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 +
filament
anode
voltage
source
high

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 Industrial 120 / 240 V 480 V

6. Voltage Required Home X-Ray Most stuff: 120 V AC / Dryer: 240 V
Door bell: 15 V
Computer 5 V
X-Ray
Filament: 8-12 V
High voltage: 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. Transformer Coil Designations
Incoming
AC Power
Primary
Coil
Secondary
Coil(s)
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. 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
NP = 20
NS = 40

14. Input DC Voltage & Current
Transformer Theory
Transformers only work when primary coil current / magnetic field is changing
Input DC Voltage & Current
Voltage
Time
Time
Zero Output
Voltage
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 NS VS ----- = ----- NP VP
Voltage Ratio = Turns Ratio # Sec. Coils Sec. Voltage
Turns Ratio = =
# Prim. Coils Prim. Voltage
NS VS
----- = -----
NP VP

17. Transformer Types Step up Transformer
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 PowerPRIM = PowerSEC VPRIM X IPRIM = VSEC X ISEC
Power = Voltage X Current
Transformer primary power = secondary power
transformer neither creates nor consumes power
PowerPRIM = PowerSEC
VPRIM X IPRIM = VSEC X ISEC

21. Power = Voltage X Current
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 Only one winding
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 NS VS ----- = ----- NP VP
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 transformer electronics cabinet
kVp adjust
mA adjust
time adjust
transformer
high voltage (step up)
filament
low voltage (step down)
electronics cabinet
support circuitry
or mAs adjust

27. High Voltage Transformer
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 volt AC single phase
240, 480 volt AC three phase

29. 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
Incoming Power Line
Generator
Circuit
Breaker

30. High Voltage Transformer
Rectifier Circuit
Variable ratio
Timer
Circuit
Fixed ratio +
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
Image Receptor
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 Cathode Anode Also called diode
Diode is two-component device
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. Rectifiers (Diodes) converts AC to pulsating DC Configurations
solid state sticks
many small diodes placed end to end
materials
selenium
silicon (most common today) DC

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

45. Three Phase Rectification
3 Phase Power
Three Phase Output
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

51. Medium Frequency Generator Operation
incoming AC converted to DC DC AC
Step #1

52. Medium Frequency Generator Operation
Smooth pulsating DC to constant voltage
Pulsating DC
Constant DC
Step #2

53. Medium Frequency Generator Operation
Regulate smooth DC to required voltage level for kVp requested
Constant DC
Exposure-level DC
Step #3

54. Medium Frequency Generator Operation
DC converted (chopped) to AC
Exposure-level DC
Chopped AC
Step #4

55. Medium Frequency Generator Operation
AC sent to transformer primary
normal transforming & rectification
high voltage smoothing

56. Medium Frequency Generators
Advantages
conducive to computer control
immune to power line fluctuations
low ripple
small size of electronics & transformer
small enough to spin on CT gantry
Today’s trend in generators

57. Surrender
We give up!