HDR102
PHYSICS FOR RADIOGRAPHERS 1
CHAPTER 8
RECTIFICATION
PREPARED BY:
MR KAMARUL AMIN BIN ABDULLAH
SCHOOL OF MEDICAL IMAGING
FACULTY OF HEALTH SCIENCES
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.
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
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
UNRECTIFIED A.C WAVE
NO RECTIFICATION
HALF-WAVE
FULL-WAVE
•
In this lesson, we will only learn TWO rectification methods:
1) Self Rectification
2) Single Phase Full Wave Rectification
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.
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.
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.
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.
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

Secondary of
High Voltage
Transformer
Voltage applied to tube
mA waveform
cathode is source of free electrons
Output of High Tension Transformer
Applied to x-ray tube
Applied to X-ray Tube
Blocked (not used)
12
• 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
Voltage applied to tube
(also mA waveform)
Actually what happens?
First Half Cycle
Second Half Cycle
+
R1
X
R2
-
R1
R4
X
R3
R2
+
X
X
R3
R4
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