A Vacuum tube diode is a two electrode vacuum tube as its name shows DI for two ODE for electrodes. The two electrodes are cathode and anode (plate). The function of cathode is to emit electrons by thermionic emission. The emission of electron is accelerated by putting a cathode in a vacuum.
Diode as Rectifier
When an alternating current is applied to the plate , it polarity will be positive during positive half cycle and at that time electron will flow to the plate. Now during negative half cycle the plate become negative which stop the plate current. It shows that plate current will flow only in one direction and hence produced rectified output.
The function of cathode is to emit electron. There are two type of cathode.
Directly Heated Cathode
It is also called the filament type cathode because in this type of cathode, cathode works as a filament.
Indirect Heated Cathode
It consist of a thin metal sleeve which is coated with the oxides. This sleeve work as a cathode and there is a heated wire of tungsten which is electrically isolated from the sleeve. This type of cathode is called indirectly heated cathode.
The function of plate to attract the electron emitted from the cathode. When positive voltage is applied to the plate the negatively charge electrons are attracted to the plate.
When electrons leave the cathode a positive charge appears on the cathode thus the cathode exerts force on the electrons and these emitted electrons are attracted back. In this way a crowed of electron appears in the vacuum tube which is called the space charge.
The characteristic curve for a typical vaccum tube shows in below figure that the plate voltage will increase the plate currents until all the electron give off by the cathode are drawn to the plate. The point on the curve at which the current has reached its highest value is called the saturation point and the plate current for this condition is called the saturation current. Further increase in plate voltage will not increase the plate current.
The cathode materials which are commonly used are as under with their work function.
Tungsten is made from pure metal and its work function is 4.54 eV (electron Volt). It can be safely operated at 2500oK. It is used in high power tubes because of its higher work function.
Thoriated - Tungsten
It is used in directly heated cathodes. It can supply electron at relatively low temprature of about 700oC to 750oC. Hence they require relatively small heating power and operate with high efficiency.
Work Function of some Important Elements
|Work Function (in eV)