APX 525/625 Generator Revision C Diagnostics
©
2005 Del Medical Imaging Corporation
7/5/2005 Page 179
On the kV Regulator Board, the feedback voltages are summed, filtered, and sent to
a comparator that compares the feedback with the kV Reference voltage and is then
used to gate the core frequency oscillator on or off, resulting in closed loop
regulation of the high-voltage being delivered to the x-ray tube. Whenever the kV
feedback falls beneath the kV reference voltage threshold, the core frequency
oscillator is gated on until the kV meets or exceeds the reference voltage. The
Feedback voltages are also sent to a pair of window comparators that are used to
detect under voltage or over voltage conditions on either the anode or cathode
supplies. An error that is detected in either window comparator latches the fault
condition, shutting off the SCR drive pulses and sending a kV fault signal to the
Logic Board. The operator must reset the machine in order to clear this fault.
The summed kV feedback voltage is also sent to another comparator, which sends a
signal to the mAs Regulator board to start the mAs timing circuit when the kV has
reached 75% of its desired value.
Filament and mA Control: (see illustration X5)
The filament preheat and mA reference voltages are developed on the Data Board
and sent to the mA/Rotor Board, where they are used to regulate the filament
preheat and mA levels for each exposure. Preheat levels for all tubes contained in
the firmware libraries are predetermined and calculated to provide proper preheat
levels for each mA level that the generator utilizes. The generator automatically
selects the highest allowable mA level for the selected combination of kV and mAs
that can be performed on the selected x-ray tube, based upon the tube’s power
rating.
Control of the filament is accomplished by switching the primary windings of an
isolation transformer on the I/O Board with at a continuous rate, while varying the
voltage applied to the switching circuit.
The preheat reference voltage is fed to a network of potentiometers, which is used to
fine tune the preheat levels for the small and large focal spots. This reference
voltage is fed to a voltage regulator circuit on the Power Supply Board, where it is
used to regulate the voltage sent to the switching circuitry on the primary side of the
filament drive isolation transformer. During standby, a small voltage is used to
develop a low “keep alive” current on the selected filament of the x-ray tube.
When PREP is initiated, the preheat reference voltage is applied to the regulator
circuit and the filament is warmed to the level of the upcoming exposure. The boost
time adjustment for the stator is also used to determine the minimum time of the
Prep cycle.
When the exposure command is received, the K1 relay is closed on the mA/Rotor
Board, bringing the comparator signal from U24 into use, in conjunction with the
filament preheat reference. Actual mA feedback current is passed through sampling
resistors and the voltage is fed to the mAs Regulator Board, where it is buffered and
fed to the mA/Rotor Board.
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