ISSN 2398-2977      

Radiography: x-ray machine circuit

pequis
Contributor(s):

Sarah Freeman


Circuits

  • X-ray machine circuits comprise three main components:
    • A circuit for heating the filament.
    • A circuit for applying a large potential difference (high voltage) between cathode and anode to accelerate electrons.
    • A timing device to control the length of exposure.

The filament circuit

  • The tungsten filament at the cathode is the source of electrons used to produce x-rays.
  • The number of electrons produced at the tungsten filament is dependent upon the temperature of the filament.
  • A tungsten filament needs to be heated to at least 2200°C to emit useful numbers of electrons.
  • Electrons are produced by thermionic emission.
  • When a metal is heated its atoms absorb energy which allows some electrons to move a small distance from the surface of the metal.
  • The filament circuit    Radiography: x-ray machine circuit - filament  consists of:
    • A step down transformer to reduce the voltage from 220V to 10V.
    • A variable resistor to control filament current and therefore filament temperature.
    • An ammeter to give an indication of filament current which is directly related to mA.
    • The tube current is directly proportional to the filament current so the ammeter is usually placed on the secondary circuit of the high voltage circuit as transformers are not 100% efficient.

High voltage circuit

  • This circuit produces a large potential difference between cathode and anode to accelerate electrons produced at the filament to high velocities.
    High tension transformers convert high Amp and low kV to mA and high kV.
  • It consists of 2 circuits (an autotransformer and step-up transformer)   Radiography: x-ray machine circuit - high voltage  .
  • The autotransformer:
    • Pre-reading voltmeter and line voltage compensation.
    • Measures incoming voltage and allows adjustments on the autotransformer so that the incoming line voltage remains constant.
    • This compensates for fluctuations in national grid.
    • Usually automatic on modern x-ray machines.
    • kV selector provides pre-determined voltage to primary turnings of the transformer.
  • Step-up transformer:
    • These alter the incoming voltage to kV.
    • The kV meter is placed across the primary circuit and measures incoming voltage but calibrated to read across the x-ray tube voltage (kV).

Transformer

  • Mains electricity is 240V and has to be modified to produce a high voltage across the x-ray tube head and low voltage to heat the filament.
  • Transformers comprise two coils of wire wound around an iron core   Radiography: x-ray machine circuit - transformer  .
  • When current flows through one coil (primary) a magnetic field is generated which induces a current to flow in the secondary wire coil.
  • The ratio of the incoming voltage to outgoing kilovolts is proportional to the number of turns on both the primary and secondary side (Vp/Vs = Np/Ns).
  • If the number of turns in the secondary coil is > the number in the primary, the voltage is increased.
  • Step-up transformer has many more turns on the secondary coil than the primary coil.

Rectification

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Timer

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