Physics Test part 4 Flashcards
Rectification
the process of changing alternating current into a pulsating direct current.
X-ray circuits
Functions to convert electrical energy into electro-magnetic energy (x-rays). There are 3 main types.
Single phase
Uses a single source of alternating current (voltage waveform is to the right).
There are two different types of single-phase x-ray machines:
1) Half wave rectified
2) Full wave rectified
1/2 wave, 1-pulse, or self rectified
Uses only one half of the normal alternating current wave.
This is accomplished by the x-ray tube itself doing the rectifying or using 1 or 2 rectifiers. Takes longer to expose, only found in dental or small portables.
Full-wave rectified or 2-pulse
Uses the full alternating current wave by incorporating 4 rectifiers that channel the pulses through the x-ray tube in the same direction. Twice as many xrays produced over 1/2 wave.
3-phase (3Φ
This type of x-ray unit is powered by three separate sources of alternating current that are out of phase with each other by 120 degrees or 1/3 of a cycle. Voltage never drops to 0, shorter exposure times, less mA can be used.
Disadvantages of 3 phase units
higher capital and installation costs (generally 3 times or more) and the larger size of the equipment due to the additional electronic components required.
3Φ units also generate more heat during x-ray production, so high-capacity rotating anode tubes must be used.
3Φ, 6-pulse
Requires 6 rectifiers and rectifies ½ of the 3Φ alternating current so that 6 individual pulses pass through the x-ray tube each cycle for a total of 360 pulses/sec.
. 3Φ, 12-pulse
Requires 12 rectifiers to operate which rectifies all of the 3Φ alternating current, resulting in 12 pulses or current passing through the x-ray tube per cycle or 720 pulses/sec.
High Frequency Systems
These systems were created in the late 1980’s and have replaced most 1Φ systems in private offices and clinics.
This unit can be supplied with either a single phase or 3-phase AC that is sent into a microprocessor circuit prior to entering the high voltage section. These cans be smaller in size and less expensive.
Voltage ripple
the amount of variation from the maximum or peak voltage that occurs during x-ray production: smaller number means more efficient
1 phase voltage ripple
always 100% since the voltage falls from its peak all the way to zero and back to the peak value (it covers 100% of all possible values)
3 phase 6 pulse voltage ripple
13 to 14 % since another peak is rising before the other can drop all the way to zero.
3 phase 12 pulse voltage ripple
– 3.5 to 4%
High frequency
– can range from <1% to 15% (comparable to 3Φ) depending on model).
Control console
contains all controls for operating the x-ray tube and accessories, as well as various meters to monitor operation.
High voltage or xray generator
contains the transformers and rectifiers
Xray tube and accessories
Last xray circuit component
Circuit breaker
Prevents circuit overload and required by federal law.
Often separate from the main unit.
On/off switch
Federal law mandates a single throw-double blade or pole switch to prevent arcing.
kVp selector
Made up of an autotransformer (also called a variac) and selects that proper voltage that will be fed to the step-up transformer.
Line voltage compensator
Functions to keep the line voltage constant during x-ray production, since the line voltage can vary as much as 5% throughout the day.
mA selector
Selects the amount of current to the filament of the x-ray tube.
Can consist of either a rheostat, choke coil, saturable reactor, or high frequency circuit, depending on the type of unit.
Filament ammeter
Measures the number of amperes being sent to the filament of the x-ray tube and found only on older units.
