1) It is simplest thermionic tube, which was invented by Sir Fleming in year 1904.
2) The working of diode valve is based upon the phenomenon of thermionic emission and Edition effect. Edition effect states that a metal plate surrounding the cathode if kept at positive potential w.r.t. cathode, the electrons emitted from cathode will be reaching the plate and there will be an electric current. If the plate is kept at negative potential w.r.t. cathode, the electrons emitted from cathode will not be reaching plate and there will be no electric current.
3) Diode valve consists of a highly evacuated glass bulb having two elements.
a) Cathode K, which may be directly or indirectly, heated tube.
b) Plate P- It is a hollow metallic cylinder of nickel-molybdenum or iron. It surrounds the filament completely. The plate serves the collector of electrons. The glass bulb is mounted on an insulating base provided with base pins to which terminals of the electrodes are connected. The bulb can be fitted into a socket with the help of base pins.
Working of diode
1) When current passed in the filament, from low tension battery the cathode gets heated and emits electrons due to thermionic emission.
2) The number of electrons emitted per second can be controlled by controlling the current in the filament.
3) When plate potential is zero, it is found that plate current is generally zero to very weak, as very few electrons emitted from cathode having high energy may reach the plate.
4) When plate potential is negative, the plate current is zero.
5) When plate potential is positive, the plate current flows in the plate circuit. In diode, battery current flows from plate to cathode whereas the electron current flows from cathode to anode.
When cathode is heated by passing current through it, the thermionic emission takes place from cathode i.e. the electrons are emitted from cathode these electron do not reach the plate if there is no potential or negative potential on the plate. The emitted electron accumulates the space around the cathode, which exert a force of repulsion on the electrons emitted by the cathode. As a result of which the plate current decreases. The negative clouds of electrons emitted by the cathode are called as space charge. When plate potential is negative, the space charge effect is prominent and there is no plate current. When the plate potential is positive but small, then few electrons from space charge reach to the plate causing plate current. The value of plate current increases with the increase in positive plate potential and finally maximum or saturated. At this stage all the thermions emitted from cathode will be reaching the plate.
Application of diode
The diode can be used as a rectifier, modulator and detector.
1) It is that circuit which is used to separate out the a.c. components from the pulsating voltage obtaining from the rectifier circuit.
2) Filter circuit can be following types:
a) Choke input or series inductance filter This circuit is used for high load currents.
b) Shunt capacitance filter This circuit is used for all values of load currents.
c) L-section filter This circuit is used for all values of load currents.
d) T-section filter This circuit is used for all values of voltages.
e) p -section filter This is the best type of filters. Note. In all the above filters, the purpose of inductance is to smooth the pulsating current and of capacitor is to smooth the pulsating voltage.