What is Vacuum Diode : Working & Its Applications

The first vacuum tube which is known as a vacuum diode was invented by a British scientist namely “Sir John Ambrose Fleming” in the year 1904. It is also called a thermionic tube or Fleming valve. This diode is made with vacuum tubes with two electrodes like anode & cathode which are enclosed in the vacuum tube. So, this article discusses an overview of a vacuum diode and its working with applications.


What is Vacuum Diode?

Vacuum diode definition is; the diode which allows the current flow from cathode to anode and blocks the current from anode to cathode is known as vacuum diode. The cathode of this diode generally emits free electrons so it is called an emitter whereas an anode gathers free electrons, so-called a collector. The vacuum diode symbol is shown below.

Vacuum Diode Symbol
Vacuum Diode Symbol

Vacuum Diode Construction

The simplest form of a vacuum tube is a vacuum diode. It includes two electrodes an anode and a cathode where the cathode generates the free electrons into the vacuum whereas the anode collects free electrons from the cathode. So free electrons will leave the cathode to enter the anode. These two electrodes are enclosed within an empty envelope of glass.

Construction
Construction of Vacuum Diode

The anode terminal is a hollow cylinder and the material used to make this terminal is nickel or molybdenum. The cathode terminal is a nickel cylinder covered with barium oxide & strontium. Here, the anode terminal is surrounded by the cathode. An empty space is present in between the two electrodes through which the electric current flows.

In a vacuum tube, the anode terminal is larger as compared to the cathode to dissipate heat without an extreme increase in temperature. Generally, cooling fins are arranged with an anode to remove the generated heat at the anode.

Working Principle

The vacuum diode works on the thermionic emission principle. Once a filament heats the cathode terminal emits electrons and these electrons will be attracted by the anode.if the positive voltage is given at the anode terminal is not enough then it cannot attract the electrons generated from the cathode because of the hot filament.

Here, the number of generated electrons by the cathode terminals mainly depends on two essential factors the amount of applied heat & work function. Once more amount heat is applied, then the number of free electrons emitted from the cathode will be more. In the same way, if less amount of heat is applied then the amount of electrons emitted by the cathode is less.

The work function can be defined as the minimum amount of energy required to remove the electrons from the metal. Generally, the metals including less work function will need less heat energy to produce the free electrons. Conversely, metals including high work functions will need a high amount of energy to produce the free electrons.

So, selecting a good material will enhance the efficiency of electron emission. Most frequently used thermionic emitters comprise tungsten, thoriated tungsten & oxide-coated cathode.

Vacuum Diode Circuit Diagram

The circuit diagram of the vacuum diode is shown below. This circuit works in three cases like the diode with forward voltage, reverse voltage, and no voltage where each circuit is explained below.

Case1: Forward Voltage

The vacuum diode with forward voltage circuit is shown below. Once the heat supply is given to the heater then it obtains some heat energy. So this energy can be transmitted to the cathode terminal. Once the flow of electrons within the cathode terminal gets ample energy, then they split the bonding from the cathode & move into a vacuum.  The electrons flow within the vacuum requires enough kinetic energy to arrive at the anode.

Vacuum Diode with Forward Voltage Circuit
Vacuum Diode with Forward Voltage Circuit

Once the voltage supply is provided to the vacuum diode in such a way that the anode terminal of the diode is given to a positive terminal whereas the cathode is given to a negative terminal then free electrons within the vacuum obtain sufficient kinetic energy to arrive at the anode. These electrons will carry the current while transmitting from the cathode terminal to the anode.

If the applied positive voltage to the anode is increased then free electrons which are attracted toward the anode terminal are also increased. So, the electric current within the diode enhances by the increase within the anode.

Case2: Reverse Voltage

The vacuum diode with a reverse voltage circuit is shown below. Once the voltage supply is given to the vacuum diode, then the anode terminal of the diode is connected to the negative terminal whereas the cathode is connected to the positive terminal. So the electrons flow within the vacuum and obtain sufficient kinetic energy to arrive at the anode. But, the anode resists the free electrons that attempt to shift towards it.

Vacuum Diode with Reverse Voltage
Vacuum Diode with Reverse Voltage

We know that if two charged particles are located near to each other then they get repelled. So in this condition, the anode terminal is charged negatively & free electrons generated from the cathode are also charged negatively. Thus, the anode repels the emitted electrons from the cathode. Consequently, there is a flow of current within the vacuum diode.

Case3: No Voltage

The circuit diagram of a vacuum diode without voltage is shown below. Once no voltage is given to the vacuum diode, then the anode performs like neutral. So it neither repels nor attracts the free electrons generated from the cathode. Thus, the free electrons generated from the cathode terminal do not attract or moved to the anode.

Vacuum Diode without Voltage
Vacuum Diode without Voltage

So, there is no flow of current within the vacuum diode. But, the huge number of free electrons produced from the cathode terminal can be built up at one position close to the cathode & forms a free electrons cloud. This free electrons cloud close to the cathode is known as space charge.

Difference between Vacuum Tube Vs Transistor

The difference between a vacuum tube and a transistor includes the following.

Vacuum Tube

Transistor

The vacuum tube is an electronic device, used to control the electrons flow within a vacuum. A transistor is one kind of semiconductor device used to conduct as well as insulate voltage or electric current.
Vacuum tubes are applicable in high-power applications. Transistors are applicable in ICs which need a low voltage supply.
The size of vacuum tubes is big. The size of the transistor is small.
It has high power consumption, so wastage of heat is high. It has low power consumption, so the wastage of heat is low.
As compared to transistors, its size is large. Its size is so small.
Vacuum tubes are expensive. Transistors are not expensive.
It is less appropriate for portable products. It is simply a portable device.
It uses a high voltage power supply, so not applicable for smaller voltage devices. It uses less voltage power supply, so applicable for smaller voltage devices.
It has less voltage gain. It has a high voltage gain.
Its physical strength is low because of the glass tube. Its physical strength is high.
Input impedance is low. Input impedance is high.
It is less dependent on temperature. It depends on the temperature.
These can be replaced by a user easily. These cannot be replaced easily by the user.

Vacuum Diode Characteristics

The space charge size mainly depends on the electrons emission from the cathode throughout space charge formation. Further, the electrons emission mainly depends on the temperature where the cathode gets heated. So if the temperature is increased then the space charge amount can also be increased. Thus the required anode voltage to defuse the space charge will also be more.

Characteristics
Characteristics

The characteristics of a vacuum diode vary at different temperatures. So here in the above diagram, we can observe the characteristics at different cathode temperatures. In the above characteristics, we have shown three graphs only where one graph is for ToC, >ToC & < ToC.

Once the voltage at the anode is increased gradually then the flow of current from the anode terminal to the cathode is increased proportionally. Since the space charge restricts the emission from the cathode terminal, the flow of current is increased proportionally by decreasing the strength of the space charge.

In the above diagram, the space charge limiting region which is shown is called a characteristics zone. Once this zone has vanished then electron emission will become stable & it is exclusively dependent on the temperature at the cathode.

Here the flow of current within the vacuum diode turns saturated. Once no voltage is given at the anode terminal then there is no flow of current within the circuit however actual case is not similar to that.

Due to the statistical fluctuation within the velocity, some flow of electrons will reach the anode even if there is no voltage supply at the anode. So the small amount of current caused through this event is called splash current.

Advantages and Disadvantages

The advantages of a vacuum diode include the following.

  • The sound quality of the vacuum diode is better.
  • These diodes‘ characteristics are extremely independent of temperature.
  • As compared to transistors circuits, this diode dynamic range is wider because of high operating voltages & overload tolerance.
  • These circuits are simple to design.
  • The capacitance of the device changes slightly through signal voltages.

The disadvantages of vacuum diode include the following.

  • These diodes are Bulky, so not appropriate for portable devices,
  • Generally, it requires higher operating voltages.
  • It uses more power.
  • As compared to metal transistors, glass tubes are delicate.
  • Cost is high.

Applications

The applications of vacuum diode include the following.

  • These are used in radios, microwave ovens, Televisions, early computers, Bluetooth, mobile phones, Wi-Fi transmissions, and even in satellite & radar communication devices.
  • Other vacuum tube devices contain cathode ray tubes, X-Ray tubes, photomultipliers & magnetrons.

Thus, this is all about an overview of a vacuum diode and its working with applications. This diode simply allows the current flow from cathode terminal to anode but does not allows it from anode to cathode. So this one-way electric flow will enable the diode to perform as a switch. If the anode terminal is positive (+ve) with respect to the cathode, then the diode performs as a closed switch. Alternatively, if the anode terminal is (-Ve) with respect to the cathode, then it performs like an open switch. Here is a question for you, what is an alternate name for vacuum diode?

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