winding. The increased magnetism created by the
higher voltage across the winding causes the lower
contacts to separate. Field current then flows through a
resistor, resulting in reduced field current. This reduced
field current causes the alternator voltage to decrease,
which decreases the magnetic pull of the voltage
regulator shunt winding. The spring causes the contacts
to close, and the cycle then repeats many times per
second to limit the generator voltage to a preset value.
As the alternator speed increases even further, the
resistor connected across the contacts is not of
sufficiently high value to maintain voltage control on
the series (lower) contacts. Therefore, the voltage
increases slightly, causing the upper or shorting
contacts to close. When this happens, the alternator
field winding is shorted and no current passes through
the winding. With no current in the field winding, the
alternator voltage decreases, which also decreases the
Figure 6-36.--Semitransistorized regulator.
magnetism in the shunt winding. The upper or shorting
contacts open. With these contacts open, field current
battery. Current passing through the field relay winding
flows through the resistor and the field winding. As the
creates magnetism that attracts the armature to the core,
voltage increases, the contacts close.
This cycle then repeats itself many times per
field winding F2 to the battery. The field circuit is
second to limit the alternator voltage to a preset value
completed to ground through the emitter-collector of
during high speeds. The regulator controls the
the transistor, the emitter-base of the transistor, and the
alternator output voltage throughout it s operating
voltage regulator contact points, which are normally
held closed by the helical spring.
When the field relay contacts close, the two
windings on the voltage regulator are also connected
across the battery. The resulting magnetism is not
strong enough, however, to overcome the adjusted
In the semitransistorized type of regulator, a single
tension of the helical spring, and causes the voltage
transistor works with a conventional voltage regulator
regulator contacts to open. The alternator field circuit
unit that contains a vibrating-contact point to maintain
is, therefore, completed to ground as soon as the
the alternator voltage at a preset level. The rotor current
ignition switch is closed.
passes through the emitter-collector of the transistor.
The rotor current is turned on and off by opening and
closing the emitter-base circuit through the regulator
current, producing a dc voltage at the BAT terminal on
contact points. In this arrangement, current passing
the alternator when it is in operation. When the
through the contact points is greatly reduced. The
alternator speed increases, the voltage increases. This
service life of this regulator is longer than that of a
voltage is impressed across the two windings on the
conventional vibrating-contact regulator.
voltage regulator unit. When the voltage reaches the
value at which the magnetism created by both windings
NOTE: For a complete and detailed explanation of
overcomes the spring tension, the armature is attracted
transistors and transistor theory, refer to NEETS,
toward the core and the contact points separate.
With the voltage regulator contact points open,
An example of this type of regulator is the
there is no emitter-base current, and consequently no
four-terminal unit shown in figure 6-36. It consists of a
emitter-collector current. The alternator field current,
field relay, a transistor, and a voltage regulator relay.
therefore, is turned off when the voltage regulator
When the ignition switch is closed (fig. 6-37), the
contacts are open. With no field current, the generated
winding on the field relay is connected across the
voltage immediately decreases. This smaller voltage