DIODE VACUUM TUBE :

It is the simplest of all vacuum tubes. It has only two electrodes. They are called cathode and anode (or plate). The operation of a vacuum diode (and also of other vacuum tubes) depends on the emission of electrons from the cathode. In almost all cases, this emission is achieved by heating the cathode.

Such emission of electrons is called thermionic emission.

CONSTRUCTION OF A DIODE:

The following figure illustrates the construction of an indirectly heated diode. It has two electrodes *, namely, cathode and anode (or plate). Usually, the cathode is a

(The heater filament is not considered as an electrode in vacuum tubes, as its function is limited only to the heating of the cathode)

cylindrical sleeve mad e of nickel coated with barium oxide and strontium oxide, Inside the cathode, the heater filament is inserted. This is made of a cylindrical sleeve made of nickel coated with barium oxide and strontium tungsten. Surrounding the cathode is another cylinder made of nickel. This to dissipate the heat produced at the anode. For the same reason, the surface serves as the anode. Often the anode is fitted

with cooling fans. This helps of the anode is blackened and roughened. The assembly of heater filament, cathode and anode is enclosed in a glass envelope. The pin connections from the electrodes and the heater filament are brought out at the bottom of the tube. The glass envelope is then evacuated. Also shows the symbol of the diode.

WORKING OF VACUUM DIODE:

Actually the operation of a diode tube is based on the basic laws of electricity as we know that the same charges repel each other and different or unlike charges attract to each other. Electrons emitted from the cathode of an electron tube are negative electric charges. These charges may be either repelled or attracted to the anode of a vacuum diode tube, depending on whether the anode is negatively or positively charged.

when the metallic cathode is heated sufficiently (indirectly or directly) an invisible cloud (or space charge) of electrons is set free in the space to form the space charge. The space charge exerts the repelling force on the electrons being emitting from the cathode of the play anode (or plate) is made positive with respect to cathode (by connecting the plate anode to the positive terminal of an HT battery and cathode to the negative terminal, as illustrated in the next down figure, an electric field is created extending from anode to cathode and the electrons from the space charge are attracted by the anode and consequently a current flow through the tube.

Upon reaching the plate the electrons continue to flow through the external circuit made up of connecting wire, millimeter and the battery. The arriving electrons are absorbed into the +ve terminal of the battery and equal number of electrons flow out from -ve terminal of battery and return to the cathode, thus refill the supply of electrons lost by emission. As long as the cathode of the vacuum tube is maintained at emitting temperature and the anode remains positive, electrons will continue to flow from cathode to anode within the tube and from plate (or anode) back to cathode through the external circuit. point to be noted that the direction flow of electrons is opposite to the conventional or assumed direction flow of current in the circuit. In case of anode is made up of negative with respect to cathode; electrons would be driven back to the cathode and no current would flow in the diode circuit. This is due neither the plate is made of suitable material for electron emission nor it is hot enough to emit the electrons.

The operation of a vacuum diode tube may be concluded as follows:

The diode conducts only when the plate or anode is made positive with respect to cathode. It will not be conduct in the opposite direction i.e. Electron flow within a vacuum diode tube takes place only from cathode to anode and never from anode to cathode. This unidirectional conduction enables the diode to act like a switch or valve, automatically starting or stopping conduction depending upon whether the plate is +ve or -ve with respect to cathode. Actually this characteristic allowed the diode to act as a rectifier.

DIODE TUBE HALF WAVE RECTIFIER CIRCUIT :

Valve / Tube Half Wave Rectifier

The simplest use of diode vacuum tube is the half wave rectifier circuit. Its consists just single one diode tube valve. However, it is not as efficient as some other forms of rectifier. 

Its maybe seen that if an alternating current waveform is applied to the diode tube valve, or diode, it conducts over half the waveform and not the other. This means that when rectifying AC waveforms, it is only 50% efficient as half the waveform is used and the other half is discarded.

Full wave diode valve rectifier 

Valve / Tube Full Wave Rectifier

In order to make use of the both (+ve) positive halves of an alternating current voltage waveform cycle, a full wave rectifier maybe used. same like this can be implemented with semiconductor diodes, the same can be achieved using diode valves tube. In fact, full wave rectifier diode valves tube is available with one device containing the two rectifiers.

In the full wave rectifier diode tube circuit, two different diodes within the rectifier circuit handle with two different halve of the waveforms. In this way both halves of the AC waveform are used. Also the fact that the time between both peaks is shorter means that smoothing the waveform is much easier.

As seen in the figure, the full wave rectifier valves tubes were also available. These contained two anodes with single cathode enabling full wave rectification to be accomplished using a single valve diode tube.

A furthermore the point to be noted that the power supply rectifier diodes tube often used a separate 5V supply, whereas the common standard for the heaters used for the equipment itself was 6.3 volts, although other voltages were often used.


Limitation and characteristics of vacuum diode tube:

After the completion of this post we should now have a basic understanding the vacuum diode tube, many of their limitation   and some of their characteristics. One of the more important concepts that we should now to understand is that most of these characteristics influence in all parts each other. as like, practically all plate characteristics are interrelated with each other’s. The one value can change the others change. Another example is heating filament voltage. vacuum tubes parameter affected by the cathode depends on proper heating voltage. Inter relationships such as these make electronics both fascinating and, frustrating at times. Many of the limiting factors that we have discussed are the same ones found in other electrical or electronics devices such as motors, stoves, toasters, and so on. Heating and overheating, insulation breakdown, and excessive current and voltage are all limitations that we have noted before. The point is that we can and should apply just about everything we have learned about electricity to electron vacuum tubes. Little is new except the environment.