The X-ray circuit: part I

By

Dr. Mohsen Dashti357 Radiologic Imaging & Processing

Lecture notes # 1

Lecture outline

• Magnetism.

• Electromagnetism.

• Controlling electrical current.

• Rectification.

Magnetism

• Magnetism is one of the fundamental forces in nature.

• What is magnetism?

• Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field.

• How is it created?

1. Through charged particles in motion known as orbital magnetic moment.

2. Through spinning electrons on their axes known as spin magnetic moment.

Magnetism

• What are the magnetic force fields?

- Lines of force or lines of flux.

- These lines of force flow not only through the magnet itself but outside the magnetic material, forming a three-dimensional field surrounding the magnet.

• What is the relationship between

magnetism and electric fields?

- Electromagnetism.

Electromagnetism

• When an electrical charge is moving or an electric current passes through a wire, a circular magnetic field is created. How was it discovered?

- Oersted’s experiment. What was the experiment?

- When there is NO current flowing in the wire, the compass needle aligns itself with the earth’s magnetic field. However, when current is flowing, the needle is deflected toward the wire until current stops.

- This experiment let to the conclusion that any moving charge produces a magnetic field.

Electromagnetism

• Oersted’s experiment.

Electromagnetism

- A series of simple aids known as “Fleming’s hand rules” were developed to understand the relationship between electricity and magnetism.

- What are the hand rules?

1. Hand thumb rules along a conductor.

2. Hand thumb rules for solenoid and electromagnet poles.

3. Hand generator effect rules.

4. Hand motor principle rules.

Electromagnetism Rule number 1: if the right hand grasp a conducting wire with the thumb in the direction of the current flow, the fingers will indicate the direction of the magnetic field lines of flux.

ElectromagnetismRule number 2: when the fingers point in the direction of the current, the thumb will point toward the north.-A solenoid is a series of loops creating a coil that helps the flux density to increase.- Electromagnet is further increase in the flux density by adding a ferromagnetic core.- The strength of solenoids and electromagnets is determined by the number of loops of wire, the current strength and the permeability of the core.

Electromagnetism

- Both solenoid and electromagnet demonstrate magnetic properties only while electric current is flowing.

- Electromagnets are used as remote control devices in circuit breaker on radiographic equipments.

- Electromagnets protect the radiographers from electrical shock by isolating control buttons on the x-ray console from the actual circuit in which high voltage is flowing.

- Solenoids are often used as detent locks to temporarily lock the tube to the center of the x-ray table or the proper distance from a film holder.

Electromagnetism

• Can a magnetic field create electrical current?

• Yes. How?

1. Move the conductor through a stationary, unchanging strength magnetic field.

2. Move lines of flux through a stationary conductor with unchanging magnetic field.

3. Vary the magnetic flux strength, causing the relative motion necessary to induce current.

Electromagnetism

Faraday’s experiment on electromagnetic induction

Controlling electrical current• Electrical current can be mainly divided into 2 types?

1. Direct current (DC): is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo or generator type.

- Direct current is used in nearly all electronic systems as the power supply.

- Direct current can be converted into alternating current by using rectifiers. (will be discussed later)

- Types of direct current include: battery, half-wave rectification and full-wave rectification.

Controlling electrical current

2. Alternating current (AC): the movement of electric charge periodically reverses direction.

- Alternating current has the ability to allow the increase or decrease of the electrical voltage. How?

- This is possible through the use of transformers.

• What is a transformer?

- A transformer is composed of two coils placed near one another but without electrical connection.

- If current is supplied to one coil, the lines of force induced will pass through the other coil to induce a flow of electrons.

Controlling electrical current

- In transformers, the coil that is supplied with current is the primary, while the coil in which current is induced is the secondary.

- The number of turns of wire in the primary coil is designed to be different from the number of turns of wire in the secondary. Why? (your answer).

- When the voltage is increased from primary to secondary it is called step-up transformer. When the voltage is decreased from primary to secondary it is called step-down transformer.

Controlling electrical current

Controlling electrical current

- Transformers are used to change voltage which is directly proportional with number of turns.

- Step-up transformer increases voltage from primary to secondary while decreasing amperage.

- Step-down transformer decreases voltage from primary to secondary while increasing amperage.

- Is it possible to construct a variable transformer by connecting both the primary and secondary coils in series instead of installing them?

- Yes and this is called autotransformer.

Controlling electrical current

• Autotransformers are not suitable for use in the high-voltage transformers in x-ray machines.

• They are suitable for controlling voltage on the low-voltage side of the x-ray circuit before it has been increased by the main step-up transformer.

• How many transformers in the x-ray circuit and what functions they do?

Controlling electrical current

1. The autotransformer: it is used to vary the incoming-line voltage to an appropriate level for the high-voltage step-up transformer.

2. The high-voltage step-up transformer: it is used to raise the incoming-line voltage to kilo-voltage range necessary for x-ray production.

3. The filament step-down transformer: it is used to decrease the incoming-line voltage to the 5-15 volt and 3-5 ampere range used to heat the x-ray tube filament.

Controlling electrical current

Transformers in the x-ray circuit

Rectification

• Liberation day gift………….

- Define the following:

- Rectification system used in the x-ray circuit.

- Anode and cathode.

- Explain the difference between half-wave and full-wave rectification.

Enjoy the holidays….