Rectification & Pulse Phases

Understanding of electronics for various types of generators is not necessary but familiarity with current waveforms is important for technique selection.
Waveform Identification:
- Half wave rectification circuits block the AC pulses traveling in the wrong direction, showing every other pulse of electricity.
- Self rectification exhibits the same waveform pattern as half wave rectification.

Mechanism:
- Self rectification operates under the principle that at lower voltages, electricity does not jump backwards from the large flat anode disc to the filament.
- At elevated voltages, this backward jump can occur, potentially damaging the x-ray tube.
Application:
- Some portable and dental x-ray units utilize self rectification.
- Both self and half wave rectification utilize only half of the electricity supply:
- For a supply of 60Hz, only 60 electrical pulses circulate through the x-ray tube due to the blocking of the other 60 pulses by the diode or, in the case of self rectification, by the x-ray tube itself.

Requirement:
- At least four diodes are required for full wave rectification.
Pulses Generated:
- Full wave rectification generates 120 pulses from a 60Hz supply.
- This allows for halving the exposure technique needed for achieving the same radiographic exposure compared to both half wave or self rectification methods.
Single Phase Power:
- Self, half wave, and full wave rectification are all forms of single phase power.
- In this context, the voltage ripple fluctuates from 0% to 100%.
Ripple Explanation:
- Full and half wave rectification exhibit a 100% ripple, indicating kilovoltage (kV) traverses from zero up to 100%.
- Average kV is one third of the maximum kilovolt peak (kVp), leading to clarification of terms:
- Kilovoltage settings are referred to as kVp (kilovoltage peak) since the effective energy utilized is reduced by the effect of ripple.

Introduction:
- Developed to provide more continuous current flow and higher average voltage by increasing the pulse generation rate.
Three Phase Six Pulse Rectification:
- Achieved by incorporating specialized circuitry that results in six overlapping pulses of electricity during each cycle.
Advantages Over Single Phase:
- Voltage minimum does not drop to zero due to pulse overlapping.
- Average kV rises from 33% to approximately 91% of the set kV, enhancing overall penetration.
- Ripple diminishes from 100% down to 14% with the three phase six pulse generator.
- Allows for increased and more consistent amperage (current).
- Enhancements in both quality and quantity when switching from a single phase to a three phase six pulse generator lead to a reduction in required radiographic techniques by half.

Three Phase Twelve Pulse Generator:
- Innovation allows for an increase in efficiency relative to the three phase six pulse generator.
- Provides an average output of approximately 97% of the set kV with an increase in pulses to 720 per second.
- Ripple is reduced from 144%, contributing to better penetration.
- Only a minor reduction in radiographic technique is required compared to the three phase six pulse generator.
High Frequency Generators:
- Introduced to further improve efficiency.
- Operate at Hertz levels between 500 to 25,000, producing a near-constant kV output with less than 1% ripple.
- The average kV is said to equal the set kV.
Advantages:
- High frequency generators lead to a further decrease in required radiographic techniques, often reflected in kV settings.
- MA (milliampere) settings can be as high as 800 to 1,200 MA, significantly reducing exposure times, which is particularly beneficial for:
- Pediatrics
- Fluoroscopy
- Interventional radiography
- Other notable benefits include affordability, efficiency, and reduced spatial requirements for high frequency generators.