Chapter 4 Theory of Operation
HF Series X-ray Generators - Service Manual Revision W
Quantum Medical Imaging 4-7
FILAMENT CONTROL REGULATOR CIRCUIT
The Filament Control Regulator circuit is a high frequency, pulse-width-modu-
lated-based system. The circuit receives signals from the KVP & Filament Control
circuit and outputs filament current to the High-voltage Tank, which then pro-
duces the small or large filament currents for the X-ray tube. The detailed opera-
tion of the Filament Control Regulator circuit (see Filament Regulator Control
Circuit Block Diagram in Chapter 6, DIAGRAMS) is as follows:
A signal from the KVP & Filament Control circuit that controls filament current is
input to Filament Current Opto-isolator U2A. Varying the voltage of this signal
will cause the output of U2A to vary between 0 and 5 volts. This varying voltage
is fed to the input of Pulse Width Modulator U1. The output of the Pulse Width
Modulator is input to Transformer Driver U3. The Transformer Driver buffers the
output of the Pulse Width Modulator in order to supply enough current to drive
Transformer T1. Transformer T1 drives an H-bridge circuit consisting of Q1-Q4.
The output of the H-bridge is a 20 kHz signal that is output through Filament
Select K1 and Q5 to drive the filament transformer in the High-voltage Tank. The
duty cycle of the 20 kHz signal is varied by the Pulse Width Modulator depending
on the amount of filament current required by the particular exposure.
In order to complete the filament current control loop, the 20 kHz signal is run
through the primary of Current Sample Transformer T2. The Current Sample
Transformer's turn ratio is configured such that 1 amp flowing through its primary
will result in 1 volt at its secondary. The output of Current Sample Transformer
T2 is fed to the input of RMS-to-DC Converter U4. The output from RMS-to-DC
Converter U4 is a dc signal that is proportional to the RMS current flowing
through the primary of the filament transformer (in the High-voltage Tank). The
RMS-to-DC Converter's output is buffered and amplified by Amplifier/Buffer U5A
before being sent to the KVP & Filament Control circuit.
U5B is configured as a full-wave rectifier whose output is filtered by R19, R32 and
C14. The resulting voltage is input to U1 as a filament current limit signal. R25
can be adjusted to limit the maximum filament current.
The large or small filament is selected by the FIL SIZE signal from the KVP & Fila-
ment Control circuit. A high level FIL SIZE signal applied to the Filament Select
circuit turns on transistor Q5, which energizes relay K1. When K1 is energized,
the small filament is selected. A low level FIL SIZE signal will keep K1 turned off
and the large filament will be selected.
KVP & FILAMENT CONTROL CIRCUIT
The KVP & Filament Control circuit controls the high voltage and tube current
supplied to the x-ray tube. The x-ray tube current is a function of its filament cur-
rent. The filament current is controlled by a feedback loop consisting of the KVP
& Filament Control circuit and the Filament Control Regulator circuit. The KVP &
Filament Control Circuit operates as follows:
The output of the kVp divider within the High Voltage (H.V.) Transformer is input
to Anode Amplifiers U1A, U1B and Cathode Amplifiers U2A, U2B as the ANODE
FDBK and CATHODE FDBK signals, respectively. The outputs of the Anode and
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