VOLTAGE MULTIPLIER RECTIFYING CIRCUITS 



arrangement it is possible to use a generator having one terminal earthed. 

 This is the Cockcroft-Walton circuit. 



Cockcroft- Walton doublet- (Figure 6.25) — When A is negative with respect 

 to earth, current flows from the generator, through diode 1, to charge Q to a 

 voltage V with the polarity shown. 



l^ou.= 2V 



V%\r\ wt 



Figure 6.25 



When A moves positive with respect to earth, diode 1 is cut off, and as A 

 reaches its peak voltage the potential of B above earth is equal to : generator 

 voltage + Ci voltage — 2V\ diode 2 conducts and allows Cg to charge to 

 this potential difference. Notice that the potential at B fluctuates between 

 zero, when A is at peak negative, to +2K, when A is at peak positive. 



Cockcroft- Walton quadrupler — Suppose we add a further section, comprising 

 two capacitances and two diodes to the top of the doubler circuit already 

 described, as in Figure 6.26. When B swings down to earth potential, diode 3 

 conducts, allowing Cg to charge from C^ to a voltage 2V. B now swings up 

 to +2K, cutting off D^ and carrying the potential at F up to +4F. D^ then 

 conducts, charging C4 so that the voltage across C4 and Cg in series is also AV. 





oul 



:4V 



l^sin wt 



feut- 



AV 



Figure 6.26 



Figure 6.27 



It is possible to multiply the generator voltage by larger factors by stacking 

 up further capacitances and diodes in a similar manner. However, due to 

 losses in the diodes the law of diminishing returns operates strongly at high 

 orders of voltage multiplication and the proportional discrepancy between 

 the desired output voltage and the output voltage actually achieved becomes 

 serious. Sextuplers are uncommon. 



The Cockcroft-Walton quadrupler is sometimes seen in an alternative form, 

 shown in Figure 6.27. The capacitances can be smaller for a given regulation 

 and ripple, but have to be rated to work at higher voltages. 



107 



