SMOOTHED RECTIFICATION 



If the load is made heavier, R is reduced, C has to supply more current 

 and discharges further between charging periods. Thus we see that on increas- 

 ing the load on a capacitor input rectifier circuit the ripple voltage increases 

 and the mean output voltage falls. The latter is in fact V — | (peak to peak 

 ripple) (Figure 6.14). 



Light load Heavy load 



figure 6.14 



Application of this circuit to power supply — The regulation of a power 

 supply is said to be good when the output voltage falls only a little with in- 

 crease of load current. On the basis of what we have said so far it looks as 

 if ripple may be reduced and regulation improved as much as we like by 

 increasing the value of the capacitance. In point of fact the ripple is reduced 

 but the regulation may get worse and the diode even be destroyed. Let us 

 see why this is. 



Referring to Figure 6. 15 it is clear that diode current only passes during 



Diode 



current 



the short period /. Since all the output current must at some time have passed 

 through the diode, it follows that the diode current must be of a magnitude 

 many times the load current. Further, it is clear that if the capacitance be 

 increased for a given load, the diode conduction time becomes even shorter 

 and the peak diode current must of necessity become larger in order that 

 power may be supplied to the load at the same rate. It is these large peak 

 currents which are damaging to thermionic diodes, though the makers of 

 selenium diodes claim that they are not harmed by large instantaneous 

 currents. 



When we take into account the resistance of the generator and the forward 

 resistance of the diode we see why the regulation suflFers. The high peak 

 currents passed by the diode produce unexpectedly large voltage drops in 

 the generator internal resistance and the diode at the instant of conduction, 

 so that point B never reaches E, only something lower. 



101 



