Electrical Engineering ⇒ Topic : Diode Valve

DIODE VALVE

This is the simplest type of thermionic valve. It consists of two-electrode tubes containing an emitting electrode called cathode K and a current collecting electrode called anode or plate P.The emitting electrode may be directly heated type or indirectly heated type. The cathode has an oxide coated filament F in a directly heated type. It becomes the source of electrons as shown in Figure 1(a) when current is passed through it. The commonly used diodes are indirectly heated as shown in Figure 1(b). In this case, cathode K is coated with barium or strontium oxides and lies very close to the filament FF which is heated by passing a current through it. Both the electrodes are enclosed in an evacuated envelope of glass or metal. The advantage of metal tube is that it has greater mechanical strength and provides permanent and complete shielding of the electrodes from the external fields. The disadvantage is that it becomes very hot during the operation. It is also difficult to know visually whether the heating element is working or not. So glass envelope is required. Its walls must be coated inside by some non-conducting material to prevent it from acquiring positive charge due to the secondary emission caused by the electrons striking the glass walls.

                                                            Figure (1) Diode valve.