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4.5 ELECTRON BEAMS
ELECTRONS AND ELECTRONICS
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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.
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ELECTRON BEAMS
• Emission of electrons from a metal surface when heated to a very high temperature is called thermionic emission.
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Cathode Rays
• Cathode rays are beams of fast moving electrons
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ELECTRON BEAMS
• VACUUM TUBES
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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
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ELECTRON BEAMS
• Deflection tubes
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ELECTRON BEAMS
• Oscilloscope:
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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
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ELECTRON BEAMSY – plates: The Y- plates move the beam vertically. This happens when an external source of voltage – for example, an AC supply is connected across the Y-input terminals
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Gain control
• The amount of vertical movement can be amplified by the gain control
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ELECTRON BEAMSX-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.
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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
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ELECTRON BEAMS
• Oscilloscope: