INDUCTANCE-INPUT RECTIFYING CIRCUIT 



following reason. In Figure 6.21 we have Figure 6. 20 again, but an additional 

 capacitance C, usually present in this type of circuit in the form of extra 

 smoothing, is connected across R and stabihzes the potential difference 

 across Rj^ at (2/77) V. At first sight it would appear that the conduction 

 time of diode (1) is that for which the potential at A exceeds (I/tt) V, and 



Figure 6.21 



similarly for diode (2) and the potential at D {Figure 6.22) : in fact it is even 

 longer. Suppose point A is positive to earth but has passed its peak and is 

 negative going: diode (1) is conducting and diode (2) is cut off. As A is 

 falling there is a tendency for the inductance current to fall, which the induc- 

 tance combats by generating a back e.m.f. across itself, the sense of which 

 is to make B more negative than E. If the potential at E is stabilized, the 

 effect of the back e.m.f. is to depress the potential at B, so that A can go 

 further negative before diode 1 is cut off. Thus the conduction time of diode 

 1 is extended. The benefit conferred by the long conduction time is very low 



Potential 

 ot/4 



Rjtential 

 ofD 



Potential 

 of /A 



Fbtential 

 at C 



Diode '^ Diode '^ Diode 



1 2 1 



conducting conducting conducting 



Figure 6.22 



peak diode current and so low voltage drops. The same applies in the case 

 of diode 2 and hence the regulation of the circuit is good. 



There is a difficulty with choke-input circuits which requires bearing in 

 mind. Referring to Figure 6.21, if the load is accidentally removed the 

 inductance current is drastically reduced and so, therefore, is the contribution 

 of the inductance to the operation of the circuit. When this happens the 

 arrangement reverts to the capacitance-input type of rectifier and the output 

 voltage rises from its normal value of about 2K/7r to V. This may cause 

 serious damage, and to prevent it happening it is necessary to ensure that 

 some load is always present. Such a load is called a 'bleeder' (R^, in Figure 

 6.23). It can be shown that if for a 50 cycle supply L > i?imax/940 the 

 output voltage will not rise above its normal value. This value of L is known 

 as the 'critical inductance'. Design procedure is therefore as follows: given 



105 



