4.5 ELECTRON BEAMS

ELECTRONS AND ELECTRONICS

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 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

Gain control

• The amount of vertical movement can be amplified by the gain control

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.

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: