SECTION 55 - ELECTRICAL SYSTEM
55-124
The basic operating principles are explained as
follows:
1. When the key switch is closed, current from the
battery flows through the indicator lamp and
resistor R6, which are in parallel, to the field coil.
From here it continues to flow on through the field
coil to ground, completing the circuit back to the
battery.
Battery Key Switch
Indicator lamp
Terminal R-
Resistor R6
Alternator
terminal L
Field Coil Tr2 Ground
2. When the alternator begins to rotate, A.C.
voltages are generated in the stator coil. The
diodes in the rectifier assembly change the stator
A.C. voltages to a D.C. voltage which appears
between the ground and terminal, B.
The stator also supplies D.C. field current
through the diode trio, the field coil, Tr2, and then
through the diodes in the rectifier assembly, back
to the stator.
Diode trio
Field Coil Tr2 Ground
3. When the generated voltage is low, no current
flows in the Zener diode (DZ) since the voltage at
point A is lower than the Zener voltage.
4. As the speed and voltage increases, the voltage
at point A also increases until it reaches the
limiting value set by the factory. As the Zener
diode (DZ) breaks down, current flows through
R1, DZ, and the base-emitter circuit of Tr1 to
ground. This renders Tr1 conductive, so that
much of the current flows through the
collector-emitter circuit of Tr1. This reduces the
base current of Tr2 thereby reducing the field
current. This means that Tr1 turns on and Tr2
turns off.
Regulator
Terminal L
R1 DZ
Tr1 Ground
Regulator
Terminal R
R6 R5
5. When the generated voltage decreases, the
Zener diode (DZ) again turns off and Tr1 also
turns off.
This cycle then repeats many times per second
and the alternator output voltage is, therefore,
regulated within a narrow limit.
In other words, the action is similar to the
conventional vibrating-contact regulator, in that
current to the field coil is varied to limit the output
voltage, but in place of the voltage coil and spring
system, there is a potential divider (R1 and R2)
and a Zener diode.
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