1. Resident Physics Lectures
Christensen, Chapter 3
X-Ray Generators
George David
Associate Professor of Radiology

2. Transformer Construction
Transformers have 2 coils of wire
no electrical contact between coils
When electric current passed through one coil
magnetic field develops around first coil
second coil near enough to feel magnetic field
Magnetic Field
Current Flow

3. Transformer Coil Designations
Primary
Coil
Secondary
Coil(s)
Incoming
AC Power
primary
coil to which power is applied
secondary
coil which feels magnetic field of primary coil

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

5. Turns Ratio Definition
number of windings of secondary coil divided by number of windings of primary coil
850 / 1200 for transformer below
NP = 1200
NS = 850

6. 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

7. Transformer Law Voltage Ratio = Turns Ratio # Sec. Coils Sec. Voltage
# Prim. Coils Prim. Voltage

8. Transformer Law
# Sec. Coils Sec. Voltage Turns Ratio = = # Prim. Coils Prim Voltage
NP = 1200
NS = 850
NS VS
----- = -----
NP VP
240
VAC
? V ?
= -----
If VPRIM = 240 Volts then VSEC = 170 Volts

9. 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

10. 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

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

12. Autotransformer
Primary
100
Input 50 80
Input 50
100 20
Primary

13. Rectification
Changes alternating current output of high voltage transformer to direct current
allows current flow in one direction only
x-ray tube is a rectifier because current will not flow from anode to cathode
no source of free electrons at anode

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

15. X-Ray Generator Supplies electrical power to x-ray tube
high voltage between anode & cathode
filament voltage
Controls exposure timing
Turns exposure on and off
High voltage switched on and off
Filament heated before exposure

16. 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

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

18. 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

19. 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

20. High Voltage Transformer
Rectifier Circuit
Timer
Circuit +
Auto-
trans-former
Line
mA regulator
Filament Transformer
Autotransformer
High voltage Transformer has fixed ratio
Autotransformer has variable ratio
Autotransformer needed to provide variable kilovoltage to tube

21. Autotransformer major kV selector to high voltage transformer primary
Line
Compensation
to high voltage transformer primary
Timer
Circuit
minor kV selector
to filament transformer primary mA
regulator
Autotransformer does line compensation & kVp selection

22. High Voltage Circuit Supplies high voltage for x-ray tube
Step-up transformer
primary from autotransformer
secondary to rectifier circuit
mA monitored at center grounded point of secondary
Auto-
transformer
Rectifier
Circuit mA
High Voltage Transformer

23. High Voltage Transformer
Grounded metal box
filled with oil
electrical insulator
Function
increases or decreases alternating voltage
Also contains rectifier circuit
changes alternating current into direct current

24. Halfwave Rectifier Circuit+ -
First Half Cycle:
Diodes closed
Voltage applied to tube
Tube current (mA) results - + - X
Second Half Cycle:
Diodes open
No voltage applied to tube
No tube current (mA) -

25. Fullwave Rectifier Four diodes 120 pulses/second
exposure times half of halfwave circuit
Secondary of High Voltage Transformer
Voltage applied to tube
(also mA waveform)

26. Voltage applied to tube
Fullwave Rectifier
Voltage applied to tube
(also mA waveform)
First Half Cycle
Second Half Cycle + - X X + -

27. Full-Wave Rectification
Rectifiers
Four diode “bridge” configuration used with single phase
both + & - half cycle of high tension transformer used
efficient
circuit reverses negative half cycle & applies to x-ray tube
Tube
Output of High Tension Transformer
Applied to X-ray Tube

28. Pulsed Radiation single phase input power results in pulsed radiation
Disadvantages
intensity only significant when voltage is near peak
low voltage heats target and produces low-energy photons
absorbed in tube, filter, or patient
can contribute to dose
Applied to X-ray Tube
Radiation Waveform

29. Three-Phase Generators
Commercial power generally delivered as 3 phase
phases 120o apart
Single Phase Power
Three Phase Power

30. Three-Phase Generators
Rectifier circuit
Inverts negative voltage
sends highest of 3 phases to x-ray tube
Input 3 Phase Voltage
Rectified
To X-Ray Tube

31. Three-Phase Generators
much higher tube ratings than single phase
more efficient than single phase
shorter exposures
lower exposure
Single Phase Power
Three Phase Output

32. Ripple variation of kilovoltage from maximum
usually expressed as percentage of maximum kV
Ripple

33. Ripple Example Ripple = 80 - 72 = 8 kVp OR 8 / 80 = .1 = 10% 80 kVp

34. Constant Potential or High Frequency Output
Ripple Typical Values
single phase
always 100 % (kV ranges from zero to maximum)
three phase
4-13%
constant potential
0 %
Medium / high frequency
very low; approx 0.
Single Phase Output
Three Phase Output
Constant Potential or High Frequency Output

35. 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

36. Exposure Timing Manual Operator sets time Automatic (Phototimed)
Equipment measures exposure
Terminates exposure when designated exposure is measured

37. Phototiming Geometry entrance type exit type detector in front of film
Sensor
Grid
Recptor
Entrance type
entrance type
detector in front of film
detector must be essentially invisible
exit type
detector behind film
obsolete except for film mammography
detector visible because of high contrast image

38. Ionization Chambers - + - thin parallel aluminum plates are electrodes
voltage applied between plates
radiation ionizes air
ions collected in air between electrodes
collected ions produce electric current
Photon + - + -

39. Solid State Detectors
PN semiconductor junction generates current when struck by radiation
small
fast response
little beam attenuation
Photon
Electric Current

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

41. High Voltage Transformer
Primary switching
High Voltage Transformer
Rectifier Circuit
Timer
Circuit +
Auto-
trans-former
Line
mA regulator
Filament Transformer
Switching (timing) almost always done using low voltage
High voltage switching used only in high end applications
Angio
Cardiac Cath

42. Primary Switch Types solid state
silicon-controlled rectifiers (SCR’s or thyristers)
turned on by voltage pulse
Can only be turned off by interrupting current through it

43. High Voltage Transformer
Rectifier Circuit
Timer
Circuit
Secondary switching +
Auto-
trans-former
Line
mA regulator
Filament Transformer
High voltage switched, not filament
Requirement
fast
smooth

44. High Voltage Transformer
Rectifier Circuit
Timer
Circuit +
Auto-
trans-former
Line
mA regulator
Filament Transformer
mA regulator
Circuitry for mA selection
Adjusts mA on the fly during exposure.

45. High Voltage Transformer
Rectifier Circuit
Timer
Circuit +
Auto-
trans-former
Line
mA selector
Filament Transformer
Filament Transformer
Steps down AC voltage from Autotransformer & mA selector to smaller AC voltage required by filament (8-12 volts typical)

46. Power Storage Generators
Application
Remote locations
Inadequate power from power line
Outlet inaccessible
Types
Battery-powered generators
Capacitor discharge generators

47. 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

48. 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
some transformers integral with x-ray tube head

49. Medium Frequency Generator Operation
incoming AC converted to DC DC AC
First Step

50. Medium Frequency Generator Operation
Pulsating DC smooth to constant voltage
Pulsating DC
Constant DC
Second Step

51. Medium Frequency Generator Operation
smooth DC regulated to voltage level proper for primary
Constant DC
Exposure-level DC
Third Step

52. Medium Frequency Generator Operation
DC converted (chopped) to AC
Exposure-level DC
Chopped AC
Fourth Step

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

54. 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