User's Manual

1.3.2.13 Fan Control CCA Block Diagram Theory
Refer to Figure 1-19. The Fan Control CCA provides control and monitoring of up to two fans in a Model
2170 High Power DME System. The Facilities CCA has an open-drain FANS_ON signal which turns the
fans on when high. The Facilities CCA monitors the active-low ~FANS_OK signal to verify both fans are
operating properly. If ~FANS_OK is high, one or both of the fans need servicing.
The Fan Control CCA has three connectors. Power signals 1_+48V, 2_+48V, and ground from the Low
Power Backplane CCA enter via header P1. The +48VDC signals are diode-OR’D, fused and filtered
before routing to voltage regulator diodes and the fans power+ connections.
Two voltage regulator diodes create +10.2V and +5.1V supplies from the diode-OR’D +48V. The +10.2V
regulator provides bias to the power transistor that is the on/off switch for the fans. The +5.1V regulator
provides power to the watchdog circuitry for fan speed monitoring.
The Fan1 and Fan2 watchdog circuitry watch for tachometer pulses. If either fan’s speed dips too low or
stops entirely, the diode-AND output of the watchdog circuitry will go low; causing the ~FANS_OK
transistor switch output to go high.
Terminal blocks TB1 and TB2 provide connections to the fans. Each fan has a power+, a power-, and a
tachometer output. The power+ of both fans connects to the fused and filtered +48VDC supply while the
power- of both fans are switched to ground by the power transistor switch whenever the FANS_ON signal
is active. The tachometer outputs are connected to their individual fan watchdog circuitry.
Figure 1-19 Fan Control CCA Block Diagram