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Transcript
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: