b. Additional devices are found in most x-ray tube filament circuits. They include
a filament limiter and a filament meter.
(1) Filament limiter. The filament limiter is an adjustable (strap-type)
resistor, which is not available to the operator. By adjusting this resistor, the equipment
repairman can limit the maximum tube current. This adjustment is needed to keep the
x-ray tube within safe operating limits.
(2) Filament meter. A meter placed in the filament circuit can be used to
good advantage. The x-ray tube filament temperature determines the x-ray tube
current. This temperature is a function of the voltage and current of the primary of the
x-ray tube filament circuit. Either a voltmeter or an ammeter on the primary circuit can
be used to predict, with accuracy, the x-ray tube current before high voltage is applied
to the tube. Normally, only one of these meters would be needed. Either meter, when
used, requires preparation of a chart correlating the meter readings with the actual
milliamperage obtained during exposure. The use of a fixed resistance selector method
eliminates the need for a filament ammeter.
The high-tension circuit (figure 3-3) uses a step-up transformer to develop the
thousands of volts needed to produce x-radiation. It is easier to discuss the high-
tension circuit, its installation and trouble-shooting, if the primary and secondary circuits
are discussed separately.
Figure 3-3. The kVp or high-tension circuit (self-rectified x-ray machine).
a. The primary high-tension (PHT) circuit includes electrical devices and
conductors from the power source to the primary winding of the high-tension
transformer and back to the power source. The power source is the autotransformer.
The electrical devices incorporated within the basic PHT circuit of any x-ray unit include