g. From leads XLO, XSO, XLU, and XSU are four shunting resistors which are
adjustable to provide sufficient current for the focal spots throughout the ma-selectable
range. These resistors do this by shunting excessive current to neutral. From the
back-up timer comes lead XBR which is active during standby only. Filament current
during standby is limited by adjustable resistor R5 called the standby-filament-current
sink. This resistor shunts excess filament current to neutral during standby to ensure the
filaments are illuminated at a low level which provides longer X-ray tube life with the
convenience of preheated filaments.
h. The positive filament-drive-feedback voltage, the positive or negative
space-charge voltage, the positive current- feedback voltage, and the negative
filament-set voltage are all summed into pin 1 of IC2. The output of IC2 at pin 12 (TP7)
will be a positive voltage which varies according to the ma and kvp selected. This
voltage is amplified and becomes the filament-drive voltage into the base of transistor
MJE340 in the cooling chimney. This transistor causes the conduction of the MJ15001
transistor, varying the conduction of the diode bridge, increasing or decreasing the
impedance of the transformer.
station is selected, the current decreases and the voltage drop across the primary
increases resulting in lower voltage to the filament transformer primaries. As the
impedance decreases, such as when a high ma station is selected, the current
increases and the voltage drop across the primary decreases resulting in a higher
voltage to the filament transformer primaries.
j. A cooling fan is provided for the components in the cooling chimney to
prevent overheating. These components are always in a conducting state, even during
standby. The filament circuit is designed in such a manner, that even if no filament
drive was available, the X-ray tube filaments would not burn out because of over
SPACE-CHARGE COMPENSATION, CIRCUIT FUNCTION DESCRIPTION 3.1
Refer to figure 2-5. The reference point, at which the space-charge
compensation is zero, is at 78kvp. This reference point is adjusted by the P30 set-point
adjust. This potentiometer (pot) is adjusted so pin 10 of the 747 op-amp is at zero volts
(TP4) at 78kvp.
a. On lead MVH is a dc voltage representative of the selected kvp which is input
to IC9 at pin 2, the inverting input. The output of IC9 at pin 6 (TP17) will be a negative
voltage proportional to the selected kvp and is input to the 747 op-amp at the inverting
input, pin 7. The output of the 747 op-amp at pin 10 (TP4) will be a positive voltage
above 78kvp and a negative voltage below 78kvp.