Transcript Physics 4 - NYCC SP-01

Chapter 4
Rectification
and
Rectification Systems
Rectification

The premise of rectification is to convert
indirect current into direct current. We have
already seen how the x-ray tube operates but
we were assuming that the current being
supplied to it (to fulfill the second
requirement for x-ray production) was such
as to make the anode positive in respect to
the cathode. This is diagrammed as follows:
Rectification

What would be the result if we were to
connect this tube into the circuit in just the
opposite way or if it was connected to
alternating current? There would be no
attraction for the electrons at the anode
target so there would be no flow of
electricity and no x-ray production. This
may also be very damaging to the tube.
Rectification
This is therefore the incorrect way to hook up
the x-ray tube. But stop a minute- what kind of
current is available for us to use on the x-ray
machine? It is a 60 cycle alternating current.
 Alternating current means that the current flow
keeps reversing direction and 60 cycle means
that there are 60 pairs of reversals per second.
Thus the state of the x-ray tube keeps changing
from the first diagram to the second and back
again at a great rate of speed.
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Rectification
The first part of the cycle may be termed useful
voltage; the second part, useless or inverse
voltage.
 We may do one of two things with this inverse
voltage:
 a) Eliminate it as best we can.
 b) Make it work for us. Any time we eliminate it
we have half-wave rectification (this is
unidirectional and very inefficient).
Rectification
 When
we make it work for us we have
full-wave rectification. Both half-wave
and full-wave rectification are direct
current.
 Rectification is the process whereby an
alternating current is transformed to a
pulsating direct current. A device that
does this is a rectifier.
Rectification
 There
are four basic rectification
methods:
 Self-rectification
 Mechanical rectification
 Valve-tube rectification
 Solid state rectification

Rectification
Self- Rectification:
During the first half-cycle the anode is positive
in relation to the cathode so current flows and
x-rays are produced. During the second halfcycle the anode is negative so there is no
current flow and no x-ray production.
 This is self-rectification because the tube does
its own rectification and is half-wave
rectification since only half of the current wave
is utilized.
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Rectification
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Self-rectification is usually limited to dental and
portable units. The highest limits are about 30 MA.
at 90 kV. They are limited so because overstepping
these bounds can cause temperature rise in the
anode target to the point of thermionic emission. In
this event the inverse voltage can drive these
available electrons towards the cathode filament
with consequent damage or destruction of the tube.
Although the concept of self-rectification is
important, it is all but obsolete today.
Rectification
ADVANTAGES OF SELF-RECTIFICATION
1. Lowest in price.
2. Less bulk - can be portable.
 DISADVANTAGES OF SELFRECTIFICATION
1. Limitation of MA. capacity.
2. Constant danger of tube destruction.
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Rectification
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MECHANICAL RECTIFICATION

Is all but obsolete today, having been replaced by the
valve-tube method. In this system a mechanical device
inserted in the circuit between the secondary coil of the
step-up transformer and the electrodes of the x-ray tube
is responsible for the rectification of the current.
This mechanical devise is a disc or wheel attached by a
shaft to a synchronous motor which is driven by the
same current that is applied to the primary coil of the
step-up transformer.

Rectification
 There
are four contact points on this
wheel, connected in two pairs. There
are two contacts at the ends of the
secondary coil of the step-up
transformer. There two contacts make
electrical connection during exposure
with two of the four wheel contacts.
Rectification

The rectifying wheel or disc directs the flow of
electrons in the following manner so that the
anode is always positive in respect to the
cathode. Mechanical rectification is therefore
full-wave rectification. Advantages of full
wave rectification over half-wave rectification
includes the fact that the electrons flow in a
shorter period of time to get to the desired
affect. The time of exposure is decreased close
to 50%.
Rectification
ADVANTAGES OF MECHANICAL
RECTIFICATION
 1.) It is full-wave rectification and higher MA. is
possible
 2.) The x-rays produced are uniform because
only peak voltage is applied to the tube. This is
because contact points are touching only at the
moment of peak current and are moving together
or moving apart as the current is building or
falling.

Rectification
DISADVANTAGES OF MECHANICAL
RECTIFICATION
 1.) Noise during exposure caused by sparking
across the gap
 2.) Ionization of air results in noxious gases as
well as making the tube gassy and inefficient
 3.) They require a large amount of space
 4.) They are not shockproof
 5.) The contact points constantly wear out

Rectification
VALVE-TUBE RECTIFICATION
 A valve-tube (or thermionic rectifier or
kenetron) is similar to a stationary anode xray tube in some respects. It is a highly
evacuated glass tube with a tungsten
filament in one end (cathode) surrounded by
a piece of molybdenum (focussing cup).
The valve-tube serves the same purpose as
does solid state rectification.
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Rectification
The filament is heated to approximately 2000 degrees
Celsius by current from the secondary coil of a
filament transformer. As high tension voltage is
applied with the anode being positive in respect to the
cathode current flows through the tube. No current
flows if the current is applied in the reverse direction.
Valve tubes are used in rectifying systems by
inserting one or more into the machine circuit
between the secondary coil of the step-up transformer
and the terminals of the x-ray tube.
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Rectification
THERMIONIC RECTIFICATION WITH ONE TUBE:
In single tube rectification one tube is inserted into the
circuit connected in series with the x-ray tube to prevent
the inverse voltage from being applied to the tube.
Single tube rectification permits the use of 100 MA. and
100 kV. It is half-wave rectification.
THERMIONIC RECTIFICATION WITH TWO
TUBES:
In two tube rectification the two tubes are hooked up in
series with the x-ray tube. One on either side of the xray tube. It is half wave rectification.
Rectification
 THERMIONIC
RECTIFICATION
WITH FOUR TUBES:
 Four-valve-tube rectification
enables us to use the inverse
voltage for the production of x-rays
and is therefore full-wave
rectification.
Rectification

The four kenetrons are arranged in a square
formation; the anodes of two tubes are
connected to each other and to the cathode of the
x-ray tube, the cathodes of the other two tubes
are connected to each other and to the anode of
the x-ray tube. The opposite electrodes of two
tubes are attached to one side of the secondary
coil of the step-up transformer; the opposite
electrodes of the other two tubes are attached to
the other side.
Rectification

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ADVANTAGES OF FOUR VALVE-TUBE
RECTIFICATION:
1. High MA. capacity
2. Full-wave rectification
3. Eliminates all the disadvantages of mechanical
rectification even though the same function is served
DISADVANTAGES OF FOUR VALVE-TUBE
RECTIFICATION:
1. High cost ($250 per valve-tube)