1. X-ray Generators and Transformers
Rad T 290
X-ray Generators and Transformers

2. Transformers are principal components of generators
Principal function of x-ray generator is to provide current at high voltage to the x-ray tube
Electrical power available to hospital/clinic provides up to about 480 V; need 20,000 to 150,000 V
Transformers are principal components of generators
Convert low voltage into high voltage through process of electromagnetic induction

4. Transformers
“Transform” alternating input voltage into alternating output voltage using principles of electromagnetic induction

5. Transformer Law
The ratio of the number of coil turns in the primary winding to the number of coil turns in the secondary winding is equal to the ratio of the primary voltage to the secondary voltage

6. Transformers (cont.) Transformer can increase or decrease voltage
Depends on ratio of the number of turns in the two coils
NS > NP: “step-up” transformer, increases secondary voltage
NS < NP: “step-down” transformer, decreases secondary voltage

8. Power
Power is the rate of energy production or expenditure per unit time (1 Watt = 1 J/s)
For electrical devices, P = IV
For an ideal transformer:
Decrease in current must accompany increase in voltage, and vice versa

9. Autotransformers
Consists of a single coil of wire wrapped around an iron core
Law of Transformers still applies
Operates on principle of self-induction rather than mutual induction
Smaller increases or decreases in secondary voltage than normal transformers
Does not electrically isolate primary from secondary circuit

11. Diodes - Rectifier
Electrical devices with two terminals that allow current flow in one direction only
Example of a diode is the x-ray tube itself
Solid-state diode contains a crystal of a semiconductor material
Crystal “doped” with trace amounts of impurity elements
Conductivity increased when voltage applied in one direction but reduced to very low level when voltage applied in opposite polarity

14. Operator Console
Operator selects the kVp, the mA (proportional to the number of x-rays in the beam at a given kVp), the exposure time, and the focal spot size
Peak kilovoltage (kVp) determines x-ray beam quality (penetrability), which plays a role in subject contrast
Tube current (mA) determines the x-ray flux
Selection of focal spot size usually determined by mA setting
Some generators support preprogrammed techniques

23. Voltage ripple
Voltage ripple of a DC waveform is defined as the difference between the peak voltage and the minimum voltage, divided by the peak voltage and multiplied by 100:

25. Timing the x-ray exposure
Digital timers have largely replaced electronic timers (high reproducibility and microsecond accuracy)
Mechanical switches only used in single-phase, low-power generators
High-voltage triode switches used in 3-phase and constant potential circuits
High-frequency inverter uses electronic switching on the primary side of the high-voltage transformer

26. Phototimers
Measure the actual amount of radiation incident on the image receptor
Terminate the x-ray production when the proper amount is obtained
Provides a consistent exposure to the image receptor by compensating for thickness and other variations in attenuation in a particular patient and from patient to patient

28. Falling-Load Generator
Works in concert with the phototiming (AEC) subsystem
Delivers the maximum possible mA for the selected kVp by considering the instantaneous heat load characteristics of the x-ray tube
Continuously reduces the power as the exposure continues
Delivers the desired amount of radiation to the image receptor in the shortest possible exposure time