Resident Physics Lectures (Year 1)

1. Resident Physics Lectures (Year 1) • Christensen, Chapter 3 X-Ray Generators George David, MS FAAPM, FACR Associate Professor of Radiology

2. Requirements to Produce X-Rays • Filament Voltage • High Voltage anode filament filament voltage source + high voltage source

3. X-Ray Generator • Supplies electrical power to x-ray tube • high voltage between anode & cathode • filament voltage anode filament filament voltage source + high voltage source

4. X-Ray Generator • Controls exposure timing • Turns exposure on and off • Filament heated before exposure • High voltage switched on and off

5. Voltage Supplied • US Commercial Power • Home • 120 / 240 V • Industrial • 480 V • 240 V

6. Voltage Required • Home • Most stuff: 120 V • AC / Dryer: 240 V • Door bell: 15 V • Computer 5 V • X-Ray • Filament: 8-12 V • High voltage: 40-150 kV (40,000V – 150,000V)

7. Problem: Voltages Don’t Match Voltage Supplied Voltage Required

8. Transformers • Devices that allow voltage to be changed to any desired value

9. Current Flow and Magnetic Fields • Magnetic field surrounds conductor carrying electric current • Magnetic field concentrated by coiling conductor Magnetic Field N S Magnetic Field Current Flow

10. Transformer Construction • 2 coils of wire • not in electrical contact with each another • When electric current passed through primary coil • magnetic field develops around primary coil • Secondary coil senses magnetic field Magnetic Field Current Flow

11. Incoming AC Power Primary Coil Secondary Coil(s) Transformer Coil Designations • primary • coil to which power applied • secondary • coil which senses magnetic field of primary coil

12. Transformer Coils • When secondary coil feels changing (increasing or decreasing) magnetic field of primary coil • power induced in secondary coil • no physical connection Incoming AC Power Primary Coil Secondary Coil(s)

13. NS = 40 • NP = 20 Turns Ratio Definition TR=NS / NP • number of windings of secondary coil divided by number of windings of primary coil • 40 / 20 for transformer below

14. Transformer Theory • Transformers only work when primary coil current / magnetic field is changing Input DC Voltage & Current Zero Output Voltage Voltage Time Time Input AC Output

15. Transformer Theory • Transformers alter both voltage & current of AC waveforms • Voltage in secondary can be > or < voltage in primary Input AC Voltage & Current Output AC Voltage & Current

16. Transformer Law • Voltage Ratio = Turns Ratio # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ------------------- # Prim. Coils Prim. Voltage NS VS ----- = ----- NP VP

17. Transformer Types • Step down Transformer • # primary coils > # secondary coils • primary voltage > secondary voltage • Step up Transformer • # primary coils < # secondary coils • primary voltage < secondary voltage

18. How does a transformer increase voltage with no source of energy?

19. Power • Power is rate of energy usage • Power defined as Voltage X Current • Units • Voltage => Volts • Current => Amps • Power => Watts • Voltage => Kilovolts • Current => milliamps • Power => Watts

20. Power • Power = Voltage X Current • Transformer primary power = secondary power • transformer neither creates nor consumes power PowerPRIM = PowerSEC VPRIM X IPRIM = VSEC X ISEC

21. Transformers Power = Voltage X Current • Electrical power not changed • Current exchanged for voltage • Voltage goes up – current goes down • Voltage goes down – current goes up

22. Transformer Law (cont.) • Current ratio is inverse of voltage ratio # Sec. Coils Sec. Voltage Prim. Current----------------- = --------------- = ----------------- # Prim. Coils Prim. Voltage Sec. Current NS VS IP ----- = ----- = ----- NP VP IS VPRIM X IPRIM = VSEC X ISEC

23. Transformer Ratio • Ratio = Output voltage / Input voltage • Most transformers have fixed ratios • X-Ray requires variable ratios • Accommodate selection of different kV’s

24. Autotransformer Taps • Only one winding • Incoming AC voltage connected across coils • primary • Output voltage proportional to # coils between taps • secondary Input NS NP Primary Secondary

25. Autotransformer • Voltage law for autotransformers same as for transformers • Variable ratio transformer • Secondary voltage adjustable by moving to different tap • changes # secondary coils NS Input NS NP NS VS ----- = ----- NP VP Primary

26. Generator Components • control console • kVp adjust • mA adjust • time adjust • transformer • high voltage (step up) • filament • low voltage (step down) • electronics cabinet • support circuitry or mAs adjust

27. + X-ray Circuit High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA selector Filament Transformer

28. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA selector Filament Transformer Line Incoming line voltage connected to generator through a circuit breaker. Typ. 220-240 volt AC single phase 240, 480 volt AC three phase

29. Generator Incoming Power Line Circuit Breaker Circuit Breaker • Generator connected to power line through a circuit breaker • Limits current from power line to generator • Allows generator to be disconnected from power line

30. + High Voltage Transformer Rectifier Circuit Variable ratio Timer Circuit Fixedratio Auto- trans-former Line mA regulator Filament Transformer • Auto transformer • Variable ratio transformer allowing operator to specify kVp • High voltage transformer • Boosts output of autotransformer by fixed ratio

31. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA regulator Filament Transformer Timer • Starts & stops exposure • Turns transformer primary (low voltage) on & off

32. Exposure Timing • Manual • Operator sets time • Automatic (Phototimed) • Equipment measures radiation • Terminates exposure when designated radiation measured

33. Phototiming • Detector in front of receptor • Must be essentially invisible Grid ImageReceptor Radiation Sensor

34. Phototiming Fields • 1, 2, or 3 • fields may be selected individually or in combination • proper positioning critical

35. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA regulator Filament Transformer Timer • Develops DC high voltage for x-ray tube

36. High Voltage Transformer • Grounded metal box filled with oil • electrical insulator • Function • Develops proper high voltage for tube • Also contains rectifier circuit

37. New Challenge • Transformers only produce AC • X-ray tube requires DC high voltage

38. Rectification • Allows current flow only in one direction • Transformers only work with alternating current (AC) • Rectifier changes alternating current output of high voltage transformer to direct current • DC voltage applied to tube

39. Rectifier • Also called diode • Diode is two-componentdevice • cathode • anode Cathode Anode

40. Diodes are One Way Streets for Electric Current • Allows current to flow from cathode to anode • Blocks current from flowing from anode to cathode No Yes C A C A

41. Diodes: A one-way valve for current • Allow current to flow through diode in cathode to anode direction • acts like closed switch • diodes connected like this said to be “forward biased” Load (tube) + Battery -

42. Diodes: A one-way valve for current • Does not allow current to flow through diode in anode to cathode direction • acts like open switch • diodes connected like this said to be “reversed biased” + - X

43. AC DC Rectifiers (Diodes) • converts AC to pulsating DC • Configurations • solid state sticks • many small diodes placed end to end • materials • selenium • silicon (most common today)

44. Three Phase Power Three-Phase Generators • Commercial power generally delivered as three phase • 3 phases • Each phase has same waveform but 120o apart in time Single Phase Power

45. 3 Phase Power Rectification Three Phase Output Three Phase Rectification Three Phase Rectification Circuit follows highest of 3 Phases

46. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA regulator Filament Transformer mA regulator • Circuitry for mA selection • Corrects mA during exposure.

47. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA selector Filament Transformer Filament Transformer Changes AC voltage to smaller voltage required by filament (8-12 volts typical)

48. Power Storage Generators • Use batteries • Application • Remote locations • Inadequate power from power line or power line not accessible • Outlet inaccessible

49. Battery-Powered Generators • Batteries used for • x-ray • transport • Independent of power line during exposure • Disadvantages • Batteries must be charged • Batteries maintenance • Heavy • Battery’s DC converted to AC for high voltage transformer to operate

50. Medium (or high) Frequency Generators • higher frequency square wave voltage sent to primary of high voltage transformer • Conventional generators use power line’s sine wave • very efficient • transformer & generator very small