Rectification

1
Chapter 4

• Rectification
• and
• Rectification Systems

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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

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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.

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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).

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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.

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Rectification

• There are four basic rectification methods
• Self-rectification
• Mechanical rectification
• Valve-tube rectification
• Solid state rectification

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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 half-cycle
  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

• 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.

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Rectification

• ADVANTAGES OF SELF-RECTIFICATION
• 1. Lowest in price.
• 2. Less bulk - can be portable.
• DISADVANTAGES OF SELF-RECTIFICATION
• 1. Limitation of MA. capacity.
• 2. Constant danger of tube destruction.

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Rectification

• 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.

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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.

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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.

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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.

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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

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Rectification

• VALVE-TUBE RECTIFICATION
• A valve-tube (or thermionic rectifier or
  kenetron) is similar to a stationary anode x-ray
  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 x-ray tube. It is half wave
  rectification.

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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.

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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.

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Rectification

• 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)

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