SOUTHERN AVIONICS COMPANY
Model SA100 Dual 2-73Theory of Operation, Radiobeacon
2.4.1.1 DC Auto Disconnect PWB
The DC Auto Disconnect PWB (DCAD) is used in SAC SA Series Transmitters with power
outputs greater than 100 watts and/or ALL Dual transmitter configurations when configured with
DC option(s). The action of this PWB circumvents the low voltage cutout circuitry associated with
the DC PWB (when required) located within each RF Group. The DCAD is utilized to
automatically power down the transmitter when the external DC supply become discharged.
The DCAD operates on, and samples the 24V external DC supply voltage at TB1, pin 7. When
the external DC supply standby voltage drops below an adjustable level set by R7, the output of
U1A, pin 1 goes high, turning on Q1. This action energizes DCAD relay K1. Upon actuation of
the relay, the external DC supply supply feeding the Auto transfer system (BTTY into TB1
position 1 and out TB1 position 3) is interrupted which de-energizes the transmitter, effectively
disconnecting the transmitter load from the external DC supply. In this state, the external DC
supply will recover to approximately 23-24VDC, and continue to supply the DCAD at a 30mA rate
until AC power is re-established or the DCAD is reset.
R7 is factory adjusted to provide the disconnect at approximately 19VDC. After actuation, the
hysterisis caused by R4 and R5 keeps the transmitter powered down. The DCAD is normally
reset by the action of the AC powered external DC supply charger supplying at least 26.2VDC to
the external DC supply line. The DCAD will also reset when the external DC supply is removed
from the unit.
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