CHAPTER 4. DIODE CIRCUITS70power is to transform, rectify, ﬁlter and regulate an AC line voltage. Power supplies makeuse of simple circuits which we will discuss presently.DC power supplies are often constructed using a common inexpensive three-terminalregulator. These regulators are integrated circuits consisting of several solid state devicesand are designed to provide the desirable attributes of temperature stability, output currentlimiting and thermal overload protection.In power supply applications it is common to use a transformer to isolate the powersupply from the 110 V AC line. A rectiﬁer can be connected to the transformer secondaryto generate a DC voltage with little AC ripple. The object of any power supply is to reducethe ripple which is the periodic variation in voltage about the steady value.4.3.2RectiﬁcationFigure 4.7 shows a half-wave rectiﬁer circuit. The signal is exactly the top half of the inputvoltage signal, and for an ideal diode does not depend at all on the size of the load resistor.V0VsRSGFigure 4.7: Half-wave rectiﬁer and its output waveform.The rectiﬁed signal is now a combination of an AC signal and a DC component. Generally,it is the DC part of a rectiﬁed signal that is of interest, and the un-welcomed AC componentis described as ripple. It is desirable to move the ripple to high frequencies where it is easierto remove by a low-pass ﬁlter.When diodes are used in small-signal applications – a few volts – their behaviour is notclosely approximated by the ideal model because of the PN turn-on voltage. The equivalentcircuit model can be used to evaluate the detailed action of the rectiﬁer under these condi-tions. During the part of the wave when the input is positive but less than the PN turn-onvoltage, the model predicts no loop current and the output signal voltage is therefore zero.When the input exceeds this voltage, the output signal becomes proportional to vs− VPN ,or about 0.6 V lower than the source voltage.The diode bridge circuit shown in ﬁgure 4.8 is a full-wave rectiﬁer. The diodes act toroute the current from both halves of the AC wave through the load resistor in the samedirection, and the voltage developed across the load resistor becomes the rectiﬁed outputsignal. The diode bridge is a commonly used circuit and is available as a four-terminalcomponent in a number of diﬀerent power and voltage ratings.