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LR3D Technical Manual
Section A
Rev. IR (5/13/2020)
Introduction
Terminal 3 - This input is used by the LR3D to accurately sense the bus voltage for the purpose of
voltage regulation and low voltage warning. If no bus voltage is provided to this input, the
regulator will not operate.
Terminal 4 - This output provides the alternator field excitation and connects to the field terminal
of the alternator. Field fault protection is provided within the LR3D to protect the LR3D if a fault
occurs on the field wire or alternator field winding. In the event of excessive field current, the
LR3D will current-limit this output and trigger the OV crowbar circuit to open the 5A alternator
field circuit breaker. The LR3D cannot be used with alternators requiring 4A or more field
current.
Terminal 5 - This output should be connected to the ground side of a warning indicator. The other
side of the warning indicator should be connected to aircraft power and can potentially be added
to another appropriate system circuit breaker if circuit breaker space is limited. The LR3D is
compatible with both incandescent and LED warning indicators and provides a pulsing ground to
the warning indicator when the low voltage warning is active. Care should be taken to use a
warning indicator that does not exceed 250mA to avoid damaging the lamp-driver circuit within
the LR3D. Do NOT connect aircraft bus voltage directly to terminal 5 and do NOT use a plain
LED without a current-limiting resistor.
Terminal 6 - This input is connected to the aircraft bus by means of a 5A circuit breaker and an
alternator switch. Field current is sourced from this input; voltage sense for OV protection is also
provided from this input. If the crowbar OV circuit is triggered due to an actual OV event, a field
fault, or a manual OV test, the LR3D internally connects this terminal to ground, causing the 5A
alternator field circuit breaker to open.
Terminal 7 - This input is connected to aircraft ground. The ground path must be capable of
providing a connection with less than 0.5 Ohms of resistance between terminal 7 on the LR3D
and the battery negative (-) terminal.
Case Ground - This redundant input is connected to aircraft ground, and should be totally
independent from the wiring associated with the terminal 7 input. The ground path must be
capable of providing a connection with less than 0.5 Ohms of resistance between the threaded
Case Ground post on the LR3D and the battery negative (-) terminal.
Voltage Adjustment - This precision potentiometer, covered by a round black cap, permits
plus/minus user adjustment of the LR3D output voltage set-point (see below, "System Function").
SYSTEM FUNCTION
The LR3D is pre-set to control alternator output voltage at 14.4V (or 28.8v on the LR3D-28). If
voltage adjustment is desired, this may be accomplished by accessing the adjustment
potentiometer located behind the round black cap on the side of the LR3D enclosure. Each full
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