POWER PACKS 



STABILIZED AC MAINS SUPPLIES 



As an alternative to stabilizing separately EHT, HT and heater supplies it 

 may be worth considering stabilizing the supply mains itself; that is, to 

 draw all one's a.c. power supplies from a single stabiUzing device which is 

 fed with unstabilized National Grid power. Stabilized a.c. is in any case 

 useful for supplying experimental apparatus of a non-electronic nature, such 

 as light sources. There are a number of commercial units which do this, and 

 designs have been published from time to time which are suitable for making 

 up. There are various methods of approach, but all — apart from the 

 constant voltage transformer — are the same in principle: a sensing device 

 detects discrepancies between the actual output voltage and that required, 

 and causes a regulator to correct them — at least partially. Thus in a device 

 due to Ackland^^ the output voltage is bridge-rectified and compared with 

 that existing across a voltage reference tube. The difference is used to actuate 

 a sensitive relay, which — via other relays — controls a motor-driven variac 

 transformer. Such a device is clearly capable of stabilizing large powers, 

 but the rate of response to a sudden change in output would not be very 

 rapid; Ackland claims a correction rate of 8 V per second. A much faster 

 type of a.c. stabilizer is due to Benson and Seaman^^. Here a saturated 

 diode is the sensing unit; the diode output is amplified by a valve whose 

 anode current is used to vary the saturation of part of the core of an auto 

 transformer, so that its effective step-up ratio is altered. The response time 

 is I second. 



STABILIZED-CURRENT POWER SUPPLIES 



It happens occasionally that a constant current, rather than constant voltage, 

 power supply is required ; for example, in the production of a stable mag- 

 netic field by a coil whose temperature, and hence resistance, is subject to 



o-l- 



Unstabilized 

 supply in 



Figure 37.25 



fluctuation (the author has in mind the tachometer-generator field winding 

 of a velodyne). 



A constant current characteristic may be had by electronic stabilizing 

 techniques very similar to those used for producing a constant voltage; 

 instead of a fraction of the actual output voltage being fed to the differential 

 amplifier, a voltage is derived proportional to the output current by passing 

 the latter through a high stability resistor (Figure 37.25). The performance 

 of circuits of this type is discussed by Sowerbyi**, who shows that in a typical 



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