Basic Circuitry and X‐ray Production Lynn C. Sadler, MSRS, R.T.(R)(QM) President, WCEC, Inc.
X‐Ray Production • What are X‐Rays? • Where do they come from? • What are some characteristics of x‐radiation? • How are x‐rays produced?
From the Beginning……. • Why learn this “stuff”? • Understanding circuits and electricity makes x‐ray production more understandable. • Understanding x‐ray production and the characteristics of x‐ radiation will make you a better technologist. – How?
Circuit • The path over which an electric current flows. • Consists of a source of energy, a conductor, and a load or resistor(s) ‐ (something that utilizes the electricity).
Circuit • Source of energy? – Battery – Generator – Outlet in wall???? • Conductor? – Gold – Silver – Copper
Types of Circuits • Series – Type of circuit where electron flow has only one path from beginning to end.
Types of Circuits • Parallel – Circuit has branches…. Electron flow has more than one path to take.
Factors in an Electric Circuit • Potential Difference or Electromotive Force – Volt • Current – Ampere • Resistance – Ohm
Potential Difference • Potential Difference is the force that “drives” electron movement in an electric circuit – Sometimes shortened to PD – Also referred to as “emf” or electromotive force • The unit of potential difference is the “ volt” – Voltage is used to measure potential difference – The higher the voltage, the greater the potential difference or emf, the greater “speed” at which electrons “move” through the circuit.
Current • Current is the number of electrons passing a given point per second in an electric circuit. – Is referred to as the “strength” or intensity of the circuit. (it’s a numbers thing) • The unit of current is the ampere. – The higher the amperage the more electrons that pass a given point in the circuit every second. – One ampere is equal to 6.25 x 10 18 electrons flowing per second. – That’s 6,250,000,000,000,000,000 electrons
Resistance • Resistance is the property of an electric circuit that hinders or opposes the flow of electrons • The unit of resistance is the ohm • The four things that determine the resistance of a conductor are: • Material • Length • Cross‐sectional area • Temperature
Factors in a cirucit • Potential difference, current, and resistance are inter‐related in a circuit. • Changing one factor will have an effect on another factor. • Ohm’s Law shows that relationship between voltage, amperage, and ohms.
Ohm’s Law • Voltage = Amperage x Resistance (V = I x R) – Remember this is a V ery I mportant R ule V I R
Ohm’s Law • If voltage is increased, and resistance is constant, what would happen to amperage? • If voltage is constant, and the resistance is increased, what would happen to amperage? • If voltage is constant, and the amperage is increased, what would happen to resistance?
Ohm’s Law • If the total voltage is 60 volts, and the total resistance is 20 ohms, what is the value of the current flowing through the conductor? • If the total resistance is 10 ohms and the value of the current is 6 amps, what is the total voltage?
But Wait…… There’s More! • One can also talk about current or a circuit in terms of power. • Electrical energy, just as any other form of energy can perform work. – Electrical energy is convertible to a definite amount of work and/or heat. – The power of a circuit is a measure of the amount of work or heat produced. – The unit of power is the watt.
Power of a Circuit • P = IV or IV = P – Power (in watts) is equal to amps x volts – Remember IVP • P = I 2 R – Since V = I x R, one can replace the V in P = IV with I x R. That’s how we get this other formula. – Power (in watts) is equal to amps squared x resistance – Amperage produces a lot of heat. This formula is usually referred to as “power loss”
Power of a Circuit • Power delivered to the x‐ray generator is essentially constant. • One can not vary the wattage of the x‐ray circuits, but can manipulate the values of amperage and voltage and/or resistance. • Just remember that amps x volts has to equal the wattage of the circuit.
The Basic X‐Ray Circuit • Two Circuits – Tube Circuit • Provides all the wiring for the production of x‐rays – Filament Circuit • Provides a source of electrons so x‐rays can be produced
Tube Circuit • Source: – Alternating current outlet (AC) – 220 Volts – Line voltage compensator adjusts incoming voltage so that 220 volts is always delivered to tube circuit.
Tube Circuit Source Main switch AC Source Image from: Stewart Bushong’s Radiologic Science for Technologists
Tube Circuit • Autotransformer – Also known as the kV selector – Allows you to vary the voltage sent to the rest of the tube circuit. – Works on the principle of electromagnetic self‐induction
Autotransformer Transformer Law N s V s = N p V p Image from: Stewart Bushong’s Radiologic Science for Technologists
Tube Circuit • Exposure timers – Determines the amount of time the tube circuit is energized. – Five main types: • Mechanical ‐ obsolete • Synchronous ‐ obsolete • Electronic • mAs timers • Automatic exposure controls (AEC)
Tube Circuit ‐ Exposure Timers Image from: Stewart Bushong’s Radiologic Science for Technologists
Tube Circuit • High Voltage Generator – Is a step‐up transformer – Increases voltage values to kilovoltage values – Works on the principle of electromagnetic mutual induction
Tube Circuit High Voltage Generator Primary Secondary Side Side Image from: Stewart Bushong’s Radiologic Science for Technologists
Tube Circuit • Rectification System – Changes alternating current (AC) to pulsating direct current (DC) – Why? ? ? ? Image from: Stewart Bushong’s Radiologic Science for Technologists
Tube Circuit • X‐Ray Tube – Where x‐ray production takes place.
Filament Circuit • Source – Same AC outlet as for tube circuit Image from: Stewart Bushong’s Radiologic Science for Technologists
Filament Circuit • “Rheostat” – Is a variable resistor used to regulate the filament current – Known as the mA selector – Modern tubes don’t use this anymore but utilize a high‐ frequency circuit to control filament current.
mA Selector in Filament Circuit Image from: Stewart Bushong’s Radiologic Science for Technologists
Filament Circuit • Filament Transformer – Is a step‐down transformer – Reduces the voltage and raises the amperage in the filament circuit Primary Secondary Image from: Stewart Bushong’s Radiologic Science for Technologists
Filament Circuit • Filament – Current of the filament circuit passes through the filament in the x‐ray tube. – High amperage of filament circuit causes the filament to become very hot. – Electrons are “boiled off” of the filament in a process called thermionic emission.
Filament of X‐Ray Tube
Putting It All Together!! Image from: Stewart Bushong’s Radiologic Science for Technologists
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