X-ray Generators
✔An x-ray generator is the device that supplies electric power to the x-ray tube.
✔An x-ray generator supplies electrical energy to the x-ray tube and regulates the
length of the radiographic exposure.
✔ The x-ray tube requires two sources of
energy, one to heat the filament and the other to accelerate electrons between the
cathode and anode.
✔ The x-ray generator has a circuit for each of these functions; the filament and
high-voltage circuits. Also, the generator has a timer mechanism, a third circuit,
which regulates the length of the x ray exposure.
✔ The filament circuit contains a variable resistance, which is the current selector,
and a step-down transformer. The cathode-anode circuit, called the high-voltage
circuit, contains an autotransformer and a step-up transformer.
✔The autotransformer serves as the kVp selector.
✔The mechanism of an x-ray generator is usually continued in two separate
compartments: a control panel or console and a transformer assembly.
✔The controls allow the operator to select the appropriate kVp, rnA, and exposure
time for a particular radiographic examination.
✔ Meters measure the actual rnA and
kVp during the exposure. One exposure button (standby) readies the x-ray tube for
exposure by heating the filament and rotating the anode, and the other button
starts the exposure.
✔The timing mechanism terminates the exposure.
✔The second component is the transformer assembly, which is a grounded metal
box filled with oil. It contains a low-voltage transformer for the filament circuit
and a high-voltage transformer and a group of rectifiers for the high-voltage
circuit.
✔The potential differences in these circuits may be as high as 150,000 V, so
the transformers and rectifiers are immersed in oil.
✔The oil serves as an insulator and prevents sparking between the various components. A transformer is a
device that either increases or decreases the voltage in a circuit.TRANSFORMERS
✔A transformer consists of two wire coils wrapped around a closed core.
✔The core may be a simple rectangle with the windings wound around opposite sides of the rectangle The circuit containing the first coil (which is connected to the available electric energy source) is called the primary circuit, and the circuit containing the second coil (from which comes the modified electric energy) is called the secondary circuit
✔When current flows through the primary coil, it creates a magnetic field within the core, and this magnetic field induces a current in the secondary coil.
✔Alternating current is used for a transformer because it is produced by a potential difference (voltage) that changes continuously in magnitude and periodically in polarity (Figure 8). Current flows in one direction while the voltage is positive and
in the opposite direction while the voltage is negative.
✔The most important
characteristic of alternating current is that its voltage changes continuously, so it
produces a continuously changing magnetic field. Therefore, an alternating
current in the primary coil of a transfer produces an alternating current in the
secondary coil.
✔A transformer with more turns in the secondary coil than in the primary coil
increases the voltage of the secondary circuit and, appropriately, is called a stepup transformer. One with fewer turns in the secondary coil decreases the voltage
and is called a step-down transforme
✔Two simple laws govern the behavior of a transformer;
1. The voltage in the two circuits is proportional to the number of turns in the two coils.
NP = number of turns in the primary coil
Ns = number of turns in the secondary coil
VP = voltage in the primary circuit
Vs = voltage in the secondary circuit
✔Example; suppose the primary coil has 100 turns and the secondary coil has
30,000 turns. If the potential difference across the primary coil is 100 V, the
potential difference across the secondary coil will be Vs = 30,000 V (check)
2. An increase in voltage must be accompanied by a corresponding decrease in current. i.e. The product of the voltage and current in the two circuits must be
equal.
Ip Vp =Is Vs
VP = voltage in the primary coil
IP = current in the primary coil
Vs = voltage in the secondary coil
Is = current in the secondary coil.
✔In the previous example the voltage across the primary coil was 100 V, and that
across the secondary coil 30,000 V. If the current in the primary coil is 30 A, then
the current in the secondary coil will be Is =100mA (check)
✔ The product of voltage and current is power. If the potential difference is in
volts and the current is in amperes, then power will be in watts:
✔ There are two basic circuits in a diagnostic x-ray unit. One circuit contains the step-up transformer and supplies the high voltage to the x-ray tube.
✔The other circuit contains a step-down transformer and supplies the power that heats the filament of the x-ray tube. A transformer called the "autotransformer" supplies the primary voltage for both these circuits
Filament Circuit
✔ The filament circuit regulates current flow through the filament of the x-ray tube.
The filament is a coiled tungsten wire that emits electrons when it is heated by this
current flow (thermionic emission). Not much power is needed to heat this
filament to the necessary high temperature; a current flow of 3 to 5 A with an
applied voltage of about 10 V are typical values. This current merely heats the
filament, and does not represent the current across the x-ray tube.
High-Voltage Circuit
✔ The circuit has two transformers, an autotransformer and a step-up
transformer. The autotransformer is actually the kVp selector and is located in
the control panel. The voltage across the primary coil of the step-up
transformer can be varied by selecting the appropriate number of turns in the
autotransformer. The kVp can be adjusted in steps from approximately 40 to
150 kVp. Rectification
✔The incoming electrical supply to the x ray generator has an alternating potential.
Rectifiers are devices (usually silicon diodes) that transmit a current in only one
direction. A rectifier changes alternating current into direct current.
✔ The high-voltage transformer provides an alternating voltage for the x-ray tube.
The simplest way to use this high voltage is to hook an x-ray tube directly to the
secondary windings of the step-up transformer, with one side of the transformer connected to the cathode (filament) and the other to the anode (target) of the x-ray tube.
✔When the cathode is negative with respect to the anode, electrons flow at
high speed from the cathode to the anode and x rays are produced. During the next
half of the electrical cycle the target (anode) of the x-ray tube is negative and the
filament positive, so electrons, if they are available, would flow away from the
target toward the filament.
✔By blocking current flows in the inverse half of the electrical cycle, the x ray tube
changes an alternating current into a direct current, so it is, in effect, a rectifier.
✔ Because only half of the electrical wave is used to produce x rays, the wave form
is called half-wave rectification. Figure 10.A shows the wave form of the incoming electrical supply, Figure 10.B shows that of half-wave rectification, and Figure 10.C shows that of full-wave rectification for comparison. Only the upper half of each electrical cycle is used to produce x rays. When the x-ray tube itself serves as a rectifier, the circuit is called "self-rectified.
✔Self-rectification has two disadvantages;
First, half of the available electrical cycle is not utilized to produce x rays, so
exposure times must be twice as long as they would be if the whole cycle were
utilized. Second, as repeated or prolonged exposures heat the anode, it may
become hot enough to emit electrons and to produce a current during the inverse
half-cycle.
✔ The electrons in this current would bombard the filament and eventually destroy it. Therefore, to protect the x-ray tube and to improve the efficiency of the x-ray production, special rectifiers are incorporated into the highvoltage circuit.
✔The principal disadvantage of pulsed radiation is that a considerable portion of the exposure time is lost while the voltage is in the valley between two pulses. The
time spent bombarding the target with low-energy electrons does little except to
produce heat in the target and to produce low energy x-rays, which are absorbed
in the patient and raise patient dose.
TYPES OF GENERATORS
✔Three-Phase Generators Three-phase generators produce an almost constant potential difference across the x-ray tube.
✔ It is easier to understand three-phase current if we think of each phase in terms of degrees rather than in terms of time. Figure 13 shows all three phases separately
and superimposed on one another (bottom). Phase two lags 120° behind phase
one, and phase three 120° behind phase two. Thus, a three-phase generator
produces an almost constant voltage, because there are no deep valleys between
pulses.
✔ The ripple factor is the variation in the voltage across the x-ray tube expressed as
a percentage of the maximum value. With a single-phase circuit the ripple factor is
100 % because the voltage goes from zero to a maximum value with each cycle
(Fig.3-22). A six-pulse circuit has a ripple factor of 13.5%, which means that at
100 kV the voltage fluctuates between 86.5 and 100 k V. A twelve-pulse circuit
has a theoretical ripple factor of 3.5%. When three-phase generators are operated
under load, the ripple factor is accentuated. This is known as the load ripplefactor, and is always greater than the theoretical ripple. The load ripple-factor of a
twelve-pulse, three phase system is about 5%.
✔Single-phase generators may have half-wave rectification (60 pulses per second).
✔Three-phase generators may be six-pulse (360 pulses per second) with a
theoretical ripple factor of 1 3.5%, or twelve-pulse (720 pulses per second) with a
ripple factor of 3.5%. Special types of generators include capacitor-discharge
generators, battery-powered generators, medium-frequency generators, and falling
load generators.
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