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Note for Basic Electrical Engineering - BEE By Suman Kumar Acharya

  • Basic Electrical Engineering - BEE
  • Note
  • Biju Patnaik University of Technology BPUT - BPUT
  • Electrical and Electronics Engineering
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Suman Kumar Acharya
Suman Kumar Acharya
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INTRODUCTION 9.1 The Cathode Ray Oscilloscope (CRO) is a very useful and versatile laboratory instrument used for display, measurement and analysis of waveform and other phenomena in electrical and electronic circuits. CROs are, in fact, very fast X-Y plotters, displaying an input signal versus another signal or versus time. The ‘stylus’ of this ‘plotter’ is a luminous spot which moves over the display area in response to an input voltage. The luminous spot is produced by a beam of electrons striking a fluorescent screen. The extremely low inertia effects associated with a beam of electrons enables such a beam to be used following the changes in instantaneous values of rapidly varying voltages. The normal form of a CRO uses a horizontal input voltage which is an internally generated ramp voltage called ‘time base’. The horizontal voltage moves the luminous spot periodically in a horizontal direction from left to right over the display area or screen. The vertical input to the CRO is the voltage under investigation. The vertical input voltage moves the luminous spot up and down in accordance with the instantaneous value of the voltage. The luminous spot thus traces the waveform of the input voltage with respect to time. When the input voltage repeats itself at a fast rate, the display on the screen appears stationary on the screen. The CRO thus provides a means of visualising time-varying voltages. As such, the CRO has become a universal tool in all kinds of electrical and electronic investigation. BLOCK DIAGRAM OF A CATHODE RAY TUBE (CRT) 9.2 The main part of the CRO is Cathode Ray Tube (CRT). It generates the electron beam, accelerates the beam to a high velocity, deflects the beam to create the image and contains a phosphor screen where the electron beam eventually becomes visible. The phosphor screen is coated with ‘aquadag’ to collect the secondary emitted electrons. For accomplishing these tasks, various electrical signals and voltages are required, which are provided by the power supply circuit of the oscilloscope. Low voltage supply is required for the heater of the electron gun for generation of electron beam and high voltage, of the order of few thousand volts, is required for cathode ray tube to accelerate the beam. Normal voltage supply, say a few hundred volts, is required for other control circuits of the oscilloscope. Horizontal and vertical deflecting plates are fitted between the electron gun and screen to deflect the beam according to the input signal. The electron beam strikes the screen and creates a visible spot. This spot is deflected on the screen in the horizontal direction (X1

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axis) with constant time dependent rate. This is accomplished by a time base circuit provided in the oscilloscope. The signal to be viewed is supplied to the vertical deflection plates through the vertical amplifier, which raises the potential of the input signal to a level that will provide usable deflection of the electron beam. Now electron beam deflects in two directions, horizontal on X-axis and vertical on Y-axis. A triggering circuit is provided for synchronising two types of deflections so that horizontal deflection starts at the same point of the input vertical signal each time it sweeps. A basic block diagram of a general-purpose oscilloscope is shown in Figure 9.1(a) and a schematic of internal parts of a CRT is shown in Figure 9.1(b). Figure 9.1 (a) Block diagram of a general-purpose CRO Figure 9.1 (b) Cathode Ray Tube(CRT) ELECTROSTATIC DEFLECTION 9.3 Figure 9.2 shows a general arrangement for electrostatic deflection. There are two parallel plates with a potential applied between. These plates produce a uniform electrostatic filed in the Y direction. Thus any electron entering the field will experience a force in the Y direction and will be accelerate in that direction. There is no force either in X direction or Z direction and hence there will be no acceleration of electrons in these directions. 2

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Figure 9.2 Electrostatic deflection Let, Ea = voltage of pre-accelerating anode; (volt) e = charge of an electron; (Coulomb) m = mass of electron; (kg) θ = deflection angle of the electron beam vox = velocity of electron when entering the field of deflecting plates; (m/s) Ed = potential difference between deflecting plates; (volt) d = distance between deflecting plates; (m) ld = length of deflecting plates; (m) L = distance between screen and the centre of the deflecting plates; (m) y = displacement of the electron beam from the horizontal axis at time t and D = deflection of the electron beam on the screen in Y direction; (m) The loss of potential energy (PE ) when the electron moves from cathode to accelerating anode; The gain in kinetic energy (KE ) by an electron This is the velocity of the electron in the X direction when it enters the deflecting plates. The velocity in the X direction remains same throughout the passage of electrons through the deflecting plates as there is no force acting in the direction. Suppose ay is the acceleration of the electron in the Y direction, therefore, 3

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As there is no initial velocity in the Y direction [Eq. (9.8)], the displacement y at any instant t in the Y direction is As the velocity in the X direction is constant, the displacement in X direction is given by Substituting the above value of t in Eq. (9.8), we have This is the equation of a parabola. Putting x = ld in Eq. (9.12), we get the value of tan θ. After leaving the deflection plates, the electrons travel in a straight line. The straight line of travel of electron is tangent to the parabola at x = ld and this tangent intersects the X axis at point O’. The location of this point is given by The apparent origin is thus the centre of the deflecting plates, the deflection D on the screen is given by From Eq. (9.16) we conclude the following: For a given accelerating voltage Ea, and for particular dimensions of CRT, the deflection of the electron beam is directly proportional to the deflecting voltage. This means that the CRT may be used as a linear indicating device. The discussions above assume that Ed is a fixed dc voltage. The deflection voltage is usually a time varying quantity and the image on the screen thus follows the variation of the deflections voltage in a linear manner. The deflection is independent of the (e/m) ratio. In a cathode ray tube, in addition to the electrons many types of negative ions such as oxygen, carbon, chlorine etc are present. With electrostatic deflection system, because deflection is independent of e/m, the ions 4

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