work and energy - xraykamarul · 2012. 4. 20. · advantages • less difference in average and...
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HDR102
SCHOOL OF MEDICAL IMAGINGFACULTY OF HEALTH SCIENCES
PREPARED BY:MR KAMARUL AMIN BIN ABDULLAH
CHAPTER 8
PHYSICS FOR RADIOGRAPHERS 1
RECTIFICATION
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Learning Objectives
At the end of the lesson, the student should be able to:-
• Explain the self rectification including the definition,
disadvantage and circuit.
• Describe the single phase full wave rectification including
circuit and advantages over self-rectification.
• Explain the use of rectifiers in x-ray circuit.
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List of Contents
8.0 RECTIFICATION
8.1 Self Rectification
8.2 Single Phase Full Wave Rectification
8.3 The Use of Rectifiers in X-Ray Circuit
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8.0 Principle
• Process by which alternating current is changed to pulsating direct
current
• A device which converts AC into pulsating DC is called rectifier.
• A rectifier can be valve type or vacuum type or it can be solid
state/semiconductor/metallic type rectifier.
• X-ray tube is a sort of valve type rectifier or diode rectifier as it has two
electrodes and allows the current to flow in one direction only
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NO RECTIFICATION
HALF-WAVE
FULL-WAVE
UNRECTIFIED A.C WAVE
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• In this lesson, we will only learn TWO rectification methods:
1) Self Rectification
2) Single Phase Full Wave Rectification
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8.1 Self Rectification
• During the first half-cycle the anode is positive in relation to the
cathode so current flows and x-rays are produced.
• During the second half-cycle the anode is negative so there is no
current flow and no x-ray production.
• This is self-rectification because the tube does its own
rectification and is half-wave rectification since only half of the
current wave is utilized.
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Disadvantages
• Self-rectification is usually limited to dental and portable units.
• The highest limits are about 30 mA at 90 kV i.e limitation of mA capacity.
• They are limited so because overstepping these bounds can cause
temperature rise in the anode target to the point of thermionic emission.
• In this event the inverse voltage can drive these available electrons
towards the cathode filament with consequent damage or destruction of
the tube.
• Although the concept of self-rectification is important, it is all but
obsolete today.
Advantages
• Low in price and less bulky so that it can be portable.
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8.2 Single Phase Full Wave Rectification
• Four-valve/diode-tube rectification enables us to use the inverse voltage
for the production of x-rays and is therefore full-wave rectification.
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Advantages
• Less difference in average and peak current.
• Both the halves (+ and -) of AC cycle and voltage are being used.
• Higher output than self or half wave rectified circuits.
• Less strain on HT cables and less insulation cost.
Disadvantages
• Costly.
• More complex.
• Heavier, not easy to transport.
• Larger in size.
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8.3 The Use of Rectifiers in X-ray Circuit
• Self Rectification It is an x-ray generator in which :
X-Ray tube acts as rectifier
Current only flows from cathode to
anode
Rarely seen
cathode is source of free electrons
Secondary of
High Voltage
Transformer
mA waveform
Voltage applied to tube
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Blocked (not used)
Applied to x-ray tube
Output of High Tension Transformer Applied to X-ray Tube
12
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• Single Phase Full Wave Rectification
In this circuit both half cycles of AC are
used to produce X-Rays by employing a
bridge of four rectifiers
Secondary of
High Voltage
Transformer
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+
-
X
X
First Half Cycle Second Half Cycle
Voltage applied to tube
(also mA waveform)
X
X
+
-
Actually what happens?
R1
R2
R3
R4R1
R2 R3
R4
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REFERENCES
• John Ball, Adrian D. Moore, Steve Turner (2008) Essential Physics
for Radiographers, Blackwell
• Richard R. Carlton, Arlene McKenna Adler (2005) Principles of
Radiographic Imaging, Delmar