CHAPTER 8 HDR102 PHYSICS FOR RADIOGRAPHERS 1 RECTIFICATION MR KAMARUL AMIN BIN ABDULLAH SCHOOL OF MEDICAL IMAGING FACULTY OF HEALTH SCIENCES

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1 HDR102 PHYSICS FOR RADIOGRAPHERS 1 CHAPTER 8 RECTIFICATION PREPARED BY: MR KAMARUL AMIN BIN ABDULLAH SCHOOL OF MEDICAL IMAGING FACULTY OF HEALTH SCIENCES

LEARNING OUTCOMES At the end of the lesson, the student should be able to:- Explain the self rectification including the definition, disadvantage and circuit.

2 LEARNING OUTCOMES 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. Slide 2 of 52

3 OUTLINES INTRODUCTION 8.1 Principles 8.2 Self Rectification 8.2 Single Phase Full Wave Rectification 8.3 The Use of Rectifiers in X-Ray Circuit 8.4 References Slide 3 of 52

8.1 Principle Rectification is a process by which alternating current is changed to pulsating direct current A device which converts AC into pulsating DC is called rectifier.

4 8.1 Principle Rectification is a 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 Slide 4 of 52

5 8.1 Principle UNRECTIFIED A.C WAVE NO RECTIFICATION HALF-WAVE FULL-WAVE Slide 5 of 52

6 8.1 Principle In this lesson, we will only learn TWO rectification methods: 1) Self Rectification 2) Single Phase Full Wave Rectification Slide 6 of 52

During the second half-cycle the anode is negative so there is no current flow and no x-ray production.

7 8.2 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. Slide 7 of 52

8 8.2 Self Rectification Figure 1: Self rectification where only half wave is being used. Slide 8 of 52

8.2 Self Rectification 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.

9 8.2 Self Rectification 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. Slide 9 of 52

10 8.2 Self Rectification Advantages Low in price and less bulky so that it can be portable. Slide 10 of 52

11 8.2 Single Phase Full Wave Rectification Figure 2: Four-valve/diode-tube rectification enables us to use the inverse voltage for the production of x-rays and is therefore full-wave rectification. Slide 11 of 52

8.2 Single Phase Full Wave Rectification Advantages Less difference in average and peak current. Both the halves (+ and -) of AC cycle and voltage are being used.

12 8.2 Single Phase 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. Slide 12 of 52

13 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 Voltage applied to tube Secondary of High Voltage Transformer ma waveform Slide 13 of 52

14 8.3 The Use of Rectifiers in X-ray Circuit Output of High Tension Transformer Applied to x-ray tube Applied to X-ray Tube Blocked (not used) 14 Slide 14 of 52

15 8.3 The Use of Rectifiers in X-ray Circuit 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 Slide 15 of 52

16 8.3 The Use of Rectifiers in X-ray Circuit Actually what happens? First Half Cycle Second Half Cycle + - R1 X R4 R1 X R4 R2 X X - R3 R2 R3 + Slide 16 of 52

17 Activity Test Your Knowledge Answer the question. What is rectification? A B C The process of increasing voltage in the x-ray tube The process of maintaining a certain voltage in the x-ray tube The process of changing alternating current to pulsating direct current Slide 17 of 52

SUMMARY Rectification is a process by which alternating current is changed to pulsating direct current. A device which converts AC into pulsating DC is called rectifier.

18 SUMMARY Rectification is a process by which alternating current is changed to pulsating direct current. A device which converts AC into pulsating DC is called rectifier. Self-rectification is when the x-ray tube does its own rectification and is halfwave rectification since only half of the current wave is utilized. Four-valve/diode-tube rectification is to use the inverse voltage for the production of x-rays and is therefore full-wave rectification. Slide 21 of 52

19 8.4 References No. REFERENCES 1 Ball, J., Moore, A. D., & Turner, S. (2008). Essential physics for radiographers. Blackwell. 2 Bushong, S. C. (2008). Radiologic science for technologists. Canada: Elsevier. Slide 22 of 52

20 APPENDIX FIGURE Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 SOURCE B7-2b.JPG pg Slide 23 of 52

CHAPTER 6 ALTERNATING CURRENT

HDR102 PHYSICS FOR RADIOGRAPHERS 1 CHAPTER 6 ALTERNATING CURRENT PREPARED BY: MR KAMARUL AMIN BIN ABDULLAH SCHOOL OF MEDICAL IMAGING FACULTY OF HEALTH SCIENCES LEARNING OUTCOMES At the end of the lesson,