Piper COMANCHE PA-24-180 Service Manual page 576

PIPER COMANCHE SERVICE MANUAL
9-6lm. Regulator (Pestolite).
W
9-61n. Reguleter Cmapnents.
Alternator output voltage can, within the limits of the design
capability of the alternator, be controlled by properly varying the average level of current flow
in the rotor winding and the Prstolite
full electronic solid state regulator is well suited for this
purpose. The Prestolite alternator, due to its design, has self-limiting current characteristics
and needs no current-limiting unit in the regulator.
a. Transistor: The transistor is an electronic switch which can turn on and turn otf the flow
of current in an electric circuit. It has no mechanical or moving parts to wear out.
b. Rectifier Diode: The rectifier diode will pass current in one direction only (forard
direction); and in ths respect, it may be compared to a one-way check valve.
c. Zener Diode: The Zener diode, in addition to passing current in the forward direction,
will pass current in the reverse direction only when a particular value of voltage
Ls
applied in
the reverse direction. It is this Zener action which makes it adaptable for use as a voltage
sensing device in the regulator.
d. Resistor: The resistor is a device which is used to limit current flow.
9-61o. Operation Of Regulator. (Refer to Figure 9-27p.)
a. When the alternator switch is turned on. battery voltage is applied to the "I" terminal of
the regulator.
b. The npn (negative-positive-negatve)
power transistor. T3, is turned on by current flow
from the ignition terminal through R6 and the collector emitter junction of T2 through D2 through
the base emitter junction of T3 to ground.
c. Whenever the power transistor, T3, is on (T2 is also on and TI is off), current will flow
from the ignition terminal through the field winding, through the collector
emitter of T3 to
ground.
^
d. With the ignition switch on, current will flow from the I'" terminal regulator ground through
a voltage dividing network consisting of RI, R2 and Pl. This network determines the system
operating voltage relative to the Zener diode, Zl, reverse conducting voltage.
e. When the system voltage connected to "I" terminal reaches a value at which the Zener
diode connected to the divider network conducts, current will flow from the "I" terminal through
Rl through Z1 and through the base emitter junction of T1 to ground. This causes the collector
emitter junction of Tl to conduct which diverts the base current of T2 flowing from ''I" terminal
through R4 to ground, turning off T2 which turns off T3, de-energizing the rotor winding; then,
when the alternator outputvoltage falls to a value which permits Zl to cease conduction, T1 will
turn off which turns on T2 and T3, reenergizing the rotor winding.
f. This sequence is performed so rapidly that the rotor current average appears as a value
usually less than full rotor current depending on rotor RPM and system load connected.
g. Each time the power transistor, T3. is turned off. current flow in the rotor winding is
reduced. This causes the rotor magnetic field to collapse which would generate high voltage at
the power transistor. T3. if a path were not provided so that the field current can decay at a
slower rate. The field suppression diode, Dl, provides this path. thus protecting the system and
regulator from possible damage.
h. Temperature compensation is flat which means the regulator will hold the alternator out-
put voltage constant with temperature increase or decrease after initial warm-up.
i. The Prestolite solid state regulator uses three npn silicon transistors.
ELECTRICAL SYSTEM
ISSUED: 8/18/72
3C6