HDR102

SCHOOL OF MEDICAL IMAGINGFACULTY OF HEALTH SCIENCES

PREPARED BY:MR KAMARUL AMIN BIN ABDULLAH

CHAPTER 8

PHYSICS FOR RADIOGRAPHERS 1

RECTIFICATION

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

NO RECTIFICATION

HALF-WAVE

FULL-WAVE

UNRECTIFIED A.C 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

cathode is source of free electrons

Secondary of

High Voltage

Transformer

mA waveform

Voltage applied to tube

Blocked (not used)

Applied to x-ray tube

Output of High Tension Transformer Applied to X-ray Tube

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

+

-

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

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