Download Generatore convenzionale (AC)

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