AP 3302 Pt. 3
Section 2
CHAPTER 10
Electronic Switching Circuits
Ignitron
Other
gas-filled valves besides the thyratron are used as switches in high-power radar
modulators. These include the ignitron and the trigatron. In very high-power
radars the modulator arrangements considered so far would require very high
applied direct voltages. The same high peak power could be obtained by using
a d.c. supply of a few hundred volts but supplying several hundred amperes of
current. Where currents of this magnitude have to be switched on and off, it
is usual to use some form of mercury pool switch.
There are a number of different types of mercury pool switches in use. The ignitron is one such valve which works on a similar principle to that of a mercury arc rectifier. In all such valves, a pool of mercury acts as the cathode. In order to make the valve conduct, an arc must first be struck between the cathode pool and a striker or igniter electrode; free ions are then made available in the vicinity of the pool for the main switching function. It is usually in the method of striking the arc that mercury pool switches differ from each other. In the ignitron (Fig 12) the valve envelope is usually made of steel, which mercury does not ‘wet’, and the arc is struck by a rod of silicon carbide or boron carbide (the igniter) which dips permanently into the mercury pool. When current is passed from the igniter to the mercury, enough mercury vaporizes to strike an arc.
Because
we now have a plentiful supply of free ions already available within the envelope,
when a positive trigger pulse is applied to the control grid the valve ‘fires’
immediately and we have a large flow of current between the main anode and the
pool. Thus the ignitron may be used in much the same way as a thyratron to provide
a rapid discharge of the delay line through the load. Compared with the thyratron
the ignitron is capable of switching very much higher currents (of the order
of hundreds, or even thousands, of amperes).
Trigatron
The trigatron is a form of cold-cathode gas-filled triode. Its basic construction is outlined in Fig 13. The two main electrodes are hemispherical caps, one of which has a hole in the middle into which is inserted a third electrode known as the trigger electrode. The spacing
