Pontiac FIREBIRD 1972 Service Manual page 781

ENGINE ELECTRICAL 6E-19
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Fig. 6E-25 2 7 SI Generator
tifier bridge connected to the stator windings contains six
diodes, and electrically changes the stator a.c. voltages to
a d.c. voltage which appears at the generator output termi-
nal. Generator field current is supplied through a diode
trio which also is connected to the stator windings. A
capacitor, or condenser, mounted in the end frame pro-
tects the rectifier bridge and diode trio from high voltages,
and suppresses radio noise.
No periodic adjustments or maintenance of any kind are
required on the entire generator assembly.
OPERATION
A typical wiring diagram is illustrated in Figure 6E-26.
The basic operating principles are explained as follows:
When the switch is closed, current from the Energizer
flows through the indicator lamp to the generator No. 1
terminal, through resistor R1, transistor TR3 (collector-
emitter) and the base-emitter of transistor TR 1 to ground,
and then back to the Energizer. This turns on transistor
TR1, and current flows through the generator field coil
and TR1 back to the Energizer.
With the generator operating, a.c. voltages are generated
in the stator windings, and the stator supplies d.c. field
current through the diode trio, the field, TRl, and then
through the grounded diodes in the rectifier bridge back
to the stator. Also, the six diodes in the rectifier bridge
change the stator a.c. voltages to a d.c. voltage which
appears between ground and the generator "BAT" termi-
nal. As generator speed increases, current is provided for
charging the Energizer and operating electrical accesso-
ries. Also, with the generator operating, the same voltage
appears at the "BAT" and No. 1 terminals, and the indica-
tor lamp goes out to indicate the generator is producing
voltage.
The No. 2 terminal on the generator is always connected
to the Energizer, but the discharge current is limited to a
negligible value by the high resistances of R2 and R3. As
the generator speed'and voltage increase, the voltage be-
tween R2 and R3 increases to the point where Zener diode
D-1 conducts. Transistor TR2 then turns on and TRl
turns off. With TRI off, the field current and system
voltage decrease, and D-1 then blocks current flow, caus-
ing TR2 to turn back off and TRI to turn back on. The
field current and system voltage increase, and this cycle
then repeats many times per second to limit the generator
voltage to a preset value.
INDICATOR S W I T C H
I - @ - - -
I
/
L A M P
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' r ' l
E N E R G I Z E R
3
B A T .
, R E C T I F I E R
I
BRIDGE
I
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J
Fig. 6E-26 SI- Regulator i n Charging Circuit