IGNITRONS The ignitron is mercury vapor rectifier in which an arc is switched between a

(usually graphite) anode and a mercury pool cathode. The discharge is initiated by an ignitor electrode which dips into the mercury

pool cathode. On application of a suitable impulse current/voltage to this ignitor an electron emitting source is formed at the point at which the ignitor contacts the pool.

This initiates the arcing between the anode and cathode. electron tube functioning as a rectifier to convert alternating current (AC)

to direct current (DC)

a mercury arc rectifier controlled by a subsidiar electrode, the ignitor, partially immersed in a mercury cathode.

a current passed between ignitor and cathode forms a hot spot sufficient to strike an arc between cathode and anode

used in a wide variety of pulse power applications, such as capacitor discharge, laser switching, magneforming, magnetizing and crowbar circuits.

Most ignitrons operate at most currents between 5 Amps and 100kA and may be suitable for voltages from a couple of hundred to 20 000 Volts.

It is important that the ignitor should be triggered correctly. The ignitor requires certain energy for successful ignition and also an 'ignitor

characteristic' application of this energy in terms of current and voltage with respect to time.

Misfiring or ignitor damage will otherwise occur. It is also vital that no significant negative voltage should appear at the ignitor with respect to the cathode else ignitor destruction will be the inevitable result.

Triggering ignitrons There are two main ways by which the trigger can be biased:

Anode excitation: common in resistance welding applications here the anode bias is connected to the ignitor by means of a switch (thyristor, thyratron etc.) and a resistor/fuse network. The ignitor current drops rapidly on ignition as the anode-cathode voltage drops very low during conduction.

Separate excitation: as the name suggests, here the ignitor circuit is largely independent of the main circuit.

Ignitrons are often used in parallel for AC power control applications.

Ignitrons must often be cooled when used continuously

Ignitrons are very limited with regards their physical orientation

they rely upon a pool of liquid at one end of the device that must be correctly positioned for the ignitor to function correctly

Thyratrons and Krytrons are sometimes used in ignitron triggering circuits along with the familiar thyristor.

Ignitrons are suited to applications where power control of high voltages or currents is required. Welding is probably the most common application.

Operation

A large steel container with a pool of mercury in the bottom that acts as a cathode during operation

A large graphite or refractory metal cylinder, held above the pool by an insulated electrical connection, serves as the anode

An igniting electrode (called the "ignitor"), made of a refractory semiconductor material such as silicon carbide,is briefly pulsed with a high current to create a puff of electrically conductive mercury plasma .

The plasma rapidly bridges the space between the mercury pool and the anode, permitting heavy conduction between the main electrodes.

At the surface of the mercury, heating by the resulting arc liberates large numbers of electrons which help to maintain the mercury arc.

The mercury surface thus serves as the cathode, and current is normally only in one direction. Once ignited, and ignitron will continue to pa ss current until either the current is externally interrupted or the voltage applied between cathode and anode is reversed.