SOFT VALVES 



Thyratron switch — The criterion of a good 'on-off' switch is that when the 

 switch is 'off' the resistance be extremely high and when the switch is 'on' 

 the resistance be low, that is, that the potential difference across it be small 



Control 

 voltage 



o+V 



+ <> 



Output 1 



Output 2 



Output 3 



k__^^. 



^^^T 



Figure 7.17 



when it is passing the load current. The thyratron is a good electronically 

 controlled switch {dvpoL — a door), for provided it is capable of passing the 

 load current the voltage dropped across it cannot exceed some 15 V, which 

 is not much in apparatus commonly supplied with voltages of around 500. 



The usefulness of the thyratron in switching direct current supplies is 

 somewhat reduced by its inability to switch 'off'. In Figure 7.18 we have a 

 direct voltage generator supplying some kind of load, an electromagnet or a 

 motor or a lamp, perhaps, and the circuit can be made by removing a heavy 

 negative bias from the thyratron grid. However it must be broken in some 

 other way, perhaps manually with a push button. Depending on the applica- 

 tion, this may or may not put the thyratron out of court. 



'Off 'button 



Jk 







200-500 

 volts 



IT 



\ 



-rwmwiv- 







350 volts 

 R.M.S. 



tlr 



Ml 



Control 

 potential 



Figure 7.18 



Control 

 potential 



Figure 7.19 



If the supply is a.c, then the grid can be used both for switching on and 

 off. For example, in Figure 7.19 the thyratron acts both as a switch and as a 

 rectifier. If the grid is at cathode potential we have virtually a soft diode 

 choke input rectifying circuit feeding d.c. + ripple to the load. When the 

 upper generator terminal is negative the thyratron anode is negative with 

 respect to the cathode and no conduction can occur. When A is positive the 



130 



