The Ignitron
The
ignitron
is a 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 (i.e., 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. (TubeGuy
Note: Some ignitrons today with proper thermal management can switch voltages
exceeding 30 kV and some devices made by Richardson
Electronics switch currents as high as 700,000 Amps.)