NON-THERMIONIC DIODES 



rectification generally have the stack structure and are usually referred to as 

 'rectifiers'. 



Unfortunately the voltage rating of a rectifier may be stated in either of 

 two ways, and this sometimes leads to confusion. Consider the circuit in 

 Figure 23.2, where we have a rectifying system of the half-wave capacitor- 



Mams 



Figure 23.2 



input type. On light load this arrangement gives an output voltage of 

 approximately {Ifl^ X 200 = 283. A little thought reveals that the peak 

 inverse voltage across the rectifier occurs in the middle of the non-conducting 

 part of the cycle and is 566 V. Sometimes it is explicitly stated that the P.I. V. 

 of a rectifier is, say, 600 V, rendering it suitable for this application, but it 

 may also be described as a '200 V rectifier, referring to the maximum trans- 

 former R.M.S. voltage. In looking at manufacturers' Hterature it is necessary 

 to be very careful which figure is being quoted. 



POWER RECTIFIERS 



These fall into three groups : 



(1) Types for supplying HT to valves, a few hundreds of volts at a few tens 

 of milliamps. 



(2) Types for supplying 'extra HT' to cathode ray tubes, image-converter 

 tubes, photomultiplier photocells and the like. These deliver a few thousands 

 volts at a few milliamps. 



(3) LT types, for making a few volts at a few amps. Important uses for 

 these are accumulator charging, and the generation of direct heater current 

 from the mains. 



HT rectifiers 



Some HT rectifiers are shown in Plate 23.1. Nos. 1, 2 and 3 are selenium- 

 iron and are rated at 120, 60 and 30 mA '125 V', i.e. the P.I.V. is about 

 350 V. Notice the cooling fins; these components should be mounted in 

 the attitude shown so that optimum convection occurs. The forward resistance 

 of these units is such that about ^ of the no-load output voltage is lost on 

 full load. Nos. 4 and 5 are copper-copper oxide and are contact-cooled, 

 that is, they are bolted against the chassis and the heat generated is removed 

 by conduction. They are rated at 120 and 60 mA respectively, 125 V, so they 

 correspond to the selenium iron units above them. No, 6 is a germanium 

 junction type. Provided it is kept properly cool (<25°C) by suitable siting 

 in the apparatus and by bolting it firmly down to a metal sheet, so that the 

 heat generated is conducted away, this small unit can handle 1 A and with- 

 stand a P.I.V. of 200. Further, the makers claim a forward voltage drop less 

 than 10 per cent of that caused by other types of rectifier. Nos. 7, 8 and 9 

 are sihcon junction types of even more astonishing performance. 7 and 8 



316 



