CPI CMP 200 Series Service Manual page 289

7
Theory of Operation
7.2.6 Filament drive and mA control (Cont)
Refer to MD-0934, sheet 2:
When an exposure is in process, X-ray tube current flows through series resistors on the HV anode and
cathode boards. The voltage developed across these resistors, which is proportional to the X-ray tube
current, is taken to J9 on the generator control board. Transient protectors on the high voltage boards
clamp the voltage across the series resistors during high voltage arcs.
The grounded mid-secondary circuit in the HT tank is connected to the mA test jacks E17 / E18 on the
tank lid board. Transient protectors on the tank lid board clamp the voltage across the mA measuring
device during high voltage arcs, and prevent the voltage at E17 and E18 from rising above approximately
15 volts if the ma test jack is removed and no measuring device is connected.
The anode mA feedback that appears at J9-1 and J9-2 on the generator control board is scaled
approximately 0.4 volts = 100 mA of anode current, and is applied to the input of differential amplifier
U68B.
The output of U68B is taken to comparator U47, which detects high anode mA. U47 will send a fault pulse
to the CPU via U78 if an anode over-current condition is detected, causing the kV output to be inhibited
immediately, thus preventing generator damage due to the over-current fault.
The output of U68B is also fed to the input of U68A, which provides a scaled mA feedback voltage. The
mA is calibrated by R160 such that 1 volt = 100 mA at the output of U68A. The output of U68A is taken to
the data bus via A/D converter U45, and also to voltage amplifier U74A, which provides a better quality
(higher gain) feedback signal at low anode currents. The mA feedback information is used by the CPU to
regulate the X-ray tube mA and to perform mA monitoring functions during exposures.
The cathode mA feedback at J9-4 and J9-3 on the generator control board is used for cathode over-
current detection only. The cathode mA feedback is fed to the input of differential amplifier U56A. The
output of U56A is taken to comparator U50, which detects high cathode mA. U50 will send a fault pulse to
the CPU via U78 if a cathode over-current condition is detected, inhibiting the X-ray exposure, thus
preventing generator damage due to the over-current fault.
7.2.7 Dual-speed starter (MD-0924)
The generator CPU decides whether the pending exposure should be made at low-speed or high-speed
operation. Based on this determination, the CPU will output the high-speed / low-speed command via
U86 and U83 on the generator control board. The output of U83 lights DS10 on the generator control
board, and energizes U12 on the dual-speed starter board. The dual-speed starter CPU monitors the
output of U12, and sets low or high-speed operation based on the state of U12.
The prep command is also output by the generator CPU and latched by U86. Driver U83 lights DS11 on
the generator control board, and energizes U13 on the dual-speed starter board. The dual-speed starter
CPU monitors the output of U13, and starts the boost cycle when the output of this opto coupler is low.
K1 on the dual-speed starter board will close at the start of the prep cycle. K4 will close at the same time
that K1 is energized. This ensures that the high voltage is isolated from the stator terminals at all times
except during normal operation of the dual-speed starter.
The dual-speed starter contains an inverter, Q1 to Q4, which produces the required stator current at 50,
60, 150, or 180 Hz by precisely switching the 560 / 650 volt DC bus. The dual speed starter CPU controls
the switching of the inverter via the driver circuit consisting of U1-U9 and T1-T4, etc. The setting of DIP
switch SW1 determines all stator drive parameters (boost voltage and boost time, run voltage, brake
voltage and brake time, etc).
Use and disclosure is subject to the restrictions on the title page of this CPI document.
Page 7-8
®
CMP 200 X-Ray Generator Service Manual
CPI Canada Inc.
Ch # 901476-09 Rev. C