[Ford-ferguson] question

[Stephen Offiler]

On Mon, Jul 14, 2008 at 9:59 PM, Ferguson,Ernie <FERG at nwmissouri.edu> wrote:

> The condenser's job is to build up (condense) electric charge (today it would be called a capacitor).  This charge is released all at once when the points close.  This causes the spark plug to arc.
>
> Ernie Ferguson
> Maryville, MO


Indeed, Ernie, a condenser is a capacitor.  However, with all due
respect, the rest of your description contains inaccuracies.

Here's the deal.  The high-voltage spark is created by the coil, which
is a transformer, with a primary (low-voltage) side and a secondary
(high-voltage) side.  The primary side connects to the battery and
points, and the secondary side connects to the spark plug.  When the
points are closed, a circuit is completed, and current flows through
the coil primary.  This "charges" the coil's magnetic field.  When the
points open, current stops, and the field collapses.  The energy of
this field is transferred magnetically to the secondary, creating a
brief high-voltage pulse as the field dissipates.

A problem is created in switching the coil on and off rapidly.  The
coil primary has very high inductance.  It is the nature of inductance
to attempt to oppose a change in current.  When the points open, the
circuit is broken therefore current wants to go to zero NOW, but, the
inductance of the coil fights this change.  As the inductance
continues to try to drag current where current can no longer flow
(points are open) a high voltage is created at the points.  By "high
voltage" we're talking a few hundred volts, nothing like the spark
plug voltage which is in the thousands of volts.  A few hundred volts
is enough to create an arc across the points, causing rapid erosion
and failure if left unchecked.

Enter the condenser.  It connects across the points to ground and
absorbs that high-voltage pulse coming out of the coil primary and
supresses the arc across the points.


Best regards,
Steve O.