DIODE CIRCUITS 



Capacitor input rectifying circuit 



This is shown in the half-wave form in Figure 6.1 1. It appears in apparatus 

 of the a.c./d.c. type for power rectification (because no transformer can be 

 used, so no centre-tap is available) and is the most usual form for signal 

 rectification. A full-wave arrangement is shown in Figure 6.12 and is almost 



B 



V s\r\ wt 



X 



I 



i 



_3 



*Vsina/f 



o 



o 



o- 



o 



o 



o 



Figure 6.11 



■Vs\nujt 



Figure 6.12 



universal for supplying power to an electronic apparatus where the current 

 is less than, say, 250 milliamps. Very few pieces of equipment consume more 

 than this so that this circuit may be regarded practically as standard. 



To see how these circuits work, consider the half-wave case. Suppose R, 

 the load, be temporarily disconnected and C discharged and the generator 

 then switched on. When A swings positive the diode conducts, B is carried 

 positive too and C is charged from the generator. When A and B reach the 

 potential V, A begins to move negative again, reversing the potential difference 

 across the diode, whose resistance therefore becomes high. The current 

 cannot now flow into or out of the capacitance C, which remains charged to 

 a potential V indefinitely. The important fact thus emerges that the no-load 

 voltage of a capacitance-input rectifying circuit is equal to the peak generator 

 voltage. 



Now let the load be reconnected and suppose the potential at A is just 

 beginning to go negative (point a in Figure 6.13). The potential difference 



Potential at B 



Earth 



Potential 

 a.\ A 



Figure 6.13 



across the diode reverses, switching it to high resistance, so that the charged 

 capacitance is left connected only to the load, to which it delivers load current. 

 The voltage across the capacitance falls in an approximately linear manner 

 with time {b in Figure 6.13) until point c is reached when A once more rises 

 above B, switching the diode to low resistance and carrying B back up to V. 

 This rhythmic fluctuation in the output voltage is called ripple. 



100 



