THYRATRON 



determined largely by the limiting resistance R (Figure 7.16), and the voltage 

 across the valve falls to the ordinary ionization potential, about 15 V. The 

 current is in fact (K— 15)/^; a typical small thyratron will pass up to 100 

 milliamps. As a result of the current of positive ions travelling towards the 

 cathode region, not only is the space charge cancelled, but also the effect of 

 the negative grid, and it is impossible to shut off the discharge by making the 

 grid more negative since the only effect this has is to attract more ions into 

 the cathode region. We say that the grid 'loses control'. Instead of fixing 



-o +v 



Figure 7.16 



the grid bias and raising the anode potential until ionization occurs, we can 

 of course fix the anode potential at some value greater than the ionization 

 potential and initiate conduction by raising the grid potential from very 

 negative to [anode potential]/[control ratio]. When this is done (bearing in 

 mind that it subsequently loses control) the grid is said to be being used as a 

 'trigger' and — extending the analogy — when ionization occurs the thyratron 

 is said to 'fire'. The only way to stop the discharge again is to reduce V 

 below the ionization potential. 



Precautionary measures to be taken with thyratrons are similar to those 

 for soft hot-cathode diodes. The cathode must be allowed to warm up 

 before the anode potential is applied, and the discharge current must be held 

 within the makers' upper limit, for fear of destroying the cathode. 



Thyratron relaxation oscillator — One difficulty with the difference diode 

 relaxation oscillator is that the amphtude of the three outputs cannot readily 

 be controlled electronically. Output (1) is fixed by the voltage-diff'erential 

 of the tube, and outputs (2) and (3) by the relative sizes of R^ and R^, {Figure 

 7.10). The thyratron relaxation oscillator can be controlled by a potential on 

 the thyratron grid {Figure 7.17). The action is similar to the difference diode 

 oscillator, but whilst the capacitance always discharges to the ionization 

 potential, the voltage to which it charges can be made greater by making the 

 grid more negative. Unfortunately the frequency of oscillation is also affected 

 and has to be corrected by an appropriate alteration in R. 



In designing these circuits it must be remembered that R^ + Rb must be 

 chosen so that the peak thyratron current is within the safe limit when the 

 grid is as much negative as it is ever likely to be, for it is then that the capaci- 

 tance charges to the highest voltages and the discharge currents are greatest, 

 that is 



Maximum anode voltage — Ionization potential 

 o I b> Maximum permissible thyratron current 



129 



