1. + - HIGH TENSION GENERATOR & RECTIFICATION SYSTEM 1 MODERATOR MR. S.C. BANSAL (LECTURER) PRESENTED BY BIVEK RAJ B. SC. MED. TECH. 3RD YEAR DEPARTMENT OF RADIO-DIAGNOSIS AND IMAGING PGIMER, CHANDIGARH-160012

3. It provides

4. high voltage between anode & cathode

5. filament voltage.

6. Controls exposure timing

7. Turns exposure on and off

8. High voltage switched on and off2

10. Current is the flow of charge.

11. It is measured in Amperes.

14. Its a sinusoidal wave which varies amplitude and periodic reversal of polarity.4

15. Battery DC A Switch A Time Direct Current Electric current which flows in one direction only through a circuit or equipment. The associated direct voltages, in contrast to alternating voltages, are of unchanging polarity. 5

19. Strength of magnetic field determines strength of induced voltage

20. Direction of magnetic field change determines the direction of electron flow

21. Process is reciprocal (B)

22. Magnitude of induced magnetic field α number of turns in coil7

24. Move magnetic lines of force

25. Vary the magnetic flux

26. Faraday law regulate the strength of the induced current

27. Strength of the Magnet

28. Speed of the motion

29. Angle of the magnet

30. Number of turns on the conduction coil8

32. For 60Hz AC, pulse length = 1/120s = 8 ms

33. Tube current is nonlinear and drops off below 40 kV

34. Capacitors hold and release charge in a time delayed manner 9

35. In polyphase AC or three phase AC, 3 coils are made to rotate simultaneously by placing at equidistance from each other in front of the magnets which will produce three separate supplies of alternating current. The voltage provided by these separate sources of electricity are out of phase e.g. 415Volt 10

36. Distribution Of Electricity The generated electricity is distributed as follows: 2 methods in which the windings of the circuit containing a polyphase generator can be connected are as below: Star type 3 phase circuit or Wye type Delta type 11

38. This type of method would result in a poor economy and great wastage. So alternative to this is by having 4 wires in place of 6 wires as shown next.

39. In this star connection coils L1, L2 and L3 are connected to a common point N known as neutral cable12

41. Combinations of wye & delta configurations in the primary and secondary windings of a transformer will give rise to a phase shift of 30.13

43. Less stress and liability to break down in regard to insulation.

44. Can supply two different voltage as per need of different types of user, e.g. single phase for domestic user and three phase for industrial user or large installations.14

46. Components of H.T. Circuit: 1.Transformer Step up transformer Step down transformer Autotransformer 2.Rectifier 15

48. Principle : It is based on the principle of electromagnetic induction.16

49. Types Of Transformer On The Basis Of Construction: 17

50. Construction Working: P. coil supplied with A.C. As the current flows through it , a changing magnetic field is set up around it in the soft iron core As the principle of e.m. induction and current is also induced in the S. coil Current flows when magnetic field is increased or decreased and not when it is stable 18

57. Step down transformer

59. It works on the principle of self induction .25

61. In comparison with two-winding transformers, autotransformers are smaller in size, are economical of copper wire and cost less.26

62. Diagram Of Autotransformer: 27 c A d 300V 450V 300V 2:2 TURN RATIO 2:2 TURN RATIO 2:3 TURN RATIO e f g B

64. Step-up transformer

65. primary from autotransformer

66. secondary to rectifier circuit

67. mA monitored at center grounded point of secondary

68. Supplies high voltage for x-ray tube28

70. filled with oil

71. oil acts as electrical insulator

72. Function:

73. Increases alternating voltage

74. Also contains rectifier circuit

75. changes alternating current into direct currentAuto- transformer Rectifier Circuit mA HIGH VOLTAGE TRANSFORMER 29

77. A step down transformer has less turns of wire on the secondary coil,which makes a smaller induced voltage in the secondary coil.

78. Placement : It is connected directly to the filament of x-ray tube.

79. Purpose: the filament of the x-ray tube needs to be heated up in order to emit the electrons (thermal electrons).

80. The Tube Current is controlled through a separate circuit called the filament circuit which is connected to filament transformer.30

81. Diagram of x-ray circuit showing filament transformer: + High Voltage Transformer Rectifier Circuit Timer Circuit Line voltage compensator Auto- trans-former TO FILAMENT TRANSFORMER PRIMARY mA selector 31

83. A device which converts AC into pulsating DC is called rectifier.

84. A rectifier can be valve type or vacuum type or it can be solid state/semiconductor/metallic type rectifier.

85. X-ray tube is a sort of valve type rectifier or diode rectifier as it has two electrodes and allows the current to flow in one direction only32

86. UNRECTIFIED A.C WAVE NO RECTIFICATION HALF-WAVE FULL-WAVE 33

88. A glass envelope enclosing a vacuum.

89. Two electrodes within the glass envelope, one of which is a heated filament.

90. The filament of a valve is heated by a step down transformer and emits electrons which are drawn across to the anode when a potential difference is applied across both the electrodes and the valve passes current.34

91. Functioning of a Diode Valve: If the valve is connected in a complete circuit such that cathode is –ve with respect to anode electrons are drawn towards the anode and valve passes current. If the cathode is positive with respect to anode, no electrons will be drawn across the valve and it blocks the current thus the supply of current to X-ray tube is unidirectional only. But these diode valves which were used earlier are replaced with solid state rectifiers. Hence, its function is to pass current in one direction only and to block any reverse flow. 35

92. Solid State Rectifier As the name solid state implies, conduction takes place by electron travel through solid materials as opposed to electron flow through a vacuum in a valve Solid materials used are semi-conductors whose characteristics place them midway between metals, which are conductors of electricity and non-metals, which mostly are non-conductor of electricity and are insulators. Semiconductors can be made either to conduct or insulate. Selenium, Silicon or Germanium are some commonly used semiconductors to rectify the high tension for an X-ray tube in place of vacuum tube. 36

93. Bands Theory of Semiconductor: 37 electron

95. A/c to adding of impurities it is of two types: 1. P-Type semiconductor 2. N-Type semiconductor 38

96. P-Type semiconductor: P-type semiconductor is obtained by adding a certain type of atoms to the semiconductor in order to increase the number of free (in this case positive) charge carriers. 39

97. N-type Semiconductor An N-type semiconductor is obtained by by adding an impurity of valence-five elements to a valence-four semiconductor in order to increase the number of free (in this case negative) charge carriers. 40

99. The block to the current in reverse direction occurs at the junctions between the two materials N type and P type.41

100. i.e. the region where the barrier exists is very thin, hence it is also called junction diode or barrier layer rectifier. A P-N JUNCTION DIODE 42

102. No filament heating.

103. More robust.

104. Smaller in size.

105. More compact, i.e. occupy less space, better for mobile units.43

107. High resistance to reverse current.

108. Ability to work at higher temperature, approx. 200oC compared to 80oC.

109. Due to higher resistance, lesser number of barrier layers are required.

110. Smaller in size and ability to withstand higher inverse voltage.44

112. When higher potential of sources is connected to p-side of diode then it is forward biased.

113. When higher potential of sources is connected to n-side of diode then it is reverse biased.45

114. Forward Vs Reverse: 46

116. Half wave rectified circuits (Single Pulse).

117. Single phase full wave rectified circuits (Two Pulse).

118. Three phase full wave rectified circuits (Six Pulse).47

120. High frequency generators.

121. Battery powered generators.

122. Capacitor discharged generators.

123. Falling load generators.

124. Anatomical programmed generator48 Contd.

126. X-Ray tube acts as rectifier

127. Current only flows from cathode to anode

128. Rarely seen

129. cathode is source of free electronsSecondary of High Voltage Transformer Voltage applied to tube mA waveform 49

131. Simple design.

132. Light in weight.

133. Less cost.

134. Simple to operate.50

136. The rating of a given X-ray tube is more limited when the tube is placed in a self rectified circuit than when it is used in any other type of HT generator.

137. Greater strain on cable.51

139. Accelerate & can destroy filament

140. Half of electrical cycle wastedWasted Used Voltage applied to x-ray tube mA waveform X-Rays Produced 52

142. Alternating voltage applied to secondary of high voltage transformerVoltage applied to tube 53

143. Half wave Rectifier Circuit: + First Half Cycle: Diodes closed Voltage applied to tube Tube current (mA) results - - X Second Half Cycle: Diodes open No voltage applied to tube No tube current (mA) + R1 - R2 R1 - 54 R2

145. only positive half cycle of high tension transformer used

146. inefficient

147. negative half cycle wasted

148. Secondary of High Voltage Transformer55

149. Single Phase Full Wave Rectified H T Generator: In this circuit both half cycles of AC are used to produce X-Rays by employing a bridge of four rectifiers Secondary of High Voltage Transformer Voltage applied to tube 56

150. Voltage applied to tube (also mA waveform) + - X X X X - + Actually what happens? First Half Cycle Second Half Cycle R1 R1 R4 R4 R2 R3 R2 R3 57

152. both the halves of AC cycle and voltage during both halves is alike.

153. both + & - half cycle of high tension transformer used

154. Short exposure time: for conventional exposure switching minimum duration for single pulse is 1/100 sec, as we are getting 100 half waves in 50 cycles in 1 sec.58

155. Higher output than self or half wave rectified circuits.Less strain on HT cables and less insulation cost. Tube Applied to X-ray Tube Output of High Tension Transformer 59

157. More complex.

158. Heavier, not easy to transport.

159. Larger in size.

160. Ripple factor is 100% as it is pulsating X-Ray beam with voltage variation between zero to peak and again to zero.60

162. Winding A and B works as a system with R2, R4, R1, R5

163. Winding B and C works as a system with R1, R6, R3, R4

164. Winding A and C work as a system with R2, R6, R3, R5.61 A B C R1 R4 R5 R2 R3 R6

166. Three Phase , Six Pulse , Twelve Rectifier The circuit is twelve pulse but this circuit has fixed potential to ground. This allows a 150 KVp generator to have a transformer that provide a voltage of -75KV to +75KV and hence, further reduces ripple. 63

167. Voltage In Respective Coils And X-ray Tubes. At any instant of time, the voltage in all coils is not the same i.e. they are out of step with each other, so two pairs which supply the tube are those pairs which have highest voltage in comparison to 3rd pair. Thus voltage across x-ray tube never falls to zero and takes the form of ripples. This is 20%. 64

168. Features Of A Three Phase High Tension Generator. Voltage wave forms: on the circuits the alternating voltage from the three phases of mains supply are fed into a triple high tension transformer. Here the voltage exists on the secondary winding as three phases of alternating voltage stepped up to the peak values required by the x-ray tube in operation. During the period of time occupied by one full cycle of voltage changes, the x-ray tube has six half waves of useful voltage applied to it. 65

170. This ripple can be reduced further by 6-pulse 12 rectifier i.e., 3%.66

171. Average And Peak Current In 3 phase circuits, there is a difference b/w avg. mA during the cycle and the peak value is reached but is narrow as compared to single phase half wave rectified circuit where peak is about 3 times and in single phase full wave rectified circuit And this difference between peak and average value of current is further reduced in 12 rectifier circuit than the six rectifier circuit as average tube current is close to the peak value of the current. 67

173. More complex in circuitry

174. Larger and occupy more space.68

175. High Frequency Generator The newest development in high-voltage generator design uses high frequency circuits. Full wave rectified power at 60 Hz is converted to a high frequency usually 500-50000 Hz. H.F. voltage generation uses inverter circuit. The DC power supply produces a constant voltage from either a single phase or three phase input line source. An inverter circuit creates the high frequency AC waveform. This AC current supplies the high voltage transformer and creates a waveform of fixed high voltage and corresponding low current. 69

176. contd. After rectification and smoothing two high voltage capacitor on the secondary circuits accumulates electron charges. These capacitors produce a voltage across the x-ray tube, that depends upon the accumulated charges according to the relation: V=Q/C where, V=voltage Q=charge(coulomb) C=capacitance (farad) 70

177. Block Diagram Of High Frequency Generator 71

180. Because of closed looped voltage regulations, autotransformers for kVp selection and input line voltage compensation are not necessary unlike other generator design.

181. More accuracy in voltage and current.73

183. 70KW to 100KW:There are 12 pulse generators with mAupto 1000-1250 and KV 150. Shortest exposure time is 0.003 sec. rate of repetition not less than 8 exp/sec. useful in angiography studies undertaken with serial film changers. Also useful in busy trauma centers and ortho departments.

184. 150KW-200KW:these are 12 pulse with mA 1200 at KV 150 as voltage. Shortest exposure are down to 0.001 sec even upto 0.0001 second and repetition rate not less than 80 films/sec. useful in cine radiography.74

186. inexpensive

187. transformer windings

188. 1 primary coil

189. 1 secondary coil

190. 100% ripple

191. 8 ms minimum exp. Time

192. 1/120th second

193. lower output intensity

194. puts less heat in tube for same technique

195. Industrial power

196. expensive

197. transformer windings

198. 3 primary coils

199. one for each phase

200. 6 secondary coils

201. 2 secondary coils induced per primary)

202. 4-13% ripple

203. slightly less patient exposure

204. <=1 ms minimum exp. time

205. higher output intensity

206. puts more heat in tube75 1f POWER SUPPLY 3f POWER SUPPLY

208. Variation of kilovoltage from maximum

209. Usually expressed as percentage of maximum kv.RIPPLE 76

210. Ripple Example: (Vmax –Vmin) ____________ X 100 % voltage ripple= 80 kVp Vmax 72 kVp Ripple = 80 - 72 = 8 kVp %=8 / 80 = .1 = 10% 77

212. always 100 % (kV ranges from zero to maximum)

213. three phase

214. 4-13%

215. high frequency

216. very low; approx 1%Single Phase Output Constant Potential or High Frequency Output 78

217. Generator Kilowatt (kW) Rating The essential function of HT generators in x-ray equipment is to provide such power as is needed by the x ray tube to which they are connected. So power output of generator is very important specification and it is expressed in Watts- unit of power or Kilowatts. But rating of HT generator is evaluated when the unit is under load and formula involve multiplying Kilovolts and the milliamperes which constitutes the load as below: 79

220. mAmax X 0.7 / 10 at 100 kVp400 X 0.7 / 10 = 28 kW (0.7 is the modification factor for single phase generator due to pulsating voltage wave form compared to ripple voltage in three phase generators.) 80

222. Remote locations

223. Inadequate power from power line

224. Outlet inaccessible

225. Types

226. Battery-powered generators

227. Capacitor discharge generators81

228. Battery Powered Generators: Some portable x-ray machines employ a series of batteries to generate the high voltage and filament currents and are useful to operate in areas where the electric supply is inadequate for the conventional generators. Each cell in the battery pack supplies potential difference of 1.5 volts, so thousands of cells are required to provide high voltages used in diagnostic radiology. 82

230. The batteries must be recharged periodically for numerous x-ray exposures.83

232. Special tube used

233. Contains grid

234. Grid close to filament

235. Low voltage on grid controls flow of electrons to anode

236. Exposure start & stop controlled by voltage on grid

237. HV from transformer charges capacitor slowly just before exposure

238. Negative voltage on grid blocks tube current from cathode to anode

239. Capacitor discharged through tube for exposure when grid turned off84

240. In a 1μf design the kV drop is around 1 kV per mAS used and the effective voltage is 1/3 of the voltage drop lower than the starting voltage. Hence the equivalent kV=Starting kV-1/3 x mAS used. Example for an exposure of 87Kv and 20mAS 87-(1/3 x 20)= 87-6.6 = 80Kv Effective. 85

242. When the exposure has been made there is still a residual charge in the capacitor.

243. To remove this a special discharge circuit is employed, the capacitor is discharged through the tube but a special lead shutter closes the exposure aperture to protect the operator and patient. This operation has to be performed if the kV selected needs to be lowered the charge cycle has to be reduced to zero and then reset.86

244. Anatomically Programmed Generator Radiographers have been traditionally selecting & settings of kV, mA & sec. But this problem is overcome by using modern circuitry in anatomically programmed generator. In this generator there is a no. Of push – button setting. Each pushbutton is designated to a particular anatomical part or region. When the chosen push- button is pressed , the circuitry associated with it automatically sets the appropriate factor s of kV , mA & selects the appropriate focal spot on the x-ray tube which is to be used. 87

246. Technologists operating unfamiliar equipment may work with greater certainty no need of use of technique chart.

247. In fluoroscopic room , if anatomical programming is used for spot filming, there is obvious benefit when exam are changed

249. Technologist must manipulate the exposure factor like in case of pt. physical build and certain known pathologies (osteoporosis, pleural effusion).

250. Well trained and experienced radiographers must be needed for evaluating the good quality of radiograph.88

251. Falling Load Generators: These are usually specially designed three phased or high frequency generators. They take full advantage of the current loading capacity of the X-ray tube by beginning the exposure with a high milliamperage and then allows it to fall during the exposure. This can be achieved with a constant potential circuit. It requires that both mA and KV be regulated independently. These must be used with automatic exposure controls or rely on mAs timer instead of independent mA and time controls. 89

253. Shorter times in heavy load situations and simpler operations.

254. Disadvantage :

255. Can shorten x-ray tube life considerably as they use higher mA settings, thus causing the filament to wear out more quickly.

256. Function with mA unknown to the operator so it is impossible for the operator to set the correct time to achieve the desired mAs, therefore, required to be used with automatic exposure controls or rely on mAs timer instead of independent mA and timer controls.mA TIME IN AN EXPOSURE 90

257. Q.C. For Diagnostic X-ray Generators: 1.Accuracy of tube voltage 2. Linearity of mA 3.Exposure time accuracy 91

258. Safety Rules for Radiological Technologist The radiological technologists operate two main categories of X-Ray equipment: 1.Permanent Installation 2.Movable Equipment Electrical hazards are most communally to be greater with movable equipment. 92

260. The R. Technologist using the equipment should at once any damage or defect.

261. R. Technologist should not put plugs into or out of sockets which are live.

262. Cables and plugs should be treated as kindly as if they are patient!

263. Do not stretch the cable.

264. Do not run the equipment on the cable.93

266. Christensen’s Physics For Diagnostic Radiology.

267. www.xray2000.uk