MadSci Network: Physics

Re: Why does an iginition coil only fire when the power connection is released?

Date: Thu Nov 2 15:39:57 2000
Posted By: Gus Calabrese aka puppet boy, Design Engineer
Area of science: Physics
ID: 972527251.Ph

Hello Andrew

Since I don't know how much you know, I may cover something you are familiar 

The ignition is a transformer.  This means it is two coils that are 
magnetically coupled to each other.  One of the coils is called the primary 
and the other is called the secondary.  The secondary might have 100 times 
as many winding as the primary.

When you attach a power source to the primary, current starts to flow in the 
primary coil.  This causes a magnetic field to be created.  The field builds 
quickly to full strength.  The field does not build instantly because the 
increasing magnetic field (in the primary) actually resists any further 
increase.  (This is called self-inductance).  Finally the primary reaches 
steady state.  The field is maximum and the current flowing in the primary 
is limited by the resistance of the primary coil and the voltage of the 
power source.

The magnetic field that is created by the primary has moved across the 
secondary coil as the field expanded.  The expanding magnetic field causes 
the secondary coil to develop a voltage to force current to flow.  Since the 
secondary coil has many more windings, it develops a larger voltage.  If the 
ratio (as I originally made up) is 100, the secondary will develop 100 times 
the voltage to drive the current.  So we might see 1200 volts from a 12 volt 
primary source.

You can test the validity of the paragraph above by winding your own 
transformer.  You can also pass a large magnet by a coil of wire and observe 
that a voltage is produced in the coil.  This is because the magnetic field 
of the permanent magnet is moving across the coil.  If there is no movement, 
there is no voltage created.

Now for the big bang. When you disconnect the power source from your coil, 
the current flow through the primary stops.  The magnetic field collapses 
very rapidly. The lines of magnetic force move past the windings in the 
secondary more rapidly in the collapse than when the magnetic field was 
built.  The secondary voltage can be many more times the 1200 volts it may 
have reached when the coil was energized.  12,000 volts might be achieved.

IF you still have questions, write back to MadSci.

Gus Calabrese

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