COLD CATHODE TRIGGER VALVE 



emission as given by the Richardson equation, and over this range the anode- 

 cathode voltage is constant at the ionization potential. 



Plotting these results in Figure 7.12 it is clear that the device is a diode in 

 the sense of Chapter 6. The shaded region requires comment. If any attempt 

 is made to draw more current than the rated maximum from the valve, the 

 extra electrons to represent this current can only be emitted by knocking 

 them out by ions, which means that the potential difference across the valve 

 must rise. Not only does this represent an increase in forward resistance, 

 nearly always a bad thing, but also the cathode will be quickly destroyed by 

 the excessive ionic bombardment. In Figure 7.13 the characteristics of soft 



'\ 



Hard thermionic diode 

 Semi-conductor diode 

 Soft thermionic diode 



w» 





,/ 



/ 



y 



Figure 7.13 



thermionic diodes are compared with those other types of approximately 

 the same rating. It is evident that at high currents the soft valve is more 

 satisfactory because the voltage drop across it is lower. For this reason the 

 application of these valves is in power rectification of high currents; they 

 tend therefore to be found in circuits employing the choke-input system. 

 In using them two precautions are important: 



(1) The current must be limited to the rated maximum by some protective 

 device such as a fuse or magnetic cut-out. 



(2) The cathode heater must be switched on and the cathode given time 

 to warm up before the high voltage supplies are switched on. If this is not 

 done there will be a period at which the cathode temperature is not yet 

 sufficient to supply all the load current thermionically, and the remainder 

 will be derived by ionic bombardment, with consequent damage to the valve. 



COLD CATHODE TRIGGER VALVE 



The cold cathode trigger valve may be regarded as a difference diode to which 

 a third, trigger, electrode has been added. In Figure 7.14, if +^is between 

 the striking and running voltage of the tube, discharge may be initiated by 

 applying a positive pulse to the trigger electrode. The trigger is given a 

 positive 'bias' by a high-resistance potential divider across the power supply 

 such that the tube just does not 'fire' : a small pulse only is then sufficient 

 to fire the tube. Once the discharge is established, it cannot be extinguished 

 by the trigger electrode; other means are necessary to achieve this. 

 In the so-called 'high speed' trigger valve it is necessary that the discharge 



127 



