Generators And The X-ray Circuit Flashcards
Flow of electrical current is in the _______ direction of the flow of electrons
OPPOSITE
Electrodynamics
Two types of electric current
- alternating current (AC) -> changes direction in cycles as the electrical potential changes
- direct current (DC) -> flows only in one direction, from positive to negative
Alternating current (AC)
-changes direction in cycles as the electric potential changes
Direct Current (DC)
-flows only in one direction (from positive to negative, opposite the direction of electron flow)
General x-ray circuit
Basic x-ray circuit can be divided into:
- main x-ray circuit
- purpose: to produce x-rays
- supplies x-ray tube with properly modified power so that it can produce x-rays
- filament circuit
- purpose: to create appropriate electron cloud
- supplies filament of x-ray tube with properly modified power to produce appropriate thermionic cloud
Components of general x-ray circuit
Main x-ray circuit
-primary side/circuit
🔹main power switch, circuit breakers, autotransformer, timer circuit and primary side of the step up transformer
-secondary side/circuit
🔹secondary side of step up transformer, mA meter, rectifier bank, x-ray tube (except filaments)
-filament circuit
🔹rheostat, step down transformer, filament
Primary circuit
- main power switch
- line compensator: used because the incoming power isn’t a consistent 220 volts, usually wired to the autotransformer, automatically adjusts the power supply to precisely 220 volts
- circuit breakers: protect against short circuits and electric shock
Primary curcuit
Autotransformer (kVp selector)
-adjustable transformer controlled by kVp selector on control panel
-when tech selects kVp setting, they are controlling this transformer
🔹determines # of turns on the secondary side to be included: which controls the output voltage sometimes called kVp selector
-primary purpose is to provide a voltage that will be increased by the step up transformer to produce the kilo-voltage selected at the operating console
Primary circuit
Step up transformer
-dividing line between primary and secondary circuits
-increases voltage from the autotransformer to the kV needed for x-ray production
🔹not adjustable, increases kV by a fixed amount
-primary coil is in primary circuit, secondary coil is in secondary circuit
Primary circuit
Timer circuit (exposure timer)
- located in primary circuit because its easier to control a low voltage vs a very high one
- purpose of the device is to ‘make or break’ the high voltage across the tube
- four types: synchronous timer, electronic timer, mAs timers, AEC
Secondary circuit
-begins with other side of step up transformer
-mA meter is there to monitor x-ray tube current
-AC is required for the transformers to work properly, but the x-ray tube requires DC current
🔹in the tube, electrons must always flow cathode to anode
Rectifiers (in secondary circuit)
- purpose is to convert AC (alternating current) to DC (direct current)
- solid state rectifier is commonly used
- consists of two semiconducting crystals joined together to form a solid state diode
Why do we need rectifiers
-when the cathode is negative and the anode is positive
🔹electrons will move across and x-rays will be produced
-if the anode of the x-ray tube is negative and the cathode positive
🔹no x-rays will be produced
🔹anode is not constructed to emit electrons, so no electrons moving to the cathode
Rectifiers
-when a positive charge is placed on the p-type crystal, and negative charge on the n-type crystal, the diode will conduct electricity
-both the traps and the electrons will move across the PN junction allowing current flow
-when AC cycle reverses, the diode will not conduct, the traps and electrons do not move to the junction and no current is conducted
-solid state p-n junction conducts electricity in only one direction (junction is called a diode)
-solid state diodes are rectifiers because they conduct electric current in one direction
🔹one way valve
🔹necessary to route the electricity through the x-ray tube correctly
Unrectified voltage
- current that passes through the x-ray tube exists only during the positive half of the cycle, where cathode is negative and anode is positive
- during the negative half of the cycle, current flows from anode to cathode, but this is not possible as the anode cannot emit electrons
Half wave rectification
- during positive portion of AC waveform, the rectifier allows electric current to pass through tube
- during negative portion of AC waveform, the rectifier does not conduct, no electric current is allowed
- result is positive pulses with gaps when current is not being conducted
- resultant electric current is rectified because the electrons flow in only one direction
- called half wave rectification because only one half of the AC waveform appears in the output
Full wave rectification
- shortcoming of half wave rectification is that it wastes half of the power supply and thus requires double the exposure time
- with 4 rectifiers, can utilize the negative half cycle
- inverts the negative half so that the anode is always positive
- energy fluctuates from max voltage to zero-100% ripple
- this is a problem for x-ray production
- x-rays produced will vary from 0 to kVp selected on console
Rectification
- not very effective when the energy level drops to 0
- one solution is to use 3 AC waveforms at the same time, but slightly out of phase or synchronized
- out of step by 1/3
- called 3 phase power
- depending on the number of rectifiers and engineering, can have ripple anywhere from 4-14%
Three phase power
-major improvement over single phase power generator where 100% ripple mean voltage supply to the tube would produce x-rays between 0-100% of the kVp set
-3 phase 6 pulse:
🔹14% ripple
🔹voltage supply to x-ray tube never falls below 86%
-3 phas 12 pulse
🔹4% ripple
🔹volatge supply to the tube never falls below 96%
High frequency generators
- the next step in improving the waveform is the use of HF generators
- takes the regular 60Hz and converts it to higher frequency (500-2500Hz) and transfers it to high voltage
- the standard 60 Hz incoming power is first fully rectified, then sent through a capacitor bank where it is smoothed
- from there it passes through an inverter circuit that ‘chops’ the DC waveform and converts it to high frequency AC
- now it is low voltage high frequency AC, which will pass through a step up transformer to turn it into high voltage AC
- then it passes through rectifiers again, to become fully rectified and finally through high voltage capacitors where it is smoothed to provide the x-ray tube with a near constant potential voltage waveform
- ripple reduced to less than 1%
Advantages of HF generators
- smaller, lightweight
- less costly
- better exposure reproducibility
Efficiency
Advantages of having less ripple are:
- greater radiation quantity and quality that results from the more constant voltage supplied to the tube
- quantity of photons is higher because the efficiency of the x-ray production is higher when the tube voltage is high
- fewer low energy photons
Falling load generator
- uses the shortest exposure time possible
- method: initial tube loading is higher, and drops during exposure
- results in achieving the set mAs, in a shorter exposure time
- principle use in areas where the shorter the exposure time, the better (interventional radiology)
In the secondary circuit
-when the exposure switch is pressed
-voltage goes through step up transformer, to increase to the kilovltage selected on control panel
-kV goes through rectifiers so that it travels through the tube correctly to produce x-rays
🔹large positive charge on anode
🔹large negative charge on cathode
-to create x-rays, need one more thing: electrons
