Lapis ISSN 2398-2969

Radiography: x-ray machine circuit

Contributor(s): Vetstream Ltd, David Vella, Elisabetta Mancinelli


  • 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/3992°F 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 220 V to 10 V. 
    • 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).


  • Mains electricity is 240 V 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 > than the number in the primary the voltage is increased. 
  • Step-up transformer has many more turns on the secondary coil than the primary coil.


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Further Reading


Refereed papers