
missile testing game at White Sand and McGregor Ranges and as a sometime
professional auto tech, I've stayed out of this until now. However,
this looks like the last post on the matter of breaker-point ignition,
so I thought I'd clear up a few points.
Barney Gaylord wrote:
BIGGA SNIP
> When the contact points open, the current in the coil primary winding >stops
>> and the magnetic field in the core suddenly collapses, >generating a HUGE
>voltage surge in the secondary winding.
When the points open current doesn't instantly stop flowing in the
primary windings of the coil. Current now flows through the condenser
and coil primary as the condenser charges up toward the 12 volt battery
voltage. As the condenser charge nears the battery voltage the current
thru it and the coil primary drop to the point where the magnetic field
in the primary collapses.
> This voltage is so high that it
> forces a current to flow across the air gaps in the dizzy and at the spark
> plug. There is a substantial amount of energy dissipated here in a very
> short period of time (the spark).
> While this big spark is happening, the contact breaker points are open
> circuit, so the remaining circuit takes an interesting path.
Yup, the condenser now discharges thru the coil secondary and the
spark plug, "powering" the spark.
> Starting with
> the battery hot terminal it goes through the ignition switch, then through
> the primary winding of the coil, then through the secondary winding of the
> coil, and on through the dizzy and spark plug to ground.
Nope. The condenser discharge current and the back EMF off the coil
primary are opposite in polarity to and greater than the battery voltage
and block any primary current from the battery.
> Also there is a
> capacitor ("condenser") wired across the contact points to ground.
Yup, about which read my explanations above.
Now
> it's time for the brain tickler and quiz.
>
> 1.) Why does the high energy spark not fry the ignition switch and all the
> other devices that draw power from it?
Because your basic premise about the points-open current flows are not
correct. Only the back EMF (Electro Motive Force) voltage induced into
the primary is present. It's small and fleeting compared to the spark
voltage.
>
> 2.) What is the function of the capacitor in this circuit?
It forms a series-tuned LC circuit with the coil secondary when the
points are opened. The discharge of the condenser thru the coil is
known as flywheel action. The condenser also serves to minimize arcing
at (and burning of) the points.
>
> 3.) Will the system generate a spark without the capacitor being connected?
In general, no. I've seen a very few that would start and run roughly
with an open condenser.
>
> 4.) Will the system generate a spark with the capacitor being shorted out
> internally?
No. It would short out the points.
> And now two questions for extra credit:
>
> 5.) Exactly how and why does the coil generate such a high voltage in the
> secondary winding when the magnetic field collapses?
The coil is a step-up transformer. Many more turns of wire in the
secondary winding than in the primary winding. When the field
collapses, the secondary produces more voltage but less current than the
primary - nothing's for free - hence the condenser to supply the current
pulse to power the spark.
> 6.) Approximately how much current is flowing in the wire from the ignition
> switch to the coil when the spark is happening (within one order of
> magnitude)?
milliamps to 0.
>
I've tried to state things for the layperson to get the drift. No
flames pls, 'cause of minor imperfections. ;^)
Charley Robinson
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