Section 3 Engine Systems
198
fi eld around the magnets induces current in the
primary windings of the ignition coil.
The base circuit of the ignition system has current
fl ow from the coil primary windings, common
grounds, resistor (R1), base of the transistor (T1),
emitter of the transistor (T1), and back to the primary
windings of the ignition coil.
Current fl ow for the collector circuit in
Figure 10-16 is from the primary windings of the
coil, common grounds, collector of transistor (T1),
emitter of transistor (T1), and back to the primary
windings.
When the fl ywheel rotates further, the induced
current in the coil primary increases. When the
current is high enough, the control circuit turns on
and begins to conduct current. This causes transistor
(T2) to turn on and conduct. A strong magnetic
fi eld forms around the primary winding of the
ignition coil.
The trigger circuit for this ignition system con-
sists of the primary windings, common grounds,
Goodheart-Willcox Publisher
Figure 10-16.
Study how this transistor circuit is used to operate the
ignition coil. Note differences and similarities.
Flywheel Ignition
coil
Secondary
windings
Primary
windings
Magnet
Spark
plug
Ignition
switch
Control
circuit
High-tension lead
ESG circuit
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Figure 10-15.
Study the components of transistor-controlled ignition systems.
Diode (D1, D2)
A K
Allows one way current from Anode “A” to Cathode “K” as
rectifier.
Flywheel Provides magnetic flux to primary windings of ignition coil.
High-tension lead Conducts high voltage current in secondary windings to
spark plug.
Ignition coil Generates primary current, and transforms primary low
voltage to secondary high voltage.
Ignition switch No spark across gap of spark plug when switch is at
“STOP” position.
Resistor (R1, R2) Resists current flow.
Spark plug Ignites fuel-air mixture in cylinder.
Thyristor (S)
A K
G
Switches from blocking state to conducting state when
trigger current/voltage is on gate “G”.
Transistor (T, T1, T2)
C
E
B
Very small current in the base circuit (B to E) controls and
amplifies very large current in the collector circuit (C to
E). When the base current is cut, the collector current is
also cut completely.
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