BORROWED FROM: Physics Applied to Radiology RADI R250 -- Fall 2003 CH 9 CIRCUITRY

BORROWED FROM:Physics Applied to RadiologyRADI R250 -- Fall 2003 CH 9

CIRCUITRY

2

X-Ray Machine Purpose:

provide a specific current (mA) & voltage (kV) to the x-ray tube

convert electrical energy to electromagnetic energy (x rays) in a controlled mannercontrol the energy of the x-ray photonscontrol the number of photons

3

X-Ray Machine Circuit

4

Main Subcircuits (1)High Voltage Subcircuitgives KE to e-

5

Main Subcircuits (2)

Filament Subcircuit thermionic emission of e-

6

Transformer SidePrimary Side Secondary Side

low voltage high voltage

7

Functional PositionControl Console Transformers Tube

8

Control Panel Functions1. ON/OFF2. Regulate incoming power3. kVp selection4. TIME selection

exposure controlexposure control

5. mA selection5. mA selection FSS selection

1.

2.

3.

4.

5.

9

Control Panel Circuit ElementsPOWER SUPPLY

60 Hz AC 120 to 480 V Panel Power On/Off

AUTOTRANSFORMER1. line compensation

a. line meterb. primary side adjustments

2. kVp selectiona. secondary side adjustments variable turns ratio

3. filament circuit power

1a.

1b.

2a.

2a.

3.

10

Control Panel Elements (cont.) on secondary side of autotransformer

1. mA selectora. precision resistors

b. meter (in transformer)

c. FSS selector

2. kVp selectora. major/minor taps

b. meter (pre-reading)

3. time selectora. circuit types

b. exposure switch

2a.

2a.

2b.

1a.

1b.

3a.

3b.

11

Timer Circuit Types (1) mechanical

spring action

synchronous motor start/stop with AC cycle 60 Hz = 60 start/stop positions shortest time = 1/cycle = 1/60 s

impulse start/stop with AC pulse 60 Hz = 120 start/stop positions shortest time = 1/pulse = 1/120 s

1/60 s

1/120 s

12

Timer Circuit Types (3) electronic timer

operationselecting time = varying resistance in rheostat to control time

it takes to charge the capacitoractivating exposure also begins storage of charge in the

capacitorwhen a precise voltage is reached in capacitor, the switch

opens & terminates the exposuremicroprocessor computer chip technologyshortest time = 1 ms = .001 s = 1/1000 s

capacitor

rheostat switch

13


14

Timer Comparison 1

mechanical shortest = 1/4 s 250.0 ms

synchronous motor shortest = 1/60 s 16.6 ms

impulse shortest = 1/120 s 8.3 ms

3 electronic shortest = 1/1000 s 1.0

ms

15

Timer Circuit Types (3)(cont.)

mAs timer type of electronic timer uses tube current (mA) to charge capacitor time to charge capacitor = time of exposure on secondary side of HV transformer

16

Timer Circuit Types (3)(cont.) Automatic Exposure

Control (AEC) timers electronic timer detects radiation that has

passed through the patient

terminates exposure after set amount of radiation reaches sensing device

based on light emitted or ionization caused

photomultiplier tube (PMT)

film/cassette etc.

patient

film/cassette etc.

patientionization chamber

17


18

Transformer Section components

1. step-up transformer

2. mA meter

3. rectification circuit

4. step-down transformers

location in “box” linked between

control console & x-ray tube

immersed in oil for insulation & cooling

TransformerSection

1.

2.3.

4.

19

High Voltage Transformer Fixed TR between 500 & 600 Primary Side (power from

autotransformer’s secondary output)

PS-AT = PP-HV

120V 10A = 120V 10A

Secondary Side V to kV

e.g. if TR = 500 & PP-HV = 120 V 10 A

then VS-HV = 500 x 120V = 60 kV

and IS-HV = 10A 500 = .02 A

amplitude of V sine wave

see text Table 7.1, page 173

= =

Voltage Sine Wave

kVV

20

mA Meter location

midpoint of secondary coil of HV transformer

face of meter may be on control console

grounded safety places 0V in center of coil each wire out of HV transformer

carries ½VT

operates only during an exposure

0V

+½VT

-½VT

21

Rectification purpose

convert HV AC to pulsed DC for x-ray tube

location between HV secondary coil

& x-ray tube

types full wave -- most common half wave -- limited use self -- rarely

TransformerSection

22

Rectification process of changing AC to pulsed DC rectifier

device that allows I to flow in only one direction

types valve tube solid state

Ie

I

+ -

Ie

I

+ -

23

Valve tube (diode) glass vacuum tube w/ 2 electrodes

1) negative electrode -- cathode thermionic emission of e-

2) positive electrode -- anode cold metallic plate

cold hot

anode cathode

24

Valve Tube Operationa) AC with + on anode & - on cathode (+ AC pulse)

e- flow from cathode to anode = I

+ -

--

-----

-

-

Ie

I+

-

V

+

-

I

+---

----- --

+

-

V

+

-

I

b) AC with + on cathode & - on anode (-AC pulse) e- drawn back to filament = no I

25

Solid State Rectifier

1) "n-type" material (donor) [similar to cathode]

contains loosely bound e-

2) "p-type" material (acceptor) [similar to anode]

spaces in molecular structure to accept e-

3) p-n junctionp-n junction union of the two types of materials

n-type- - - --

--- - --

--

p-type

p-nj

26

Solid State Rectifier Operationa) AC with + on p-type & - on n-type (+AC pulse)

e- move across junction to the spaces = I

+

-

V

+

-

I

b) AC with + on n-type & - on p-type (-AC pulse) e- drawn away from the junction = no I

+

-

V

+

-

I

+ -I

Ie

- - --- --- - --

- -

+---

- ---

--- -- --

27

Rectification Process rectifiers placed in circuit to convert AC

current to pulsed DC methods

suppress unwanted part of the AC (half wave rectification)

redirect unwanted part of AC (full wave rectification)

+

-

+

-

+

-

+

-

V

V

I

I

28

Full-Wave RectificationPositive HV pulse Negative HV pulse

+

-

I

+

- I

+

-

+

-

V

+

-

I

Ie

x rays

+

-

I+

-

+

-

V

+

-

I

Ie

x rays

29

Half-Wave Rectification

Positive HV pulse Negative HV pulse

+

-

+

-

V

+

-

I

+

-

V

+

-

I

x-rays

I

e

I

+

-

+

-

+-

+ -

no Ie

noI

+

-

+

+

-

-no

x-rays

+-

30

Self RectificationPositive HV pulse Negative HV pulse

+

-

V

+

-

I

+

-

V

no Ie

noI

+

-

+

-

nox-rays

x-rays

I

e

I

+

-

+

-

+

-

I

+

-

31

Step-Down (Filament) Transformer purpose

adjust current to cathode filament to produce the heat required for the thermionic emission of e-

control # x rays IF = heat heat = # e-

# e- = # x rays

TransformerSection

32

Filament Transformer (cont.) operation

primary current adjusted by precision resistors (Ohm’s Law)

fixed TR = .10 to .05 secondary current to 3

to 5 A required for thermionic emission

2 selectable transformers small filament transformer large filament transformer

33

Voltage Ripple

% ripple voltage fluctuation in waveform

+ V

- V

+ V

- V

0 0

34

Single Phase – Full rectifiedVoltage

1Ø AC has a 100% voltage ripple

+ V

- V

+ V

- V

0 0

35

Filament Circuit vs. Tube CircuitIT anode to cathodeP from kVp & mA

IF around filamentP from V & A

36

Circuit Summary 2 major subcircuits

filament circuit thermionic emission of e- at filament

high voltage circuit PD to accelerate e- from cathode to anode

Other circuit parts timing method rectifiers meters

37

Single vs. Three Phase Power Single phase (1Ø)

power supplied or used one cycle at a time

unrectified

200%

+

-

V

1/60s

V

+

-

100%

1/60s rectified

1 or 2 + pulses in each 1/60s cycle

100 % voltage ripple

1 + & 1 - pulse in each 1/60s cycle

200 % voltage ripple

38

Single vs. Three Phase Power (cont.) Three phase (3Ø )

power supplied/used w/ a new cycle beginning each 120° results in three overlapping sine waves 3 + pulses & 3 - pulses occur in the length of time of one complete

cycle (1/60s) each wave can be rectified (3Ø6p) pulses overlap reducing voltage ripple & Vmin

1/60s +

-

V

1/60s +

-

V

39

3Ø Voltage Ripple

pulse overlap decreases ripple to 12-15% on 3Ø6p 3-5% on 3Ø12p

effective voltage nearly = maximum voltage

1/60s +

-

V

12 - 15%

40

X-ray Machine Power Rating kilowatt rating

electrical power of the machine power formula adapted to compensate for:

dimensional prefixes used in x-ray machines effectiveness of 3 vs. 1

P IxV

PmAxkVp

PmAxkVp

x

=

=

=

3

1

1000

10007

Ø

Ø .

41

Kilowatt Rating ProblemWhat is the power rating of a 1Ø machine that can operate at 150 kVp and 500 mA?

P1Ø = ?? kVp = 150 mA = 500

P1Ø = [(kVp x mA) / 1000] x .7

= [(150 x 500) / 1000] x .7

= 52.5 kW = 50 kW

42

3Ø comparison to 1Ø 3Ø more efficient than 1Ø 3Ø requires more complex circuitry 3Ø more expensive to install

43

High Frequency Power

machine designed to have a low ripple (<1%) & use 1Ø power

1Ø AC rectified then smoothed & chopped into high frequency output (kHz)

44

Rectification ReviewType # rectifiers % Ripple Wave Form (2 cycles)

1ØSelf 0 100%

1Ø1p 1 or 2 100%

1Ø2p 4 100%

3Ø6p 6 or 12 12-15%

3Ø12p 12 3-5%

+

-

+

-

+

-

+

-

+

-

45

Single vs. Three Phase Power (cont.) Three phase (3Ø )

power supplied/used w/ a new cycle beginning each 120° results in three overlapping sine waves 3 + pulses & 3 - pulses occur in the length of time of one complete

cycle (1/60s) each wave can be rectified (3Ø6p) pulses overlap reducing voltage ripple & Vmin

1/60s +

-

V

1/60s +

-

V

46

1Ø vs. 3Ø HV Transformers

Each phase must have step-up transformer

1Ø

primary secondary

3Ø

primary secondary

47

3Ø Transformer Coils

1/60s +

-

V

Wye -- delayed output

Delta -- either side

Wye -- secondary only

Delta Coils Wye Coils

48

3 Transformer Configurationssimple 3 6 pulse: delta/wye ()

primary secondary

15% voltage ripple

49

3 Trans. Configurations (cont.)

complex 3 6 pulse: delta/wye/wye ()

primary secondary

12% voltage ripple

50

3 Trans. Configurations (cont.)

3 12 pulse: delta/wye/delta ()primary secondary

2 secondary waves from each primary3-5% voltage ripple

51

3 Rectification each secondary coil requires 2 rectifiers 3Ø6 with coils = 6 rectifiers

positive pulse negative pulse

-+

-+

+-

-+

52

3 Rectification (cont.) 3Ø6 with coils = 12 rectifiers


-+

-+

-+

-+

-+

-+

53

3 Rectification (cont.) 3Ø12 with coils = 12 rectifiers


-+ -

+

+ -

-+

-+

+ -

54

Ripple Factor (%) variation in tube V during an exposure What is the ripple of the following waveform?

1/60s

+

-

V

80 kV

68 kV

= look in book for comparisons

55

Generator SummaryType p/c p/s # % Wave Form (2 cycles)

rectifiers Ripple

+

-

+

-

+

-

+

-

+

-

1ØSelf 1 60 0 100%

1Ø1p 1 60 1 or 2 100%

1Ø2p 2 120 4 100%

3Ø6p 6 360 6 or 12 12-15%

3Ø12p 12 720 12 3-5%

56

Medium/High Frequency Generators

x-ray tube nearly constant ripple <1%

small in size Expensive – most efficient

rectifiers smooth choppedHVT

rectifiers smooth tube

57

Comparison of X-ray Machines1Ø 3Ø High freq.

V ripple 100% < 15% <1%

p/s 60 or 120 720 13,000

X-ray Quantity “X” more highest

X-ray Quality “X” higher min time 1P (~8 ms) 1 ms machine $ “X” higher highest

operation $ “X” lower ??

generator size moderate largest smallest

58

Capacitor Discharge Generator

capacitor bank charged by rectified high voltage during exposure capacitors provide kV across tube problems

as capacitors drain kV drops (~1 kV/mAs) any residual charge may shock operator

HVT

rectifiers capacitor tube

59

Falling-Load Generators

operates at shortest time + highest mA uses series of steps (mA + t) to achieve mAs

60

Miscellaneous Terms (cont.) kilowatt rating (review)

power output of the generator power that the tube can use power formula

P IxV

PmAxkVp

PmAxkVp

x

=

=

=

3

1

1000

10007

Ø

Ø .

Documents

BORROWED FROM: Physics Applied to Radiology RADI R250 -- Fall 2003 CH 9 CIRCUITRY