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ELECTRONS AND ELECTRONICS 4.5 ELECTRON BEAMS Electron Beams • Given enough energy, electrons can escape from a conductor and move through a vacuum space.Beams of electrons are used in oscilloscopes, TV picture tubes, computer monitors and X –Ray tubes. ELECTRON BEAMS • Emission of electrons from a metal surface when heated to a very high temperature is called thermionic emission. Cathode Rays • Cathode rays are beams of fast moving electrons ELECTRON BEAMS • VACUUM TUBES Cathode Ray tube • Cathode- it is a tungsten filament that is heated to 20000C • A 6V supply is used to heat the cathode that emits electrons • The electrons are attracted to the anode and a current starts to flow • With air in the tube, rather than vacuum, th electrons would collide with gas molecules and the white hot filament would burn up ELECTRON BEAMS • Deflection tubes ELECTRON BEAMS • Oscilloscope: Cathode Ray Oscilloscope • The CRO uses a narrow beam of electrons to trace out waveforms and other signals on a fluorescent screen • A bright spot is formed on the screen where the beam strikes it • If the beam is deflected, the spot moves • If the spot moves fast enough, it appears as a line • To deflect the beam, there are two sets of deflection plates ELECTRON BEAMS Y – plates: The Yplates move the beam vertically. This happens when an external source of voltage – for example, an AC supply is connected across the Y-input terminals Gain control • The amount of vertical movement can be amplified by the gain control ELECTRON BEAMS X-plates: The X-plates move the beam horizontally.. The movement is produced by a circuit called the time base inside the oscilloscope. The time base automatically applies a changing voltage across the plates so that the spot moves from left to right across the screen at a steady speed. Measuring with an oscilloscope • The Y input is connected to an AC supply • The gain control is set at 5V/cm • If the amplitude of the waveform is 2cm, the peak voltage will be 10V • The time base control is set at 10ms/cm • The horizontal peak to peak is 4cm, so the time period is 40ms = 0.04s • Frequency = 1/time pd = 25Hz ELECTRON BEAMS • Oscilloscope:
