Define Vacuum Diode?
What is Vacuum Diode?
In 1904, Sir John Ambrose Fleming developed the primary vacuum tube called vacuum diode. It is additionally called Fleming valve or thermionic tube. Vacuum diode is an electronic gadget that permits the electric current in one direction (cathode to anode) and blocks the electric flow toward another path (anode to cathode).
Two electrodes of vacuum diode
Vacuum diode is the most simplest type of vacuum tube. It comprises of two electrodes, a cathode, and an anode or plate. The cathode emits the free electrons. Thus, it is called as emitter. The anode collects the free electrons. Thus, it is called as collector.
The cathode and anode are encased in a empty glass envelope. The anode is an empty cylinder made of molybdenum or nickel and cathode is a nickel cylinder covered with strontium and barium oxide. The anode encompasses the cathode. In the middle of the cathode and anode a vacant space is available, through which the free electrons or electric current flow.
What is electrode?
Electrode is a conductor through which free electrons or electric current leaves or enters. In vacuum diode, cathode is an electrode or conductor from which the free electrons are discharged into the vacuum. Then again, anode is an electrode that collects the free electrons discharged by the cathode. In other words, free electrons leave the cathode and go into anode.
Electron emission depends on the amount of heat applied and the work function
The quantity of free electrons emitted by the cathode is relies or depends upon two variables: amount of heat applied and work function.
In the event that more amount of heat is applied, more number of free electrons is emitted. Similarly, in the event that less amount of heat is applied, less number of free electrons is emitted.
The minimum amount of energy expected to eliminate the free electrons from the metal is called work function. Metals with low work function will require less amount of heat energy to emit the free electrons. On the other hand, metals with high work function will require enormous amount of energy to emit the free electrons.
Hence, picking a decent material will expand the electron emission efficiency. Most usually used thermionic emitters include oxide-covered cathode, tungsten, and thoriated tungsten.
Directly and indirectly heated cathode
At the point when the cathode is by indirectly or directly heated, free electrons are emitted from it.
In the directly heated cathode, the heat energy is provided directly to the cathode. Hence, a modest quantity of heat energy is enough to emit the free electrons from the cathode. At the point when the heat energy is directly provided to the cathode, enormous number of free electrons gain adequate energy and breaks the holding with the cathode. The free electrons that break the holding with the cathode are emitted into the vacuum. These emitted free electrons are attracted towards the anode.
In the indirectly heated cathode, no electrical connection is available between the cathode and the heater. Hence, the cathode isn’t heated directly. The heat energy is provided to the heater and the heater will move its heat energy to the cathode. At the point when the heat energy applied to the cathode is expanded to an ideal level, the free electrons in the cathode gain adequate energy and break the holding with the cathode. The free electrons that break the holding with the cathode are emitted into the vacuum. These emitted free electrons are attracted towards the anode.
Vacuum diode with forward voltage
At the point when the heat is provided to the heater, it acquires heat energy. This heat energy is moved to the cathode. At the point when the free electrons in the cathode acquires adequate energy, they breaks the holding with the cathode and bounces into vacuum. The free electrons in the vacuum need adequate kinetic energy to arrive at the anode.
Assuming voltage is applied to the vacuum diode, in such a way, that anode is connected with a positive terminal and cathode is connected to a negative terminal (anode is more positive with respect to the cathode), the free electrons in the vacuum acquires sufficient kinetic energy to arrive at the anode.
We realize that, if two opposite charged particles are put near one another they get attracted. For this situation, anode is positively charged and free electrons emitted from the cathode are negatively charged. Hence, the free electrons that gain sufficient kinetic energy will move or pulled in towards the anode. These free electrons convey the electric current while moving from cathode to anode.
If the positive voltage applied to plate or anode is expanded, the quantity of free electrons pulled in towards the anode is also increased. In this manner, the electric current in the vacuum diode increases with increase in the anode or plate voltage.
Vacuum diode with reverse voltage
If voltage is applied to the vacuum diode, in such a way, that anode is associated or connected with the negative terminal and cathode is connected with the positive terminal (anode is more negative regarding cathode), the free electrons in the vacuum acquires sufficient kinetic energy to arrive at the anode. However, anode repulses the free electrons that attempt to move towards it.
We realize that if two like charged particles are put near one another they get repulsed. For this situation, anode is negatively charged and the free electrons radiated or emitted from the cathode are additionally negatively charged. Hence, the anode repulses the free electrons that are emitted by the cathode. Therefore, no electric current flows in the vacuum diode.
Vacuum diode with zero voltage
In the event that no voltage is applied to the vacuum diode, anode or plate acts as neutral. It neither draws in nor repulses the free electrons emitted from the cathode. Hence, the free electrons emitted or transmitted from the cathode don’t move or pulled in towards the anode.
Therefore, no electric current occurs in the vacuum diode. However, the enormous number of free electrons emitted from the cathode is develops at one spot close to the cathode and structures a haze of free electrons. This haze of free electrons close to the cathode is called space charge.
Therefore, the vacuum diode permits the electric current from cathode to anode and don’t permit the electric current from anode to cathode. This one-way direction of the electric current empowers the vacuum diode to carry on like a switch. In the event that the anode or plate is positive with respect to cathode, the vacuum diode behave like a closed switch. Then again, assuming that the anode is negative as for the cathode, it act like an open switch.