physics session 5&6 (tony)
There are 125 turns on the primary side of a transformer and 90,000 turns on the secondary side. If 110 V AC is supplied to the primary winding, what is the voltage induced in the secondary winding?
90,000/125 = x / 110 V 720 = x / 110v x= 79200 volts x= 79.2 kV this is a step up transformer
Solid state p-n junction
Conducts electricity in only one direction Solid state diodes are rectifiers because they conduct electric current in only one direction
The primary circuit
Consists of the main power switch (connected to the incoming power supply), circuit breakers, the autotransformer, the timer circuit, and the primary side of the step-up transformer.
The secondary circuit
Consists of the secondary side of the step-up transformer, the mA meter, a rectifier bank, and the x-ray tube (except for the filaments).
Circuit Sections
Control Console Line monitor Autotransformer Line compensator kVp selection mA Selection Timing circuit Time selection High Voltage X-ray Tube Circuit Step-up transformer Rectification circuit mA meter X-ray tube Filament Circuit Step-down transformer Focal spot selection Filaments
Tube current
Crosses from cathode to anode Measured in milliamperes (mA) Number of electrons emitted by the filament is determined by the filament temperature Filament temp is controlled by filament current which is measured in amperes 3-6 A is normal filament current Thermionic emission - Release of electrons from a heated filament
Voltage rectification
Current is 60Hz from wall outlet Changes direction 120 times a second X-ray tube can only operate in DC - Electron flow in one direction Rectification converts AC to DC Transformers operate with AC
Unrectified voltage
Current that passes through the x-ray tube only exists during the positive half of the cycle It cannot exist during the negative half - The anode is not constructed to emit electrons
Circuit Diagram of Imaging System
Left side is low voltage because it hasn't gone through a step-up transformer yet Mid is high voltage because it stepped over the autotransformer
Falling load generator
Max mA, anode heats, mA drops - Minimum exposure time
mA meter
Monitors tube current Placed halfway on the secondary windings - (0 volts because halfway between 60Hz AC) Meter not in contact with high voltage
Semiconductors
N- type - Loosely bound electrons P-type - "holes" where there are no electrons mobile like electrons
The secondary side of a transformer has 300,000 turns; the primary side has 600 turns. What is the turns ratio?
N_p = 600 N_s = 300,000 Ratio = 300,000/600 Ratio = 500:1 (500:1 for every 1 turn of the primary side of the transformer there are 500 turns on the secondary side of the transformer)
filament current amount to produce tube current
Normally, a rather large filament current of 5-7 amperes is required to produce a tube current in the range of milliamperes.
Turn ratio
Ns/Np is known as the turns ratio P = primary S = secondary
Quantity
Number of x-rays Intensity of x-ray beam Milligray (mGy)
Filament circuit
Precision resistors are used to reduce the voltage that corresponds to the selected millamperage Fixed stations that provide tube currents of 100, 200, 300, 400 mA
Autotransformer picture
Primary side (left side) Secondary side (right side) Mutual induction tells us that the moving charged particle creates a magnetic field around it Creates a current on the right side by the movement of a charged particle (electron) moving through the charged coil on the left side
Full-wave rectification
Rectifies the entire AC waveform Contains at least 4 diodes The anode is always positive because the negative portion of the waveform is reversed (allows exposure times to be cut in half)
Filament transformer
Step down transformer - Voltage decreased, current increased 12V provides the current Secondary windings heavily insulated because on the high voltage side - 12V and current of 5-8 A Primary windings are thin copper - 0.5-1A and 150V
From the rheostat, electricity then travels to the step-down transformer.
The adjusted current from the step-down transformer then travels directly to the filament located within the focusing cup of the x-ray tube. This current heats the filament to the point where electrons are literally boiled off.
current vs voltage
The change in current across a transformer is in the opposite direction from the voltage change but in the same proportion: an inverse relationship. For example, if the voltage is doubled, the current is halved.
The change of voltage or current through a transformer:
The change in voltage or current through a transformer is an inverse relationship. If voltage is increased, current decreases and vice versa (*Critical concept- Inverse relationship between current and voltage)
At this point, we return to the autotransformer to fill in the other half of the process.
The filament circuit draws electricity from the autotransformer, which then travels to the rheostat.
Half-wave rectification
The inverse voltage is removed from the supply to the x-ray tube by rectification Half-wave is a condition where the voltage is not allowed to swing negatively (Wastes half of the supply of the power Requires twice the exposure time)
Primary Circuit
The main power switch is simply an on-off switch for the unit and is connected to the power supply of the facility.
Working in concert with the rheostat is the exposure timer.
The rheostat controls filament temperature and the rate at which electrons are boiled off of the filament. The timer determines the duration of this process. Together, they determine the quantity of electrons boiled off of the filament and available for x-ray production.
When the turns ratio is less than 1
The transformer is a step-down transformer - because the voltage is decreased or stepped down from the primary side to the secondary side.
X-ray Circuit sections
The x-ray circuit may be divided into three sections: 1 the primary circuit, 2 the secondary circuit 3 the filament circuit.
When the turns ratio is greater than 1
This is a step-up transformer - because the voltage is increased or stepped up from the primary side to the secondary.
P-n junction
Tiny crystals of n-type placed in contact with p-type If higher potential on the p side then the electrons and "holes" start to travel and move across the junction
transformer law
V_s/V_p = N_s/N_p P = primary S = secondary (A way for us to be able to tell if our voltage is initially increasing when being placed through our transformer) Step up- takes incoming voltage and makes it higher Step down - makes incoming voltage lower
Line compensator
is a device usually wired to the autotransformer and automatically adjusts the power supplied to the x-ray machine to precisely 220 volts. (makes sure there is a 22- volts being applied, compensates for variability in the outlet. Makes sure there is a steady 220V going to the system.) *variability usually around 5% (11V)
The autotransformer
is an adjustable transformer controlled by the kilovolt peak (kVp) selector on the operating console (Autotransformer is single coil) The lines on the side are the kVp selectors. *How many coils are needed to make 60k volts or etc *there is a major and minor kVp selector , allows to tap into the amount of coils needed for select kVp Typical kVp (as told by colleen is 140)
The timer circuit (exposure timer)
is located in this section (primary circuit) because it is easier to control (turn on and off) a low voltage than a very high one.
A step-down transformer
is used in the filament circuit to increase the current by reducing the voltage that is applied to the filament.
The step-up transformer
is used to increase the voltage from the autotransformer to the kilovoltage necessary for x-ray production (This transformer is the dividing line between the primary and secondary circuits. The primary coil is in the primary circuit and the secondary coil is in the secondary circuit (hence the names).)
The large negative charge on the cathode serves to:
keep the electrons crowded together; otherwise, they would repel each other and scatter throughout the tube.
After passing through the rectifiers:
the electricity creates a large positive charge on the anode of the x-ray tube and a large negative charge on the cathode focusing cup
How is voltage affected by turns ratio?
the voltage change across the transformer is proportional to the turns ratio.
3 principle parts of imaging system
1 X-ray tube 2 Operating console 3 High-voltage generator
The final piece of the filament circuit is the filaments.
(A general-purpose radiographic tube typically has two filaments. They are represented on the operating console by the "large focal spot" and "small focal spot.") The goal of the filament circuit is to literally boil electrons out of the filament wire.
# of diodes in half-wave
0,1, or 2 diodes The x-ray output from half-wave high voltage generator pulsates - produces 60 x-ray pulses per second
Major and Minor kVp selection
220V in primary output between 100-400V This becomes the input to the high voltage step up transformer Which then increases the voltage to kilovoltage kVp meter - On output terminals, measures voltage not kilovoltage - Pre reading kVp meter - Monitors voltage
Operating Console
Allows for control of x-ray tube current and voltage Need x-ray beam to be of proper quality and quantity
The filament circuit
Consists of a rheostat, a step-down transformer, and the filaments
Safety feature OF Exposure Timers
Guard timer- terminate after a prescribed time Not possible to irradiate forever
Exposure Timers
Radiographer starts the exposure and the timer ends the exposure
circuit breakers
are included in the primary circuit to protect against short circuits and electric shock.
The rectifiers Within the x-ray tube
are needed to convert AC to DC. The rectifier commonly used in today's x-ray circuit is a solid-state rectifier that is made of two semiconducting crystals. One is a p-type crystal and the other is an n-type crystal. (Rectifier takes alternating current (normal in outlets) and turns it into a Direct Current (allows current to only flow in one direction))
current
current flows in the opposite direction of electrons REMEMBER: arrow head points in direction of conventional electric current which is opposite to the flow of electrons
Filament Circuit
filament circuit begins with the rheostat. This variable resistor is controlled by the mA selector on the operating console. The parameter "mA" is an abbreviation for "milliampere" and is called "tube current" because it reflects the rate of flow of electrons passing through the x-ray tube during an exposure.
The mA meter
is simply a device placed in the secondary circuit that monitors x-ray tube current.
Solid-state rectifiers are used to:
route electricity through the x-ray tube correctly
The kVp selected adjusts:
the autotransformer and determines the number of turns on the secondary side necessary to produce a voltage, through self-induction, that will be sent to the step-up transformer.
In a step-down transformer
the current on the secondary side (Is) is larger than the current on the primary side (Ip)
In a step-up transformer
the current on the secondary side (Is) is smaller than the current on the primary side (Ip)
The step-up transformer increases
this voltage by a fixed amount and through mutual induction produces the kilovoltage selected on the operating console. (Step up transformer is what actually achieves the set kVP, turns the voltage from x volts to y kVp)
The turns ratio of a filament transformer is 0.125. What is the filament current if the current through the primary winding is 0.8 A?
use inversed turns ratio to solve for I_s directly I_s = I_p (N_p/N_s) I_s = 0.8 A (1/0.125) I_s = 6.4 A OR: I_p/I_s = N_s/N_p 0.8 / I_s = 0.125 0.8/0.125 = I_s I_s = 6.4 A This is a step down transformer
autotransformer
(important to the x-ray machine) Operates on the principle of self-induction. Has only one coil of wire around a central magnetic core serving as both the primary and secondary coil. The outside wires are attached at different points along the coil, and the induced voltage varies depending on where the connections are made.
Secondary Circuit
circuit begins with the rest of the step-up transformer.
The rheostat is a variable resistor
controlled by the mA selector on the operating console and when the mA station was selected, the appropriate filament was also selected
The kilovoltage applied to the x-ray tube
created a large positive charge on the anode and a large negative charge on the cathode (focusing cup). The large positive charge attracts the electrons boiled off the filament, giving them tremendous kinetic energy in the process. The electrons travel across to the anode and interact there to produce x-rays until the timer circuit terminates the process.
Autotransformer
Electromagnetic induction Primary and secondary connections The voltage it receives and the voltage it supplies are related directly to the number of turns enclosed by the connections Autotransformer law is the same as the transformer law
Space charge effect
Electron cloud near filament As kVp increased, anode becomes more attractive to the electrons - Electrostatic force
Diodes
Electronic device that contains 2 electrodes
Tube crystals
When flow needs to be restricted, use same sign crystal When flow needs to be pulled, use opposite sign crystal
Current in the x-ray tube
Within the x-ray tube, current must always flow from anode to cathode and electrons from cathode to anode, and rectifiers are used to achieve this. (The x-ray tube can only operate on direct current (DC) [turns a sine way of electricity into a consistent positive line]) ***Alternating current is current that goes from positive to negative back and forth (up and down sine way)
