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Conventional generators (AC)

Conventional generators (AC). The mains alternating current (50 Hz) is applied to a circuit (basically a step-up transformer) which elevates it from 230V to 70.000V (70kV) the high voltage (still at 50Hz) is applied to a rectifier circuit which:

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Conventional generators (AC)

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  1. Conventional generators (AC) • The mains alternating current (50 Hz) is applied to a circuit (basically a step-up transformer) which elevates it from 230V to 70.000V (70kV) • the high voltage (still at 50Hz) is applied to a rectifier circuit which: • eliminates the negative portion of the waves (half wave rectifier), or • converts the negative half-waves into positive (full wave rectifier)

  2. Conventional generator (AC) kV • Initial waveform • Half wave rectifier • Full wave rectifier 70 0 t kV 70 0 t kV 70 0 t

  3. Conventional generator(AC) • The portion of wave useful for image formation is limited to the “crest”. • All the rest doesn’t produce film blackaning, but it is noxious for the patient. Useful X-ray portion kV 70 0 t

  4. HF and DC generator • The HF generator employs a specific circuitry (INVERTER) which generates a waveform of much higher frequency than the mains. • Typical operating frequency of an HF generator is in the order of several tens of kilohertz (kHz) • A specific circuits “flattens” the crests of the waves to further increase the amount of useful radiation. • Such almost flat waveform is called “constant potential”, or DC.

  5. Comparison of waveforms kV AC generator HF generator t t

  6. DC Generator Residual oscillation (RIPPLE) t If the ripple is smaller than 4%, the IEC standards allow to give the generator the “constant potential” or DC attribute.

  7. HF technology advantages • Tubehead more compact and lighter, because the high voltage transformer is smaller • Patient dose reduced by 25%, because the “soft” X-ray portion is reduced • Complete line voltage fluctuation insensibility • Exposure times shorter

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