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.
It is important that the ignitor should be triggered correctly. The ignitor requires a 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.
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 (ie. Not single shot as in capacitor discharge) Water cooling is commonly employed. It is vital that Ignitrons must be used in the correct temperature range to hot or to cold can be very bad news for these devices- (cold leads to mercury vapor condensing on the anode.)
Ignitrons are very limited with regards their physical orientation. This reason being simple that they rely upon a pool of liquid at one end of the device that must be correctly positioned for the ignitor to function correctly. Positioning the device so that it leans over at an angle of more than 2 or 3 degrees from the vertical is fatal.
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.
Thyratrons and Krytrons are sometimes used in ignitron triggering circuits along with the familiar thyristor.
Ignitrons are suited to applications were power control of high voltages or currents is required. Welding is probably the most common application.