New Holland LS140 Workshop Manual page 548

The basic operating principles are explained as
follows:
1. When the key switch is closed, current from the
battery flows through the field excitation resistor
and resistor R6, which are in parallel, to the field
coil.
The current continues to flow on through the field
coil to ground, completing the circuit back to the
battery.
Battery Key Switch
Alternator Field Coil
terminal L
2. When the alternator begins to rotate, A.C.
voltage is generated in the stator coil. The diodes
in the rectifier assembly change the stator A.C.
voltage 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
ground.
Diode trio Field Coil
SECTION 55 - ELECTRICAL SYSTEM
3. When the generated voltage is low, no current
4. As the speed and voltage increases, the voltage
Field Excitation
Resistor
Terminal R-
Resistor R6
Tr2 Ground
5. When the generated voltage decreases, the
Tr2 Ground
55-123
flows in the Zener diode (DZ) since the voltage at
point A is lower than the Zener 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
R1 DZ
Terminal L
Regulator
R6 R5
Terminal R
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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Tr1 Ground