SYSTEM DESCRIPTION (cont)
DIM / TEST
AFT FUEL CELL
PILOT / CPG
28 VDC BUS 1
28 VDC BUS 2
MANUAL DIM / BRIGHT
AUTOMATIC DIM / BRIGHT
AUX PWR LAMP TEST
LOW FUEL WARNING
LOW FUEL WARNING
TGT 1 SENSOR DATA
TGT 2 SENSOR DATA
EMERGENCY 24 VDC
SDC Functional Block Diagram
(g) The FSC (fig. 1019) operates on 28 VDC from the SDC except during refueling. During refueling
the FSC operates on 24 VDC from the battery.
(h) The fuel cells act as the sensing capacitor for the system. The capacitive value is determined by the
level of fuel between the inner and outer tubes of the sensor, density of fuel, and the shape of the inner tube.
(i) The FSC derives its operating voltages from an internal power supply. A 6 kHz oscillator provides a
LO-Z voltage across the fuel tanks. The amount of current the tank passes from the LO-Z unshielded line to the
HI-Z shielded line is directly proportional to the mass of fuel in the tank. The forward and aft signal conditioners
amplify and rectify the HI-Z signals, producing zero to 5 VDC to the fuel indicators. Internal empty adjustment is
made at transformer centertaps at the oscillator. Internal full adjustment is made in the dc amplifier. The FSC
contains built-in-test (BIT) circuitry. BIT provides a known signal to functionally check the FSC by full fuel cell
simulation. BIT is activated by an external test switch on the FSC.
(j) The pilot fuel quantity indicator receives operating and lamp supply voltages from the SDC. The FSC
supplies aft and forward fuel cell data to the indicator. The pilot dim/test panel provides display lamp test, digital
blanking and display lamp dim/bright automatic control. The pilots indicator edge-lighting is controlled by the pilot
EXT LT/INTR LT panel.