Some uses of diodes

Chapter Some uses of diodes

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • 20CHAPTERSome uses of diodesTHE TERM DIODE MEANS “TWO ELEMENTS.” IN THE EARLY YEARS OF ELEC-tronics and radio, most diodes were vacuum tubes. The cathode element emitted elec-trons, and the anode picked up electrons. Thus, current would flow as electronsthrough the tube from the cathode to the anode, but not the other way.Tubes had filaments to drive electrons from their cathodes. The filaments wereheated via a low ac voltage, but the cathodes and anodes usually wielded hundreds oreven thousands of dc volts.Today, you’ll still hear about diodes, anodes, and cathodes. But rather than large,heavy, hot, high-voltage tubes, diodes are tiny things made from silicon or other semi-conducting materials. Some diodes can handle voltages nearly as great as their tubecounterparts. Semiconductor diodes can do just about everything that tube diodescould, plus a few things that people in the tube era probably never imagined.RectificationThe hallmark of a rectifier diode is that it passes current in only one direction. Thismakes it useful for changing ac to dc. Generally speaking, when the cathode is negativewith respect to the anode, current flows; when the cathode is positive relative to the an-ode, there is no current. The constraints on this behavior are the forward breakover andavalanche voltages, as you learned about in the last chapter.Suppose a 60-Hz ac sine wave is applied to the input of the circuit in Fig. 20-1A.During half the cycle, the diode conducts, and during the other half, it doesn’t. This cutsoff half of every cycle. Depending on which way the diode is hooked up, either the pos-itive half or the negative half of the ac cycle will be removed. Figure 20-1B shows theoutput of the circuit at A. Remember that electrons flow from negative to positive,against the arrow in the diode symbol.The circuit and wave diagram of Fig. 20-1 show a half-wave rectifier circuit. This370Copyright © 2002, 1997, 1993 by The McGraw-Hill Companies. Click here for terms of use.