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If I may, I would like to offer a bit of clarification on the recent AC vs DC
discussion as it pertains to ignition coils. An ignition coil is, very
basically, a simple transformer. A transformer will, believe it or not, work
on DC just as well as AC, PROVIDED.....
The key to inducing a current into a coil of wire, whether it be the
secondary of a transformer or the stator windings of an alternator, is the
relative motion of the coil with respect to a magnetic field. The wire must
"cut" the magnetic field. In an alternator, this motion is rotary in nature,
and is produced by the rotation of the engine. In a transformer (coil), the
relative motion is provided by the build up and decay of the magnetic field
produced by the current in the secondary. As the magnetic field strengthens,
the lines of magnetic force are expanding, thus producing relative motion
with respect to the secondary windings. When the field decays, the lines of
force contract, again producing relative motion with respect to the
secondary.
In an AC system, the current flows first in one direction, and then in
another, producing an expanding and contracting magnetic field in the primary
winding. If you rectify this AC current, you will produce a DC current, but
it will not be a steady current, as in a car battery. What you will have is a
cyclic DC current, going from zero to a maximum value, and back to zero
again, repeatedly. The only difference between this cyclic DC and AC is that
the current never changes direction. (Ken Streeter is posting an article on
the VTR web site, which I wrote, titled "understanding alternators," which
will have a picture of this.)
To a transformer, though, it looks just like AC, as the magnetic field
expands and contracts just the same as AC. You only need to go back and look
at the vacuum tube radios used in automobiles prior to about the 60s for a
perfect example of this. As the vacuum tubes required about 300 volts DC to
operate, and the car only had 6 or 12 volts, a way had to be devised to
produce 300 volts within the radio itself. This was accomplished by the use
of a "chopper," a electro-mechanical device that turned the 12 volt supply to
the radio off and on very rapidily, acting as a switch. This produced a
series of 12 volt pulses, which were then fed to a transformer and stepped up
to the required 300 volts. This 300 volt output was also pulsed, but a very
large capacitor was used in a filter circuit to convert to a smooth DC, as we
usually think of DC.
This is exactly the way a coil works. The 12 volts DC is switched on and off
by the points, producing the required expansion and contraction of the
magnetic field around the primary, which is coupled to the secondary to
produce the high voltage output.
As you can see, it is not the reversal of current flow that is the factor
with AC transformers, but it is the build up and decay of the magnetic filed
that is important. If you can create this same cyclic action with DC, it will
work just as well.
I hope this has been of interest to some, and has not been an irritant to
any!
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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