Lesson 1, Topic 1
In Progress

X-Ray Circuit

April 11, 2024

X-Ray Circuit

Learning Objective: Examine the x-ray circuit.

X-ray circuits help you to understand how three relatively complex electric circuits are integrated to produce x-rays. As indicated in FIGURE 37.13, the x-ray circuit is divided into three sections or subcircuits: the low-voltage, filament, and high-voltage.

Low-Voltage Circuit

Learning Objective: Define low-voltage circuit.

The low-voltage circuit is the subcircuit between the AC power supply and the primary (input) side of the high-voltage (step-up) transformer. The current flows from the AC power supply to the step-up transformer. From the transformer, it returns to the power source, forming an enclosed loop. Except for the step-up transformer, all of the devices in the subcircuit are located within the control console. The control console is where the operator sets all exposure factors, such as kVp and mAs. They include the main switch, autotransformer, kVp selector, exposure switch, and exposure timer.
The AC power supply is wired into the wall, providing electric power from the local power company. The autotransformer is a single-coil transformer that serves three functions: it provides for kVp selection, provides form compensation for the incoming line voltage fluctuations, and supplies power to other parts of the x-ray circuit. The autotransformer’s primary purpose is to vary the voltage to the primary side of the step-up transformer. This is accomplished by the kVp selector, which is on the secondary (output) side of the autotransformer. The autotransformer varies the kVp to the tube by controlling the input to the step-up transformer.
The exposure switch closes the circuit, allowing electric current to flow through the primary side of the step-up transformer. When this occurs, the current is induced to flow through the secondary side of the transformer, creating voltage across the x-ray tube. This voltage causes the electron stream to flow across the x-ray tube, producing x-rays. The exposure timer terminates the exposure and is set by the operator on the control console.

Filament Circuit

Learning Objective: Define filament circuit.

The filament circuit is another circuit within the main x-ray circuit as seen in FIGURE 37.13. This circuit is divided into two parts by the step-down transformer. The filament circuit supplies low current to heat the x-ray tube filament for thermionic emission. When the operator adjusts the mA on the control console, the filament circuit is activated.
The primary side of the circuit begins and ends with the contacts on the autotransformer. Current in this circuit flows from the autotransformer, the mA selector, the primary side of the step-down transformer, and back to the autotransformer. The secondary side begins and ends with the secondary side of the step-down transformer conducting current through the x-ray tube filament. The step-down transformer reduces the voltage on the secondary side, providing an appropriate current to heat the filament.
The mA selector controls amperage in the filament circuit. Because the current through this circuit controls filament heat, this setting determines the number of available electrons at the x-ray tube filament. It determines the mA in the high-voltage section that includes the x-ray tube.

FIGURE 37.13  Schematic of a circuit of an x-ray imaging system. From Bushong SC: Radiologic science for technologists: physics, biology, and protection, ed 12, St. Louis, 2021, Mosby.

High-Voltage Circuit

Learning Objective: Define high-voltage circuit.

The high-voltage circuit begins and ends with the secondary side of the step-up transformer. The rectifiers and the x-ray tube are located in the high-voltage section. During an exposure, the current is flowing. The high-voltage transformer is a step-up transformer because it increases the incoming voltage in proportion to the transformer ratio. The primary purpose of the high-voltage circuit is to supply the x-ray tube with voltage high enough to create x-rays. The autotransformer, step-down transformer, and high-voltage transformer are all located in a cabinet in the x-ray room.