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
Controls exposure timing
Turns exposure on and off
Filament heated before exposure
High voltage switched on and off +
filament
anode
voltage
source
high

4. Voltage Voltage from US power company Voltage required Home Industrial
Most stuff: 120 V
AC / Dryer: 240 V
Door bell: 15 V
Computer 5 V
X-Ray
Filament: 8-12 V
High voltage: kV (40,000 – 150,000)

5. Transformers
Magical devices that allow voltage to be changed to any desired value

6. 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

7. Transformer Construction
Transformers have 2 coils of wire
coils not in electrical contact with each another
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

8. 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

9. 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)

10. 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

11. Input DC Voltage & Current
Transformer Theory
transformers do not work with direct (unchanging) current (DC)
Current induced in secondary coil only when primary coil current / magnetic field is changing
Input DC Voltage & Current
No Output
Voltage
Voltage
Time
Time

12. 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

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

14. 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

15. How Does a Transformer Magically Increase Voltage without Some Source of Power?

16. 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

17. 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

18. 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

19. 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

20. Transformer Ratio Ratio = Output voltage / Input voltage
Most transformers have fixed ratios
X-Ray requires variable ratios
Accommodate selection of different kVs

21. 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

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

23. 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

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

25. 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

26. 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

27. 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

28. 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

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

30. Phototiming Detector in front of receptor
Must be essentially invisible
Grid
Image Receptor
Radiation
Sensor

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

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

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

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

35. 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.

36. 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)

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

38. Battery-Powered Generators
Batteries used for
x-ray
transport
Independent of power line during exposure
Disadvantages
Require charging
Heavy