Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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Magnetism 65
TRANSFORMERS
Transformers (Figure 4-27) are electrical devices
that work on the principle of mutual induction.
Transformers are typically constructed of a primary
winding (coil), secondary winding (coil) and
a common core. When alternating current or pulsating
direct current is applied to the primary
winding, a voltage is induced in the secondary
winding. The induced voltage is the result of the
primary winding’s magnetic field collapsing. The
principle of a transformer is essentially that of
flowing current through a primary coil and inducing
current flow in a secondary or output coil.
Variations on this principle would be coils that are
constructed with a movable core, which permits
their inductance to be varied.
Transformers can be used to step up or step
down the voltage. In a step-up transformer, the
voltage in the secondary winding is increased over
the voltage in the primary winding, due to the secondary
winding having more wire turns than the
primary winding. Increasing the voltage through
the use of a transformer results in decreased current
in the secondary winding. An ignition coil is
an example of a step-up transformer operating on
pulsating direct current. A transformer that steps
down the voltage has more wire turns in the primary
winding than in the secondary winding.
These transformers produce less voltage in the
secondary but produce increased current.
Mutual induction is used in ignition coils
(Figure 4-28), which are basically step-up transformers.
In an ignition coil, low-voltage primary
current induces a very high secondary voltage
because of the different number of turns in the primary
and secondary windings.
ELECTROMAGNETIC
INTERFERENCE
(EMI) SUPPRESSION
Until the advent of the onboard computer, electromagnetic
interference (EMI) was not a
source of real concern to automotive engineers.
The problem was mainly one of radiofrequency
interference (RFI), caused primarily by the use
of secondary ignition cables containing a lowresistance
metal core. These cables produced
electrical impulses that interfered with radio and
television reception.
Radiofrequency interference was recognized
in the 1950s and brought under control by the
use of secondary ignition cables containing a
high-resistance, nonmetallic core made of carbon,
linen, or fiberglass strands impregnated
with graphite. In addition, some manufacturers
Primary Coil
Secondary Coil
Figure 4-27.
Transformer
Transformer.
Figure 4-28. Mutual induction in the ignition coil produces
voltage across the spark plugs.