DIODE CIRCUITS 



will level off at something lower. The detector efficiency is defined as the 

 output voltage divided by the peak carrier voltage required to produce it. 

 It is not a good term because it implies that signal rectifier circuits with poor 

 detector efficiencies must be somehow unsatisfactory. If the efficiency is low 

 because a bad diode has been used having a high forward resistance this is 

 perfectly true. If it is low because a small value of C has been chosen in the 

 interests of faithful response, then it is not. 



INDUCTANCE-INPUT (OR CHOKE INPUT) 

 RECTIFYING CIRCUIT 



This is used for power rectification where the load current is so high that 

 the capacitance input circuit would have excessive ripple and exceedingly 

 poor regulation. It is always used in a full-wave system {Figure 6.20). The 



Figure 6.20 



inductance and the load resistance may be regarded as forming a low-pass 

 filter which will allow the steady direct-voltage component of the potential 

 at v4 = (2/77) V (as we saw in the section on multi-range meters) to pass on 

 to the load R^^ at B, but which will attenuate the ripple frequency component, 

 so that the load current will be relatively steady. The cut-off frequency of 

 this arrangement will be at coq = RjL, and as for the best attenuation of the 

 ripple frequency, wc must be much lower than the ripple frequency, we have 

 that, for good smoothing, L must be large or R must be small. It follows that 

 choke input smoothing works best when the load is heavy (contrast this with 

 capacitance-input filtering). It can be shown that for a 50 cycle supply the 

 load ripple current will not exceed 10 per cent of the load steady current if 

 L in henries is greater than or equal to RjAlO, where R is in ohms. This 

 is the 'optimum inductance'. 



If the inductance is sufficient to satisfy a certain criterion of smoothing 

 when the load is light, then it will be unnecessarily large when the load is 

 heavy. In order to achieve greater economy in material here a special type 

 of inductor known as a 'swinging choke' is commonly used, which makes 

 use of saturation effects in the core. When the load is fight, the current is 

 small, the flux is small, the permeability of the iron is maximal and so is 

 the inductance. As the current rises the iron partially saturates and the 

 inductance falls, but the ratio L/7? is approximately maintained. If a normal 

 inductor were used which would maintain its inductance value up to the 

 highest load current, a great deal of iron would be necessary and a very bulky 

 component would be required. 



The regulation of choke input rectifying circuits is very good for the 



104 



