Transformers and impedance matching

Chapter Transformers and impedance matching

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

  • 18CHAPTERTransformers andimpedance matchingIN ELECTRICITY AND ELECTRONICS, TRANSFORMERS ARE EMPLOYED IN VARIOUSways . Transformers are used to obtain the right voltage for the operation of a circuit or sys-tem. Transformers can match impedances between a circuit and a load, or between two dif-ferent circuits. Transformers can be used to provide dc isolation between electronic circuitswhile letting ac pass. Another application is to mate balanced and unbalanced circuits, feedsystems, and loads.Principle of the transformerWhen two wires are near each other, and one of them carries a fluctuating current, acurrent will be induced in the other wire. This effect is known as electromagnetic in-duction. All ac transformers work according to the principle of electromagnetic induc-tion. If the first wire carries sine-wave ac of a certain frequency, then the inducedcurrent will be sine-wave ac of the same frequency in the second wire.The closer the two wires are to each other, the greater the induced current will be,for a given current in the first wire. If the wires are wound into coils and placed along acommon axis (Fig. 18-1), the induced current will be greater than if the wires arestraight and parallel. Even more coupling, or efficiency of induced-current transfer, isobtained if the two coils are wound one atop the other.The first coil is called the primary winding, and the second coil is known as thesecondary winding. These are often spoken of as simply the primary and secondary.The induced current creates a voltage across the secondary. In a step-down trans-former, the secondary voltage is less than the primary voltage. In a step-up transformer,the secondary voltage is greater than the primary voltage. The primary voltage is ab-breviated Epri, and the secondary voltage is abbreviated Esec. Unless otherwise stated,effective (rms) voltages are always specified.The windings of a transformer have inductance because they are coils. The required327Copyright © 2002, 1997, 1993 by The McGraw-Hill Companies. Click here for terms of use.