Electrical components and circuits

Introduction

• Radiographers require understand the basic elements of X-ray machines and each process step.
• Radiographers are responsible for patient safety.
• Understanding how X-ray machines operate allows radiographers to use them with maximum efficiency.

The Nature of Electricity

• Electrostatics studies of stationary electric charges.
• Electric charge refers to a property of matter.
• The coulomb is used to measure electrical charges.
• 6.25 × 10^18 electrons is equivalent to one coulomb of electrical charge.
• Good conductors contain many free electrons.
• Good insulators contain few free electrons.
• Electrodynamics studies the motion of electric charges, also known as electricity.
• An electric potential must exist for an electric current to move.

Electric Potential

• Electric potential refers to the ability to do work resulting from a charge separation.
• Volts measures electric potential.
• One volt is the potential difference needed to maintain a current of 1 ampere in a circuit with 1 ohm resistance.
• A Volt expresses of the electric potential difference between two points.
• A volt is also equal to the amount of work in joules that can be done for each unit of charge.
• A volt is the ratio of joules to coulombs, written as volt = joules/coulombs.

Current

• Current expresses the flow of electrons in a conductor and its unit is the ampere.
• Electric current needs a potential difference between two electrodes and a suitable medium to flow.
• There are 2 types of current:
  • Direct Current (DC) flows in one direction and an example is batteries.
  • Alternating Current (AC) changes direction as the electric potential of the source changes and an example is home electricity.
• In basic X-ray production, both AC and DC are used.

Resistance

• Opposition to the current flow in a circuit is resistance, measured in ohms.
• The composition of the circuit dictates the resistance present.
• Four factors affect resistance amount:
  • Conductive material
  • Conductor length
  • Cross-sectional diameter
  • Temperature
• Resistance is useful and important in x-ray production.
• Conductors contain many free electrons, facilitating relatively free electricity flow.
• Insulators consist of atoms with tightly bound electrons and poor electricity conduction, even with potential differences.
• Semiconductors conduct electricity, but not as well as conductors, and insulate, but not as well as insulators.

Electric Circuit

• An electric circuit functions as a closed pathway, composed of circuit elements and wires allowing electricity to flow.
• A closed circuit is a complete pathway that electricity must travel
• An open circuit has a pathway that is broken, like when a switch is turned off.
• Circuit elements wired along a single conductor form series circuits.
• Parallel circuits contain circuit elements that "bridge" or branch across a conductor.
• An X-ray circuit is a complex version with different voltages and currents flowing through different sections.

Common Circuit Devices

• Battery use: produces electrons through chemical reaction, stores long term electric charge, provides electric potential.
• Capacitor use: temporarily stores electric charge.
• Diode use: a "one-way valve" device allowing electrons to flow in only one direction.
• Protective devices (fuses, circuit breakers) use: emergency devices that breaks/opens the circuit if excess electricity is being used.
• Resistor (and rheostat) use: inhibits electron flow/precisely regulate the flow of electricity; a rheostat is an adjustable/variable form of resistor.
• Switch use: opens circuit, breaks pathway.
• Transformer use: increases/decreases voltage by a predetermined amount.

Magnetism

• Magnetism is the ability of a material to attract cobalt, iron, or nickel.
• Magnetic materials have orbital electrons of their atoms spinning in a mostly singular direction.
• Magnetic dipoles are created by atoms.
• Groups of similarly aligned atoms or atomic magnets form magnetic domains.
• Flux:
  • A magnetic field consists of lines of force in space.
  • Lines of flux travel from the south pole to the north pole inside the magnet. Outside the matter, the travel from north to south.
  • Lines of flux in the same direction repel and flux in opposite direction attract.
  • Magnetic fields are distorted by magnetic materials but nonmagnetic materials do not affect them.
• Three laws of magnetism help to explain electromagnetism:
  • Every magnet has a north and south pole.
  • Opposite poles attract, like poles repel.
  • Attraction/repulsion force varies directly with pole strength and inversely with distance squared between them.
• The SI unit, Tesla (T), measures the strength of a magnetic field.
• Magnetic resonance imaging (MRI) units for medical imaging are defined by their magnetic field strength, usually 0.5 - 5 T.
• Magnetic classifications:
  • Nonmagnetic materials like glass, plastic, and wood are not attracted to magnetic fields.
  • Diamagnetic materials like mercury, gold, and water are weakly repelled by magnetic fields.
  • Platinum, gadolinium, and aluminum are examples of Paramagnetic materials that are weakly attracted to magnetic fields.
  • Ferromagnetic materials like iron, nickel, and cobalt are strongly attracted to magnetic fields.

Electromagnetism

• Magnetism and electricity represent one basic force.
• Any flow of electrons in space or in a conductor creates a surrounding magnetic field.
• A moving magnetic field can create an electric current.
• Hans Oersted:
  • He first identified electromagnetism.
  • Intensifying the magnetic field surrounding a conductor can be done by fashioning it into a coil (solenoid).
  • Further intensification involves adding an iron core to the coil (electromagnet).
• Michael Faraday found that an electric current is induced in a conductor when moving it through a magnetic field (electromagnetic induction).
• Mutual induction occurs when a moving magnetic field induces electricity in secondary coil.
• Electricity begins to flow within that secondary coil if a moving magnetic field is next to it.
• Self-induction:
  • Needs understanding of Lenz's law: an induced current goes in a direction opposing the action that induced it, as well as the changing magnetic field.
  • A coil carrying electrical current then creates a magnetic field, which expands outward from the coil's center.
  • The "cutting" creates a current within the same conductor that opposes the original as it "cuts" through the coil's turns.

Generators, Motors, and Transformers

• Electric generators convert mechanical energy into electrical energy.
• Electric motors convert electrical energy into mechanical energy through electromagnetic induction.
• Transformers increase or decrease voltage/current through electromagnetic induction.
  • A step-up transformer increases voltage.
  • A step-down transformer decreases voltage.
• The change to voltage and current have an inverse relationship.
• Closed-core/shell-type transformers use ferromagnetic cores to maximize efficiency.
• Autotransformers: -They operate on the principle of self-induction.
  • They have a single wire coil around a central magnetic core serving as the primary and secondary coil.

General X-Ray Circuit: The Primary Circuit

• The main power switch connects the unit to the facility's power supply and is an on/off switch.
• The circuit breakers in the primary circuit protect against electric shock and short circuits.
• Autotransformer is an adjustable transformer that is controlled by the kVp selector.
• Selection of a kVp setting will determine the transformer to be used.
• The autotransformer's purpose is to provide the voltage for the step-up transformer to produce the kilovoltage chosen at the operating console, it is also called the kVp selector.
• Since it's easier to control a low voltage that a very high one, the timer circuit is located in the timer circuit.
• The step-up transformer increases the voltage from the autotransformer to the kilovoltage required for x-ray production.
• Unlike the autotransformer, the voltage is not adjustable, and the vote increases by a specified number.

General X-Ray Circuit: The Secondary Circuit

• The secondary circuit's components include:
  • Remaining components of the step-up transformer.
  • Milliampere (mA) meter, which is responsible for monitoring the current of the X-ray tube.
  • Rectifiers to convert AC to DC.
  • The X-ray tube; excluding the filaments.

Secondary Circuit

• In order to convert AC to DC, rectifiers are needed:
  • Rectifiers are set up in pairs to allow this conversion.
  • In half-wave rectification, one half of the AC cycle flow via the x-ray tube, while the other half is suppressed.

General X-Ray Circuit: The Filament Circuit

• Filament circuit components include:
• Rheostat which adjusts filament temperature and thus, the rate electrons are boiled off of the filament.
• Regulating milliamperage (mA) on the operating console adjusts this rheostat, changing resistance in the filament circuit and current applied to the tube's filament.
• To increase the current, reduce voltage applied to the filament by using a step-down transformer.
• Filaments:
  • Two are typically found in general-purpose radiographic tubes.
  • The operating console denotes a “large focal spot" and "small focal spot" representing them.

Principles of X-Ray Circuit Operation

• The operating console allows for selecting the focal spot size, kVp, mA, and exposure time.
• The radiographer specifies kVp, mA, exposure time (or mAs), and focal spot when starting the exposure technique.
• The autotransformer is adjusted with the selection of kVp, and determines the number of secondary turns needed to create a voltage, through self-induction, to be distributed to the step-up transformer.
• Step-up transformer uses mutual induction to produce kilovoltage that is selected in the operating console by increasing the voltage by a value that has been set.
• It transitions to the secondary circuit form the primary circuit.
• Solid-state rectifiers route electricity through the x-ray tube and are used for to rectify the kilovoltage.
• These "one way valves" route flow of current from (+) to (-), and electrons from (-) to (+).
• Passing through the rectifiers creates a large positive charge on the anode, and a large negative charge on the portion of the cathode that surrounds the filaments.
• Electricity is drawn from the autotransformer, traveling to the rheostat, for the filament portion of the process.
• This variable resistor: rheostat, is controlled on the operating console, and when mA is selected, an appropriate filament is selected.
• Electricity form the rheostat transfers to the step-down transformer.
• Direct filament location in focusing cup of x-ray tube recieves current from the step-down transformer.
• Filament heat is increased to the point that allows electrons to boil off current.
• The kilovoltage applied to the x-ray tube creates a larger positive charge on the anode and negative charge on the cathode.
• In the process, large positive charge attracts boiled off filaments, allowing them to create tremendous kinetic energy, traveling across to interact on the anode to produce x-rays until time circuit closes.

X-Ray Generators

• Generators supply power to create X-rays.
• Several designs of X-ray generators of varying complexity and cost exist, including: single-phase, multiphase, high frequency, capacitor discharge and battery-operated generators for mobile units.
• Single phase generators are in older equipment.
• When full-wave rectification is used, produce 2 voltage ripple pulses per hertz/120 pulses per second.
  • 2 usable pulses with a ripple of 100% is produced.
  • This drops the voltage twice each cycle/period.
• When full-wave rectification is applied to Multiphase Generators, with its 3 currents, it produces 6 pulses.
• Three-phase power is six-pulse which produces ripple of 13-25%.
• Voltage in the tube never dips below 75-87% peak kilovoltage.
• A full wave rectified, three-phase, six-pulse waveform releases 35% more average photon energy than a full-wave rectified, single phase.
• High frequency generators: the incoming-line voltage frequency is change from 60 Hz to the thousands of Hz with AC and CD converters.
  • Supplying high-frequency current through the use of full-wave rectified power can produce roughly 12-13 kHz.
  • Sent pulses in circuit controlled by an oscillator/inverter unit increases the sending frequency and decreases the close together sending.
  • At 10% time span as three-phase generators and only 3-4% voltage ripple, the peak kilovoltage is achieved by the use of this wave when applied to the X-ray tube.

Test your knowledge about electricity. This quiz covers important electrical components like diodes, capacitors, AC, and insulation. It also includes questions about parallel and series circuits.