Download The X-ray circuit - Dr. Mohsen Dashti

The X-ray circuit: part I
By
Dr. Mohsen Dashti
357 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 threedimensional 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.
Electromagnetism
Rule 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 stepdown 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 highvoltage transformers in x-ray machines.
• They are suitable for controlling voltage on the lowvoltage 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 incomingline 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….