SIGNAL DIODES 



have P.I.V.s of 200 V and can pass respectively 2 A and 8 A if the tempera- 

 ture is kept down to 25°C, and — an important point with silicon — will operate 

 in a temperature of 100°C, when they can pass 1 and 4 A respectively. No. 9, 

 by a different manufacturer, only passes a maximum of | A in an ambient 

 temperature of 25°C, falUng to 150 mA at 100°C; but a P.I.V. of 400 V is 

 possible. 



EHT rectifiers {Plate 23.2) 



The smaller object is a rectifying stack of the selenium-iron type rated at 

 1 mA, 600 V. The larger is copper-copper oxide, mean forward current 

 8 mA, 760 V. Voltage rating for voltage rating, selenium rectification is seen 

 to produce a considerably shorter component. 



LT rectifiers 



Nowadays these seem usually to employ selenium rectification. LT rec- 

 tifiers are made up by stacking junctions or 'plates' on to a threaded rod, 

 securing them with nuts and using the ends of the rod as a fixing bolt. The 

 object of the stacking is to secure appropriate voltage rating and rectifier 

 function. Units are available ready connected up in any of the configurations 

 of Figure 22.1, of which the bridge connection is the most important. The 

 reverse voltage rating per plate is such that a single bridge consisting of 4 

 plates can be supplied from a transformer of up to about 18 V R.M.S. output. 

 For higher transformer voltages the number of plates should be increased 

 pro rata. Plates are made in a range of diameters, suitable for rectifying 

 currents between about | and 8 A. The output of bridge-connected rectifiers 

 therefore ranges from 1 to 16 A. The specimen in Plate 23.3 is a bridge- 

 connected unit rated at 1 A, 18 V R.M.S. input. 



SIGNAL DIODES 



Germanium types 



In germanium signal diodes connection is made to a block of N type of 

 germanium and to a metal cats-whisker which presses on it. During manu- 

 facture a carefully controlled burst of heavy current is passed through the 

 device which is found to convert the germanium in the region of the cats- 

 whisker from N to P, thus forming the requisite junction {Figure 23.3). 



Glass 



envelope 



Region of P Germanium 

 at tip of whisker 



Crystal ( N Germanium ) 

 "■^Crystal mounting block 



Figure 23.3 



Diodes formed in this way will operate up to radio frequencies and have a 

 P.I.V. ranging from 20 to 100 V. They can pass forward current up to about 

 50 mA. With one volt across them in the forward direction they mostly 



317 



