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? 1
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. 2
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. 3
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. 4
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? 5
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. 6
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 7
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) 8
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 Side Secondary Side Image from: Stewart Bushong’s Radiologic Science for Technologists 9
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 10
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 11
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 12
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