Editing Diode (section)

==Vacuum tube diodes==
{{Infobox electronic component
| name              = Thermionic diode
| image             = 2-50A 2 (2).JPG
| image_size        = 225px
| caption           = A high power vacuum tube diode used in [[radio]] equipment as a [[rectifier]]
| type              = Active
| working_principle = [[Thermionic emission]]
| inventor          = [[John Ambrose Fleming]]
| invention_Year    = 1904
| pins              = [[Plate electrode|Plate]] and [[Hot cathode|cathode]]
| symbol            = [[File:Vacuum diode.svg|100px]]
| symbol_caption    = The symbol for an indirectly heated vacuum tube diode. From top to bottom, the element names are: ''plate'', ''cathode'', and ''heater''.
}}

A '''thermionic diode''' is a [[vacuum tube]] consisting of a sealed, evacuated glass or metal envelope containing two [[electrode]]s: a [[hot cathode|cathode]] and a [[plate electrode|plate]]. The cathode is either ''indirectly heated'' or ''directly heated''. If indirect heating is employed, a heater is included in the envelope.

In operation, the cathode is heated to [[red heat]], around {{cvt|800–1000|C}}. A directly heated cathode is made of tungsten wire and is heated by a current passed through it from an external voltage source. An indirectly heated cathode is heated by infrared radiation from a nearby heater that is formed of [[Nichrome]] wire and supplied with current provided by an external voltage source.

[[File:5U4GB.agr.jpg|thumb|upright|100px|left|A vacuum tube containing two power diodes]]
The operating temperature of the cathode causes it to release [[electron]]s into the vacuum, a process called [[thermionic emission]]. The cathode is coated with [[oxide]]s of [[alkaline earth metal]]s, such as [[barium]] and [[strontium]] [[oxide]]s. These have a low [[work function]], meaning that they more readily emit electrons than would the uncoated cathode.

The plate, not being heated, does not emit electrons; but is able to absorb them.

The alternating voltage to be rectified is applied between the cathode and the plate. When the plate voltage is positive with respect to the cathode, the plate [[electrostatically]] attracts the electrons from the cathode, so a current of electrons flows through the tube from cathode to plate. When the plate voltage is negative with respect to the cathode, no electrons are emitted by the plate, so no current can pass from the plate to the cathode.