Hologic DISCOVERY A, Technical Manual

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Discovery QDR Series Technical Manual
2-19
capacitors. The current load in the H-bridge is sensed and adjusted. The output voltage
from the H-Bridge is up to 200 volts peak.
The Filament circuit is a regulator. It receives current feedback from the tank indicating
the tube current. 10 volts at the feedback input is equivalent to 10 milliamps of current in
the tube. The 10 volts are reduced to 5 and then fed to an A/D circuit to be converted to a
digital value for use by the control circuits. The value into the A/D is sampled towards the
end of the pulse where it is stable. After conversion, the digital value is fed as an address
to an EPROM. If the current is correct, the output of the EPROM will be 0 volts. Anything
other than 0 volts (“+” = high current, “-“= low current) will be added and accumulated in
two stages forming an error integrator. This digital error number is converted back to an
analog value and then used as the threshold of a comparator. The output of the comparator,
through additional circuitry, adjusts the duty cycle of the modulated filament voltage.
Refer to Table 2-11 for pin assignments.
2.9 X-Ray Source Unit
The X-Ray Source (commonly referred to as the Tank assembly) consists of the X-Ray
tube, the X-Ray tube filament transformer, the high voltage transformer, the high voltage
rectifier circuit, and the sensing circuits that monitor the high voltage applied to the X-ray
tube and the beam current.
Table 2-12 describes the interconnections between the X-Ray Controller Assembly and
the X-Ray Source unit. The table also identifies the X-Ray Controller Assembly board
connector and pin assignments for each interconnection signal.
Table 2-12.
X-Ray Controller Assembly/X-Ray Source Unit Interface
* A thermal overload protector is installed in series with the filament transformer
primary winding, between TB1-6 and TB1-8.
Signal Description XRC Board Controller Pin Tank
Terminal
HV_X
HV_XFMR_PRI-
High Voltage Transformer
Primary H-Bridge (JP1) JP1-1, JP1-2
JP1-3, JP1-4
TB1-3
TB1-4
FIL_XFMR_A
FIL_XFMR_B
Filament Transformer Primary I/O & Logic
(JP6) JP2-7
JP2-8
TB1-6*
TB1-9
IF+ Beam Current Sense "+" I/O & Logic
(JP6) JP2-4 TB1-10
IF- Beam Current Sense "-"
(Chassis) I/O & Logic
(JP6) JP2-5 TB1-7
TP1 Anode pulse monitor I/O & Logic
(JP6) JP2-2 TB1-12
TP2 Cathode pulse monitor I/O & Logic
(JP6) JP2-1 TB1-11