a means of adjusting the potential applied to the high-tension primary winding, thus
varying the potential applied to the x-ray tube, a provision for closing and opening the
PHT circuit, and some means of predicting the potential to be produced across the x-ray
(1) In figure 3-3, the PHT circuit begins at the autotransformer (the major
kilovoltage selector), through the primary winding to the contact points, then to the
minor kilovoltage selector, and back to the autotransformer.
(2) The major and minor kilovoltage selectors are rotary tap switches. They
provide a means of increasing or decreasing the peak kilovoltage across the x-ray tube
by varying the potential across the primary winding of the high-tension transformer. The
major kVp-selector usually adjusts in increments of 10 kVp, whereas the minor kVp-
selector adjusts in increments of 1-2 kVp. Therefore, if the major kVp-selector were
provided with eight steps and the minor with ten steps, it would yield a possibility of 80
(3) The contactor is used to close and open the primary high- tension circuit.
As long as its points are open, there will be no high voltage impressed across the x-ray
tube, and there will be no x-radiation by the tube. The contactor is normally an open
relay, the coil of which is located in the operating circuit.
(4) The kilovoltage selectors must not be rotated when the PHT circuit is
energized; if this precaution is ignored, severe arcing will damage the selectors.
(5) The prereading kVp meter is connected across the primary of the PHT.
Although this meter is monitoring the low voltage input to the high-tension transformer, it
is scaled to indicate the high voltage produced in the secondary of this transformer.
b. The secondary high-tension (SHT) circuit consists of the secondary winding
of the step-up transformer, the x-ray tube, the milliammeter, and rectifiers.
(1) The step-up transformer multiplies the voltage so that the high EMF
(voltage) needed to operate the x-ray tube is produced. This means that the secondary
winding has a great many more turns than the primary winding.
(2) The milliammeter is necessary in this circuit as an indicator and has
been discussed previously.
(3) Rectification is essential, since x-ray tubes require direct current for their
operation. A complete discussion of various systems of rectification has been
presented earlier in this subcourse. It is suggested that the student review this material.
In brief, rectification is any method of changing AC to DC. In a self-rectifying x-ray
machine (this would be a small, old-fashioned field unit), rectification is accomplished by