Species: Canine   |   Classification: Miscellaneous


  • 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 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.


  • The production of x-rays is dependent on the current flowing in one direction across the x-ray tube.
  • The step-up transformer on the high voltage circuit requires alternating current (AC) to function.
  • The current to the tube changes from AC to direct current (DC) by the process of rectification.
  • Ideally voltage across the tube head is kept constant so that x-ray production is consistent.
  • Half wave rectification:
  • These circuits contain two rectifiers Radiography X-ray machine circuit - half wave rectification and prevent the retrograde flow of electricity from anode to cathode.
  • For 50% of the cycle there is no voltage Radiography X-ray machine circuit - half wave rectification current across the tube head and x-rays cannot be generated.
  • Full wave rectification:
    • If a circuit includes four rectifiers Radiography X-ray machine circuit - full wave rectification it can be constructed so that current flows from cathode to anode unimpeded and retrograde current is reversed to ensure current flows continously.
    • The voltage across the tube head varies with time Radiography X-ray machine circuit - full wave rectification current.
  • This may be important if a large number of exposures are made rapidly, eg angiography.
  • A constant voltage may be achieved by placing condensors across the output from the rectifier circuit.
  • The condensor supplies voltage when the rectifier output is less than maximal.
  • Capacitor discharge unit:
    • A capacitor is charged from the mains to a preset kV.
    • During exposure kV decreases with time.
  • High frequency machines use a high frequency alternating current copper coil - this reduces the ripple effect and allows a higher kV compared with traditional machines.
  • Three phase rectification:
    • This circuit consists of 3 separate transformers linked to the mains supply with each out of phase by 120° Radiography X-ray machine circuit - 3 phase rectification.
    • A special mains supply is required.
    • As the voltage from one transformer starts to drop the current is boosted by the other two.
    • This results in an almost constant voltage across the tube head Radiography X-ray machine circuit - 3 phase rectification current.
  • The kV selector provides a variable but pre-determined voltage to the primary turnings of the step-up transformer.


  • X-ray machines haveelectronic timerswhich are accurate down to less than 0.01 seconds - which is important for measuring the length of exposure.
  • Electronic timers are based on a thyrotron tube (a gas filled triode).
  • Older machines may use mechanical timers based onspring loaded timersorsynchronous timerswhich use an electric motor to turn a cam.
  • These are less accurate than electronic timers and have to be reset between exposures so that rapid serial exposures cannot be taken.
  • Phototimerslimit the exposure after a predetermined amount of radiation has reached the film, or a set mA has passed through the tube.