Fundamental Electrical and Electronic Principles, Third Edition

Alternating Quantities 209It is the electronic equivalent of a mechanical valve, for example thevalve in a car tyre. This device allows air to be pumped into the tyre,but prevents the air from escaping.Fig. 6.8The circuit symbol for a diode is shown in Fig. 6.8 . The ‘ arrow head ’part of the symbol is known as the anode. This indicates the directionin which conventional current can flow through it. The ‘ plate ’ partof the symbol is the cathode, and indicates that conventional currentis prevented from entering at this terminal. Thus, provided that theanode is more positive than the cathode, the diode will conduct. Thisis known as the forward bias condition. If the cathode is more positivethan the anode, the diode is in its blocking mode, and does not conduct.This is known as reverse bias.Note: The potentials at anode and cathode do not have to be positiveand negative. Provided that the anode is more positive than the cathode,the diode will conduct. So if the anode potential is (say) 10 V, and thecathode potential is 8 V, then the diode will conduct. Similarly, ifthese potentials are reversed, the diode will not conduct.6.9 Half-wave RectifierThis is the simplest form of rectifier circuit. It consists of a singlediode, placed between an a.c. supply and the load, for which d.c. isrequired. The arrangement is shown in Fig. 6.9 , where the resistor Rrepresents the load.linputRoutputinputRoutputFig. 6.9Fig. 6.10Let us assume that, in the first half cycle of the applied voltage, theinstantaneous polarities at the input terminals are as shown in Fig. 6.10 .Under this condition, the diode is forward biased. A half sinewave ofcurrent will therefore flow through the load resistor, in the directionshown.In the next half cycle of the input waveform, the instantaneouspolarities will be reversed. The diode is therefore reverse biased, andno current will flow. This is illustrated in Fig. 6.11 .