1. X ray Generator, Circuits
&
Exposure timer
Presentor: Anjan Dangal
B.Sc. Medical Imaging Technology
National Academy of Medical Sciences,
Bir Hospital,Kathmandu, Nepal
February 6 , 2018
2. Presentation Objectives
• Understand the Basic Components of X ray Circuits.
• Understand the basic working Mechanism of X ray Circuit.
• Understand the Exposure Timer and Automatic Exposure Control.
( AEC)
3.
4. X ray Generator
• An x ray generator is a device that supplies electric power to the x ray
tube.
• Remember: Function of x ray generator is to transform the electrical
energy in an appropriate form necessary for X ray production but not
to generate .
• To permit operator to control, kv, MA and Exposure time.
5. • Our X ray tube requires electrical energy for two primary purpose:
1. To boil the electrons from the filament.
2. To accelerate these electrons from cathode to Anode.
6. • X ray generator have circuits for these task and are:
1. High Voltage Circuit
2. Filament Circuit
3. Timer Circuit
• What is circuit?
Interconnection between some electrical
elements is called circuit. Its components are
battery ,wire , resistor, capacitor etc.
7. Electromagnetic Induction
• Change in Magnetic flux in a conductor induces voltages in an near by
conductor
• AC current produces an alternating magnetic field.
8. Transformer
• Transformer uses Electromagnetic Induction to vary the voltage.
• The basic transformer
consists of an iron core, a primary winding
circuit, and a secondary winding circuit.
• An AC flowing through the primary winding
produces a changing magnetic field,
which permeates the core and induces
an alternating voltage on the secondary
winding.
• This mutual electromagnetic
induction is mediated by the containment
of the magnetic field in the iron core and
permeability through wire insulation.
9. Types of Transformer
There are three types of Transformer used in X ray circuit:
• Auto transformer: Autotransformers allow the selection of
input voltage to step-up and step-down transformers.
• Step Up Transformer : Step-up transformers are used to provide high
voltage to the x-ray tube.
• Step down transformer: Step-down transformers are used to
• provide high current to the x-ray tube filament. the incoming line
voltage drops to 5 to 15 V and Amperage increases to 3 to 5 A.
10.
11. • In a step-up transformer the voltage is increased but the
amperage must decrease to keep the power constant.
• Conversely, a step-down transformer will decrease voltage
from the primary coil to the secondary coil, with a
corresponding increase in amperage.
12. Transformer Efficiency
• Although transformer eficiency is typically above 95%,several factors influence
how much energy is lost.
Copper loss: Electric current in copper wire experiences resistance that results in heat generation.
Hysteresis loss: Results from energy expended as the continually changing AC current magnetizes,
demagnetizes, and remagnetizes the core material.
Eddy currents loss: These currents oppose the magnetic field that induced them, thereby creating a loss of
transformer efficiency.
The current flowing through the coils produces heat in the transformer, which
must be dissipated. X-ray transformers are usually placed inside a metal box.
The box is filled with oil to provide electrical and thermal insulation to
prevent electric shock and to cool the transformer.
13.
14. The High Voltage Transformer
• Has a step up ratio of 1000: 1 [Ns/Np]
• Current flows in 1 A to 1mA.
• Converts voltage to kilovoltage.
15. Rectifiers
• Rectiifers are solid-state devices that allow current to flow in only 1 direction.
• convert high-voltage AC from the secondary side of the step-up transformer to
high- voltage DC, which is applied to the x-ray tube.
• Current flows from + to – but in x ray tube current flows from – to + so it best
Works with DC otherwise it may result in no x ray production or damage to tube.
16. • Half-wave Rectifcation
• Half-wave rectiifcation uses solid-state diodes to effectively suppress the
negative portion of the AC sine wave. The positive portion of the sine wave is
utilized to produce pulsating DC
• Full-wave Rectiifcation
full-wave-rectiifed circuit because the negative part of the AC sine wave is
converted to positive current. With fullwave rectiifcation, the positive flow
remains the same but the negative portion is converted to positive current
flowing in only one direction, or DC. This produces more uniform pulsating DC sine
wave.
17. Types of Generators:
• Single Phase generator
• Three Phase generator
• High Frequency generator
18. Circuits
• Single Phase Circuit : Single phase power with 60 and 120. Zero Max Zero
• Three phase Circuit: Addition of two more transformer and rectifiers.
19.
20. High Frequency Generator
• High-frequency circuits first change the input frequency
from 60 Hz to a higher frequency (500–3,000 Hz)
21. Voltage Ripple
• Amount of the variation between the maximum and minimum
voltage.
Type of circuit Voltage Ripple %
Single Phase 100
Three phase 6 pulse 13
Three phase 12 pulse 3
High Frequency 1
23. Filament Circuit
• Cathode is heated by current from the low voltage supply.
• Output is controlled by MA selector.
• Increasing MA= More Heating Current to Cathode= Increased
temperature= Increased thermoionic emission.
• Two level of heating of Cathode:
1. Standby heating
2. During Exposure heating
24. Exposure timer
• Control the duration of x ray exposure.
• Terminates after the preset time have elapsed or the when the
receptor have received a specific level of exposure.
• Operator control switches and timers turns radiation ON and OFF.
• Of Two Types:
1. Manual Timers
2. Electronic Timer
3. Automatic Exposure control.
25. Manual Timer
• Manual Timer: is shut off after machine effectively counts in an
synchronous motor.
A solenoid whose winding
is in series with the timer
circuit.
The core, which is
magnetized when current
flows through the
solenoid.
27. • Electronic Timer :
Based upon the time it fully takes to fully charge the capacitor. When capacitor
limit is reached the electricity allows to run off from it . It is accurate and produce
the exposure time as short as 1 ms.
28. Automatic Exposure Control ( AEC) or Phototimer
• AEC measures the actual amount of radiation exposure incident on IR and
terminates the x ray production when required amount is obtained.
• Its components are:
radiation detectors,
an amplifier,
a film density or digital SNR
variable selector,
a termination switch, and
a backup timer safety
shutoff switch
29. • Has three photocells.
• They are 5 or 6 cm long by 3–4 cm wide.
• For PA Chest, the two outside
chambers are usually activated,
and the x-ray beam transmitted
through the lungs determines
the exposure time.
• This prevents signal saturation in
the lung areas, which can occur
when the transmitted x-ray flux
is otherwise measured under the
highly attenuating mediastinum
with the center chamber.
32. Bibliography,
• Sprawls P, et al, Physical Principals of Medical Imaging, Maryland:
Aspen: 1987,111-125
• Fosbinder R, Orth D et al, Essentials of Radiologic Science,
Pennsylvania: Lippincott Williams and Wilkins : 2011, 61- 73
• Bushberg J, et al, The Essential Physics of Medical Imaging, California:
Lippincott Willim and Wilkins, 2011, 65- 97
• Clarks, et al, Essential Physics in Imaging, United Kingdom, Churchill,
2013 , 42 - 46
33. Questions
• The automatic exposure control circuit:
A. turns off the x-ray beam when the proper exposure
is reached
B. measures the amount of radiation striking the
patient
C. measures the amount of radiation leaving the
cathode
D. turns off the filament cooling circuit when the
proper exposure is reached
34. 2nd year
• A rectiifer in an x-ray circuit
A. prevents positive charge from reaching the
anode
B. converts AC to DC
C. prevents excess grid bias on the anode
35. • 1st and 2nd Year
• A transformer can operate only on
A. AC
B. DC
WHY?
36. 1st year and 2nd Year
• An x-ray tube can operate only on
A. AC
B. DC
WHY?
37. 2nd Year
• An autotransformer functions as
A. line-voltage compensator
B. kVp or mA selector
C. filament transformers
D. automatic exposure controller
38. 2nd year
• Ripple measures
A. total tube voltage
B. variation between maximum and minimum mA
C. variation between maximum and minimum voltages
D. total mA
39. 2nd year
A transformer with more turns in its primary
winding than in its secondary winding would be
expected to:
A. increase the voltage and decrease the amperage
B. increase the voltage and increase the amperage
C. decrease the voltage and decrease the amperage
D. decrease the voltage and increase the amperage
40. • Describe the process of mutual induction.
2nd year or 3 rd year
41. 2nd year or 3 rd Year
• What is the relationship between the number of
coils in a winding and the amount of voltage and
amperage that is produced?
42. • Explain the purpose of oil in a transformer
2nd year
