AC and DC generators Conventional generators AC The

AC and DC generators Conventional generators (AC) • The mains alternating current (50 Hz) is applied to a circuit (basically a step-up transformer) which elevates it from 230 V to 70. 000 V (70 k. V) • the high voltage (still at 50 Hz) 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) A Global Technologies Company June 2002

AC and DC generators Conventional generator (AC) k. V • Initial waveform 70 0 t k. V • Half wave rectifier • Full wave rectifier 70 0 k. V 70 0 A Global Technologies Company t t June 2002

AC and DC generators 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. k. V Useful X-ray portion 70 0 A Global Technologies Company t June 2002

AC and DC generators 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 (k. Hz) • 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. A Global Technologies Company June 2002

AC and DC generators Comparison of waveforms k. V AC generator t HF generator t A Global Technologies Company June 2002

AC and DC generators 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. A Global Technologies Company June 2002

AC and DC generators 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 A Global Technologies Company June 2002