Piper SEMINOLE PA-44-180, Maintenance Manual

Page: 221 / 881

3. Zener Diode: The zener diode (Symbol “Z”) in addition to passing current in the forward direction,
will also pass current in the reverse direction when a particular value of reverse voltage is applied.
This property makes it useful as a voltage reference device in the regular.
4. Capacitor: The capacitor (Symbol “C”) is a device which will store electrical energy for short periods
of time. This property makes it useful as a filter element to smooth variations of voltage.
5. Resistor: The resistor (Symbol “R”) is a device which is used to limit current now.
LAMAR MODEL NO. B-00288-1 REGULATOR OPERATION.
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 a p p l i e d
to the field of the alternator is controlled automatically by action of the regulator in response to alternator output
as described below.
Current flow through R6 and Z1 now establishes a reference voltage across Z1 . 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. Thereafter, whenever alternator output voltage (as applied to the “BUS” terminal) falls below the
desired regulation value, the comparison transistor Q1 will supply increased current to driver transistors Q2
Q3, which in turn will drive power transistor Q4 to a higher value of field current. This will result in alternator
output voltage increasing to a value which will restore balance between the two voltages applied to Q1.
C o n v e r s e l y, if alternator output voltage (as applied to the BUS terminal) increases due to a greater engine
speed or reduced loading of the electrical system, the comparison transistor Q1 will act to reduce current now
to the driver transistors Q2/Q3 and thus reduce the drive to power transistor Q4. This will result in a reduction
of alternator field current and automatically restore balance between the two voltages applied to comparison
transistor Q1. Capacitors C1 and C2 function, together with their related transistors, in a way to smooth
a l t e rnator output ripple and voltage spikes so that the alternator field current is controlled at a steady value.
The Lamar B-00288-1 solid state regulator controls alternator field current to a steady value as required
by the electrical load conditions and engine speed. It does not continuously switch field current between high
and low values as do mechanical regulators and the switching type of electronic regulators. The unit is
designed to provide an alternator output voltage that does not vary with ambient temperature.
REPARATION FOR TESTING LAMAR MODEL NO. B-00288-1 VOLTAGE REGULATORS.
Test regulators using the aircraft's alternator or an alternator test stand.
1. Disconnect the battery ground cable at the battery before:
A. Connecting or disconnecting a test ammeter or other test equipment
B. Before making wiring changes in the electrical system.
2. Voltmeters with test probes or clips are not recommended. Fully insulated bolted terminal connections
are best, and should be attached when all power is removed as described above.
3. When installing a battery in an aircraft, be sure that the battery negative terminal is in a position so that
this terminal can be connected to the battery ground cable for negative ground systems.
4. The regulator under test is to be mounted on a grounded metallic surface using three No. 8 screws
pulled up tight. For extended test periods the heat transfer from regulator to the mounting surface is
significant .
PIPER AIRCRAFT
PA - 4 4 - 1 8 0 / 1 8 0 T
AIRPLANE MAINTENANCE MANUAL
24 -23-05
Page 24-17
Revised: June 20, 1995
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