COLD CATHODE DIODE 



series stabilizing, or 'ballast' resistance must always be used {Figure 7.1). 

 It is then found that, on applying a voltage larger than the 'striking' voltage, 

 the glow is established and the potential difference across the diode falls to a 

 lower 'running' voltage. The arrangement is now stable because any tendency 



X 



Figure 7.1 



for the glow to thicken and the current to increase is met by an increased 

 voltage drop in the ballast and hence a reduced difference of potential applied 

 to the valve. The difference between the striking and running voltage is 

 called the 'voltage differential' of the tube. Tubes having a large differential 

 will be called 'refractory'. With proper care in manufacture the running 

 voltage can be made very constant despite considerable changes in the glow 

 current. This is because as the current is varied the thickness of the glow 

 column varies, but the potentials involved are unchanged. This property is 

 made use of in the voltage reference and voltage stabilizer tubes. The upper 

 limit of current which may be passed through these tubes is set by the point 

 at which the glow column completely surrounds the cathode surface. No 

 further increase in current is then permissible or the potential difference 

 across the tube will suddenly rise. The cathode is then hable to destruction 

 by ionic bombardment. 



Voltage reference tube 



This is for producing a stable reference voltage from a supply which is 

 subject to fluctuations (Figure 7.2). It is not intended that power be drawn 



+ 



Unstabilized 

 supply, V^j 



Stabilized 

 potential 

 difference V^ 



Figure 7.2 



from the output terminals (though small amounts may be); the device is 

 intended to be used in the same sort of way as a Weston standard cell. 

 Reference tubes are designed to operate with low glow currents of the order 

 of a milHamp. 

 The running voltages of reference tubes he within the range 60-100 V 



119 



