Rectification 2Nd Module

RECTIFICATION OF HIGH TENSION

Voltage Rectification

• Transformers operate with alternating current.
• X-ray tubes operate on direct voltage ( electron moving in one direction).

To convert AC to DC we use rectifiers.

RECTIFICATION

• changing alternating current into direct current.
• The device used is called a rectifier.

A symbol for rectifier

RECTIFICATION …

• Methods of rectification
• Self rectification
• Half wave rectification
• Full wave rectification

Three phase full wave rectification

Self Rectified Circuit

• Secondary of High Voltage Transformer
• mA waveform
• The x ray tube is direct connected to secondary
• winding of the high tension transformer.
• X-Ray tube acts as rectifier
• Current only flows from cathode to anode
• cathode is source of free electrons
• Rarely seen

Voltage applied to tube

Self-rectification Disadvantages

• hot anode can emit electrons
• accelerate & can destroy filament
• half of electrical cycle wasted
• Voltage applied to
• x-ray tube
• mA waveform
• Used
• Wasted

X-Rays Produced

Halfwave Rectifier Circuit

• +
• X-ray tube connected to secondary of high voltage transformer through diode rectifiers
• Alternating voltage applied to secondary of high voltage transformer

Voltage applied to tube

Halfwave Rectifier Circuit

• +
• X

Second Half Cycle:

• Diodes open
• No voltage applied to tube
• No tube current (mA)
• +

First Half Cycle:

• Diodes closed
• Voltage applied to tube
• Tube current (mA) results

–

Halfwave Rectified Circuit

• Secondary of High Voltage Transformer
• 60 pulses per second
• only positive half cycle of high tension transformer used
• inefficient
• negative half cycle wasted
• Blocked (not used)
• Applied to x-ray tube
• Output of High Tension Transformer

Applied to X-ray Tube

Fullwave Rectifier

• Four diodes
• 120 pulses/second
• exposure times half of halfwave circuit
• Secondary of High Voltage Transformer
• Voltage applied to tube

(also mA waveform)

Fullwave Rectifier

• +
• X

First Half Cycle

Second Half Cycle

• Voltage applied to tube
• (also mA waveform)
• X

+

Full-Wave Rectification

• Rectifiers
• Four diode “bridge” configuration used with single phase
• both + & – half cycle of high tension transformer used
• efficient
• circuit reverses negative half cycle & applies to x-ray tube
• Applied to X-ray Tube
• Output of High Tension Transformer

Tube

Pulsed Radiation

• single phase input power results in pulsed radiation
• Disadvantages
• inefficiency of radiation production due to the pulsating waveform
• not providing enough voltage to produce x-rays for a portion of the time.
• the inability to select short exposure times.
• Applied to X-ray Tube

Radiation Waveform

Three-Phase Generators

• Commercial power generally delivered as 3 phase
• phases 120o apart

Single Phase Power

Three Phase Power

Three-Phase Generators

• Rectifier circuit
• Inverts negative voltage
• sends highest of 3 phases to x-ray tube

To X-Ray Tube

Input 3 Phase Voltage

Rectified

Three-Phase Generators

• much higher tube ratings than single phase
• more efficient than single phase
• shorter exposures
• lower exposure

Three Phase Output

Single Phase Power

Ripple

• variation of kilovoltage from maximum
• usually expressed as percentage of maximum kV

Ripple

Ripple Example

• Ripple = 80 – 72 = 8 kVp
• OR
• 8 / 80 = .1 = 10%
• 80 kVp

72 kVp

Ripple Typical Values

• single phase
• always 100 % (kV ranges from zero to maximum)
• three phase
• 4-13%
• constant potential
• 0 %
• Medium / high frequency
• very low; approx 0.

Three Phase Output

Single Phase Output

Constant Potential or High Frequency Output

Wave Forms of Different Generator Types

• As the ripple effect decreases, the efficiency increases.

There is one more type of generator. It uses is called stored energy.

The X-ray Circuit

Filament Transformer

• Intended to lower voltage and increase current
• Allows for thermionic emission to occur at the filament
• Ultimately, provides the electrons necessary for x-ray production.

Tube current measured in mA.

Step-up Transformer

This transformer is responsible for producing the high voltages necessary for x-ray production.

Turns ratio of 500:1 or 1000:1

Autotransformer

The autotransformer works on the principle of self-induction. It has a single core and is responsible for varying the voltage.

Because of its ability to adjust voltage, the autotransformer can be either a step-up or step-down transformer.