Chapter 10 Ignition Systems
197
Goodheart-Willcox Publisher
Figure 10-12.
The flywheel magnets induce a low-voltage alternating current in the charge coil. As the alternating current passes
through the rectifier, it is changed to direct current. The direct current continues to the capacitor, where it builds up
a charge. When the capacitor nears its full charge, the flywheel magnets induce a small current in the trigger coil.
The current briefly activates the silicon controlled rectifier (SCR), which allows the 300V stored in the capacitor
to discharge through the primary windings of the spark coil. This induces a much higher voltage in the secondary
windings, which fires the spark plug.
N S
CDI module
Magnet
Alternating
current (ac)
Rectifier
Direct
current
(dc)
Capacitor
Charge coil
Direct current
(dc) maximum
300V
Spark coil
Spark plug
High voltage
maximum
30,000V
Magnet
S N
Trigger coil
To operate SCR
1//2V to 1V
SCR
Goodheart-Willcox Publisher
Figure 10-13.
At low speeds, the flywheel magnets induce a small
current in the trigger coil, which turns on a silicon
rectifier at BTDC for easy starting.
Retard
N
S
Flywheel
Magnet

Counter
weight
Goodheart-Willcox Publisher
Figure 10-14.
At 800 rpm, stronger trigger coil current turns on the
silicon rectifier at 29° BTDC for satisfactory ignition
during normal engine operation.
N
S
Flywheel
Magnet
29°
Counterweight
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