Rectifiers play an important role in the x-ray circuit, this presentation gives a brief idea about rectifiers, generators and transformers.

1. RECTIFIERS
PRESENTER – DR Y MADHUSUDANA

2. Contents
• Brief introduction to Generators and Transformers
• Rectification
• Rectifiers
• Types of Rectifiers
• Summary

3. Generators
• An x-ray generator is the device that supplies electric power to the
x-ray tube.
• The tube requires electrical energy for two purposes:
1. to boil electrons from the filament and
2. to accelerate these electrons from the cathode to the anode.

4. • The mechanism of an x-ray generator is usually continued in two
separate compartments
1. a control panel or console and
2. a transformer assembly

6. Transformers
• Transformers are used to change the potential difference of the
incoming electric energy to the appropriate level.

7. • A transformer consists of two wire coils wrapped around a closed
core.
• The circuit containing the first coil (which is connected to the
available electric energy source) is called the primary circuit.
• The circuit containing the second coil (from which comes the
modified electric energy) is called the secondary circuit.

8. Current induction by transformer

9. Transformers in x-ray circuit
• The transformer assembly , is a grounded metal box filled with oil.
• It contains
1. a low-voltage transformer for the filament circuit and
2. a high-voltage transformer and a group of rectifiers for the high-
voltage circuit.
• Autotransformer

11. Rectification
• Rectification is the process of changing alternating current into
direct current.
• The high-voltage transformer provides an alternating voltage for the
x-ray tube.

12. • When the cathode is negative with respect to the anode, electrons
flow at high speed from the cathode to the anode and x rays are
produced.
• During the next half of the electrical cycle the target (anode) of the
x-ray tube is negative and the filament positive, so electrons, if they
are available, would flow away from the target toward the filament.

13. The Circuit for self-rectification

14. • It would be highly undesirable to have electrons moving from the
target to the filament for two reasons:
1. such electrons would not produce useful x rays, and
2. such electrons would further heat the filament and reduce its
lifetime.

15. Rectifier
• A rectifier is a device that allows an electrical current to flow in one
direction but does not allow current to flow in the other direction.
• Rectifiers are incorporated into the x-ray circuit in series with the x-
ray tube.

17. Types
1. Vacuum tube rectifiers :
2. Solid-state rectifiers : Solid-state rectifiers are smaller, more
reliable, and have a longer life.
Selenium was the first material used for solid-state rectifiers. Today
most x-ray generators use silicon rectifiers.

18. • A single silicon rectifier (called a cell) will resist a reverse voltage of
about 1000 V, which is 10 to 20 times higher than a selenium
rectifier.
• Silicon rectifiers can withstand a temperature of up to 392° C,
considerably higher than selenium at 266° C.
• A silicon rectifier is made up of a number of cells, or individual
diodes, connected together to form a cylindrical stack that might
have dimensions of 20 to 30 cm long by 20 mm diameter. Such a
rectifier can operate up to 150 kVp and 1000 mA.

19. Vacuum tube and silicon rectifiers

21. Semiconductor
• The heart of a solid-state rectifier is a semiconductor, which is
usually a piece of crystalline silicon.
• Silicon contains four valence electrons.
• The valence electrons must lose or gain energy to move from one
energy level to another.
• Electrons in the conduction band (which corresponds to unfilled
energy levels) are relatively free from atomic bonding and may
move freely through the semiconductor material.

22. • If there is no forbidden region at
normal temperature and pressure,
the material is called a conductor.
• If, the forbidden region is in the
order of an electron volt, the
material is called a semiconductor.
• If the forbidden gap is of the order
of 10 eV the material is an insulator.

23. • At absolute zero temperature, the semiconductor behaves as an
insulator.
• At room temperature, some of the electrons are thermally raised to
the conduction band and are available to support a current.
• So the word "semiconductor" describes material that at low
temperatures acts like an insulator, but at normal room
temperature acts like a conductor.

24. N-type Semiconductor
• Silicon contains four valence electrons. If a material with five
valence electrons is added as an impurity to the silicon crystal, the
added atoms will take the place of some silicon atoms throughout
the crystal.
• This unbound electron can move about in the crystal much easier
than one of the bound electrons.
• The impurity is called a donor since it donates an extra electron .
The crystal resulting from the addition of the donor is called N-type,
with N derived from the negative charge of the surplus electron.
• Arsenic and Antimony

25. N-type Semiconductor

26. P-type Semiconductor
• If an impurity with only three valence electrons is added to silicon,
the impurity atom will have only three electrons to share with four
surrounding silicon atoms.
• One silicon atom now has an electron that is looking for another
electron with which to form a covalent bond.
• The absence of this electron is called a "hole." Since the hole is a
positive particle, as compared to the negative electron, the material
is called a P-type semiconductor.
• Indium, Gallium, and Aluminium

27. P-type semiconductor

28. P-N Junctions
• P-N junction can be formed only by a complex process in which the
P and N materials are diffused into a single crystal.
• When the junction is formed, electrons diffuse across the junction
• When electrons leave the N-type material, the junction area is left
with a net positive charge. Similarly, the P-type material acquires a
negative charge. This creates what is called a "depletion layer."
• The depletion layer has a junction potential that is opposite in sign
to the designation of the materials (i.e., the junction potential is
positive on the side of the N-type and negative on the side of the P-
type material).

29. • The device formed by a P-N junction is called a diode. Solid-state
rectifiers are diodes.
• If a voltage is applied to a diode, current will flow or not flow
depending on the polarity.
• If the polarity of the applied voltage is opposite that of the junction,
electrons will flow from the N-type material across the junction
barrier to the P-type material and current will flow.

31. Forward bias
• The negative pole of a battery is connected to the N-type material
and the positive pole to the P-type. This is called forward bias.

32. Reverse bias
• If the polarity of the applied voltage were reversed, with the
negative pole of a battery being connected to the P-type material,
the junction potential would be augmented and no current would
flow. This is called reverse bias.

33. Types of rectifiers
1. Single phase
 Half-wave rectifier
 Full-wave rectifier
 Bridge-wave rectifier
2. Three phase
 Six pulse, six-rectifier
 Six pulse, twelve-rectifier
 Twelve-pulse rectifier

35. Half-wave rectifier
• Electrons flow through the x-ray tube from the cathode to the
anode.
• When the voltage reverses during the inverse half of the alternating
cycle, the rectifier stops current flow.
• When rectifiers are used in this manner they produce halfwave
rectification .

36. Circuit for Half-wave rectifier

37. Half-wave rectifier

38. Full-wave rectifier
• Both halves of the alternating voltage are used to produce x rays, so
the x-ray output per unit time is twice as large as it is with half-
wave rectification .

39. Full-wave rectifier

41. Full-wave bridge rectifier

43. Six pulse, six-rectifier
• This design employs a delta-wound primary transformer with a
wye-wound secondary transformer. The output of the secondary
windings is rectified with six solid-state rectifiers.
• There are three maximum and three minimum voltages in one
complete cycle (1/60 sec). When rectified, there will be six positive
maximum voltages per cycle. Thus the term "six pulse."

44. Six pulse, six-rectifier

45. Six pulse, twelve-rectifier
• This circuit is still a six-pulse circuit
• This circuit has a fixed potential to ground, an advantage over the
six-rectifier circuit.

46. Six pulse, twelve-rectifier

47. Twelve-pulse rectifier
• Looks similar to the six-pulse, the difference is that the secondary is
not a double wye connection; it is a wye and a delta connection.
• When a delta and a wye winding are connected together in the
secondary, the output of the delta will lag the wye by 30°. The
result of this is that the output of one winding will fill in the ripple
of the other, resulting in a twelve-pulse rather than a sixpulse
output

48. Twelve-pulse rectifier

49. Ripple factor
• The ripple factor is the variation in the voltage across the x-ray tube
expressed as a percentage of the maximum value.
• With a single-phase circuit the ripple factor is 100% because the
voltage goes from zero to a maximum value with each cycle.
• A six-pulse circuit has a ripple factor of 13.5%.
• A twelve-pulse circuit has a theoretical ripple factor of 3.5%.

50. Ripple factor

51. • When three-phase generators are operated under load, the ripple
factor is accentuated.
• This is known as the load ripple-factor, and is always greater than
the theoretical ripple.
• The load ripple-factor of a twelve-pulse, three phase system is
about 5%.

52. Summary

53. References
• CHRISTENSEN’s PHYSICS OF DIAGNOSTIC RADIOLOGY 4th edition