Download 1) Half Wave Rectification

Rectification – The process of changing
alternating to direct current.
Necessary in order to prevent reverse bias
in the x-ray tube.
- Current must be allowed to pass in only one
direction in the tube (cathode to anode) to
prevent damage to the tube filament.
Can be accomplished by:
1) Half Wave Rectification – Suppressing the
negative half of the AC (1 pulse)
2) Full Wave Rectification – Inverting the
negative part of AC to a positive pulse
and using both pulses. (2 pulse)
Half Wave Rectification
Full Wave Rectification
All rectification systems are located between the
secondary side of the step-up transformer and
x-ray tube.
The Two Methods of Rectification:
1) Self-rectification
2) Valve tube or solid state diode rectification
1) Self-rectification – Performed by the
x-ray tube. Results in half-wave rectification
Voltage and current only flow when current
flows from cathode (-) to anode (+). One half
of the wave is suppressed (half-wave rectification)
2) Diode Rectification – Using a device (either
valve tube or solid state) that only allows
current to flow in one direction.
- diodes only allow current to flow from
cathode to anode.
Cathode
Anode
One rectifier diode results in half-wave rectification.
Two rectifier diodes also result in half-wave
rectification, but have better efficiency resulting
in less chance of reverse bias
Full-Wave (4 Diode) Rectification
Positive Cycle
high potential
Lower Potential
Negative Cycle
Results in 2 pulses
Four Diode Tube Rectification
Provides full-wave rectification (2 pulses/cycle)
Advantages
1) Allows us to use all of the current, which doubles
the amount of radiation produced for each
cycle of AC
2) Increases the heat loading capacity of tube
Disadvantage
1) Increased percentage of production of low
energy x-rays
Reduced by:
- Tube Filtration
- Three Phase or High Frequency Generation
Solid State Diode Rectification
Do you recall the difference between:
Conductors?
Insulators (dielectrics)?
Semiconductors?
Conductivity in Matter Depends on:
1) Energy State – Energy level of a particular
electron in an atom
- Electrons further from the nucleus of
an atom have higher energy levels
2) Energy Bands – Groups of atoms with the
same energy states grouped as a crystal
lattice.
The Three Energy Bands
1) Valence Band – Contains loosely bound (outer
shell) electrons
2) Forbidden Band – Opposes the movement
of electrons (no electrons can remain here)
3) Conduction Band – Place where electrons
can move freely
Large Band Overlap Large Forbidden Band
Small Energy Gap
(electrons flow easily
from valence to
conduction band)
(some electrons flow
& we have more
control over current)
(electrons do not flow due
large energy gap)
Solid State Diode Rectifiers
Stacked silicon diodes used to make current
unidirectional.
- Conductivity is related to coated impurities
- The Octet Rule is basic to understanding
why impurities are important
Octet Rule – Atoms tend to share electrons so
the outer shell of each contains 8 electrons.
Silicon (4 outer shell electrons)
fulfills the rule through covalent bonding with
other silicon atoms
Solid State Diode Rectifiers
Adding arsenic (As) with 5 valence electrons
to silicon leaves one extra electron not needed
for bonding (valence +1)
- It is used to make n-type silicon
N-Type Silicon
Adding gallium (Ga) with 3 valence electrons to
silicon results in a deficit of one electron for
bonding (valence -1)
- It is used to make p-type silicon (accepts electrons)
P-Type Silicon
Solid State Diode Rectifiers
By combining n-type and p-type silicon, we can
selectively control and rectify a circuit.
1) n-type silicon behaves as negative charges
2) p-type silicon behaves as positive charges
or holes to be filled
3) During forward bias, electrons enter the n-type
silicon from the source & p-type silicon drift
toward source at the n-p junction, completing
the circuit
4) During reverse bias, electrons from the n-type
pass towards the positive side of the circuit &
p-type silicon towards the negative side. This
breaks the circuit & current can no longer flow.
The Stick Rectifier
Consists of stacked diode modules
• Each can withstand 1,000 V.
• 200 kVp generator requires 200 stacked diodes
Advantages of Silicon Diode Rectifiers (Stick
Rectifier):
1) Compact
2) No filament burnout
3) Low voltage drop
4) Low reverse bias
5) long Life
Rectifier Failure
• mA meter on x-ray machine will indicate ½
of selected value
• Radiographs appear underexposed
Checked with spinning top for single phase machine
Shows both rectifier and
timer failure
Checked with synchronized (motorized) spinning
top for 3 phase or high frequency
Checks rectifier only
Critical Thinking Questions
1) Why is rectification necessary in an x-ray
tube?
2) What are the disadvantages to half-wave
rectification?
3) On a single phase, fully rectified machine, a
.25 sec. exposure produces yields 12 dots.
Is the machine functioning properly or is there
a problem? If there is a problem, what is it?
4) Explain what prevents reverse bias during the
negative half of the AC cycle when using silicon
diode solid state rectification?
5) Explain how doping semiconductors helps
control the flow of current in solid state
electronics.
Study Questions
1) What is rectification?
2) Why is rectification needed in an x-ray tube?
3) List the two primary mean of rectifying an x-ray tube.
4) Draw a diagram of a 4 diode rectification system without
referring to the book (remember to put labels on all parts
of the diagram, including the x-ray tube)
5)Where is the rectification system located on an x-ray tube?
6) What is meant by energy bands and how do they relate
to conductivity in semiconductors?
7) Define the term “solid state” diode rectification and what are
it’s advantages over valve tube rectification?
8) What are the two signs of valve tube failure?
9) What is the expected number of dots with a single phase, full
wave rectified unit using a .15 sec. exposure? How would this
be different for a half wave rectified unit? How many degrees
of arc would be expected if it were a 3 phase unit?
10) What is meant by 3 phase x-ray equipment?