Piper Navajo PA-31 Service Manual page 732

PIPER NAVAJO SERVICE MANUAL
11-44. OPERATION OF REGULATOR (PRESTOLITE). (Refer to Figure 11-32.)
a. When the alternator switch is turned on, battery voltage is applied to the "I"
terminal of the regulator.
b. The npn (negative-positive-negative) 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 T1 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 R1, R2 and P1. This network
determines the system operating voltage relative to the Zener diode, Z1, 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 R1 through Z1 and through the base emitter junction of T1 to ground.
This causes the collector emitter junction of T1 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 output voltage falls to a value
which permits Z1 to cease conduction, T1 will turn off which turns on T2 and T3,
re-energizing 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, D1, 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
output voltage constant with temperature increase or decrease after initial warm-up.
i. The PRESTOLITE solid state regulator uses three npn silicon transistors.
j. Capacitor, C1, is used to filter ripple and alternator diode switching spike when
operating batteryless.
k. Neon lamp, L1, provides transient voltage protection acting as a surge suppressor.
1. Control P1 is used to provide a limited range of voltage adjustment.
11-45. OPERATION OF REGULATOR (LAMAR). (Refer to Figure 11-33.)
a. When the alternator is turned on, battery voltage is applied to the "BUS" terminal
of the regulator and via Q4 through the "FIELD" terminal of the regulator to the alternator
field terminal F2. The amount of voltage applied to the field of the alternator is controlled
automatically by action of the regulator in response to alternator output as described below.
b. Current flow through R6 and Z1 establishes a reference voltage across Z1.
c. Resistors R1 and R2/R3 comprise a voltage divider which is adjustable by means of
the variable portion R3. Voltage at the junction of R1 and R2 and the reference voltage
across Z1 are applied to comparison transistor Q1. R3 is adjusted so that these voltages are
balanced with the desired alternator output voltage present on the "BUS" terminal of the
regulator.
Reissued: 10/12/79
ELECTRICAL SYSTEM
4C6