1. Resident Physics Lectures
Christensen, Chapter 3B
X-Ray Generator Circuit
George David
Associate Professor
Medical College of Georgia
Department of Radiology

2. 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

3. 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

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

5. 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

6. Incoming Power Line affects generator performance
diameter of wire
length or wire
other devices sharing branch circuit
Resistance of power line wires can reduce generator voltage during exposure affecting
power available to x-ray tube
calibration

7. 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

8. Line Voltage Compensation
Incoming voltage can vary during day
Generators need to correct for changes in line voltage
power line fluctuations affect calibration
Incoming Power Line
Generator
Circuit
Breaker

9. Line Voltage Compensation
Compensation may be
automatic
most new & high end equipment
manual
user must make adjustment
Line
Line
Compensation

10. 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

11. 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

12. Generator Voltages Input line voltage Autotransformer
single or three phase
Volts AC
Autotransformer
provides variable voltage to primary of high voltage transformer 1f 3f
Auto
Transformer
High Voltage
Transformer
Timer
Circuit
Power
Line

13. 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

14. 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

15. Self (tube) Rectified Circuit
X-Ray tube acts as rectifier
Current only flows from cathode to anode
cathode is source of free electrons
Rarely seen
Secondary of High Voltage Transformer
Voltage applied to tube
mA waveform

16. Self-rectification Disadvantages
Wasted
Used
hot anode can emit electrons
accelerate & can destroy filament
half of electrical cycle wasted
Voltage applied to
x-ray tube
mA waveform
X-Rays Produced

17. Halfwave Rectifier Circuit
X-ray tube connected to secondary of high voltage transformer through diode rectifiers
Alternating voltage applied to secondary of high voltage transformer + -
Voltage applied to tube

18. 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) -

19. Halfwave Rectified Circuit
60 pulses per second
only positive half cycle of high tension transformer used
inefficient
negative half cycle wasted
Secondary of High Voltage Transformer
Blocked (not used)
Applied to x-ray tube
Output of High Tension Transformer
Applied to X-ray Tube

20. 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)

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

22. 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

23. 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

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

25. 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

26. 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

27. 3f Generator Circuits pulses windings
number of peaks per 1/60 second (16.6 msec) power line cycle
windings
3 primary coils (one for each phase)
3 or 6 secondary
with 6 secondaries, 2 secondary coils induced per primary
Three Phase Output

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

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

30. 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

31. Three Phase Transforming
3 coils can be hooked up in 2 ways
Delta
Wye

32. 3-phase generator Primary windings Secondary windings generally delta
may be delta or wye
Primary
Secondary

33. 3-phase generator Six pulse six rectifier one primary delta
one secondary wye
six rectifiers
One on each side of each secondary coil
13.5% ripple
Primary
Three Phase Output
Ripple
Secondary

34. 3 Phase Generator 6-Pulse Twelve Rectifier 1 delta primary
2 wye secondaries
6 secondary windings
two diodes per winding
13.5% ripple
Primary
Ripple
Secondary
Secondary
Three Phase Output

35. 3 Phase Generator 12-Pulse Twelve Rectifier 1 delta primary
2 secondaries, 1 wye, 1 secondary
30o phase difference between secondaries
6 secondary windings
2 diodes per winding
3.5% ripple
Primary
Ripple
Secondary
Secondary
Three Phase Output

36. 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.

37. 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)

38. to filament transformer primary
mA selection
Allows selection from available discrete mA stations.
Applies correct voltage to primary of filament transformer.
Line
Compensation
to filament transformer primary
10 mA
25 mA mA
stabilizer
50 mA
100 mA
200 mA
300 mA
400 mA

39. mA Stabilization During Exposure
On first trigger
mA regulator supplies anticipated voltage to filament transformer primary
mA monitored during exposure
Corrections made to filament voltage during exposure as necessary
if mA low, filament voltage boosted
if mA high, filament voltage lowered

40. Generator kilowatt (kW) Rating
measured under load
kW rating changes with kVp
Standard
measure at 100 kVp

41. Generator kW Rating three phase kV X mA / 1000 mAmax / 10 at 100 kVp
kVp kVp kVp
kVp
600 / 10 = 60 kW

42. Generator kW Rating single phase kV X mA X 0.7 / 1000
mAmax X 0.7 / 10 at 100 kVp
kVp kVp kVp
kVp
400 X 0.7 / 10 = 28 kW

43. 1f vs. 3f Generators 1f 3f Typical home & small business power
inexpensive
transformer windings
1 primary coil
1 secondary coil
Industrial power
expensive
transformer windings
3 primary coils
one for each phase
6 secondary coils
2 secondary coils induced per primary)

44. 1f vs. 3f Generators 1f 3f 100% ripple 8 ms minimum exp. Time
1/120th second
lower output intensity
puts less heat in tube for same technique
4-13% ripple
higher average kVp
slightly less patient exposure
<=1 ms minimum exp. time
higher output intensity
puts more heat in tube

45. Exposure Time Control mechanical electronic, measuring
obsolete
electronic, measuring
time (crystal)
power line pulses
automatic (phototiming)
terminates exposure based on radiation received by receptor
عفا عليها الزمن

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

47. Phototiming Radiation Detectors
screen & photomultiplier tubes (PM Tubes)
obsolete
ionization chambers
solid-state detectors

48. Ionization Chambers Almost always entrance type Notes - + -
thin parallel aluminum plates are electrodes
voltage applied between plates
collect ions produced by radiation in air between electrodes
collected ions produce electric current
Photon + - + -

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

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

51. Phototiming Notes must be calibrated for particular film-screen system
some generators allow selection from several preset film/screen combinations

52. Phototiming Notes phototimer must correct for safety rate response
kVp response of
film/screen system
phototiming sensor
Higher kVp beam more penetrating
Less attenuated by phototimer detector
safety
exposure limited to 600 mAs if phototimer does not terminate exposure (2000 mAs for < 50 kV)