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

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

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

6. AC and DC generators Global Technologies Company June 2002 A 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. AC and DC generators Global Technologies Company June 2002 A 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