The power transformer

Chapter The power transformer

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • Second, the ac is rectified, so that it becomes pulsating dc with a frequency of ei-ther 60 Hz or 120 Hz. This is almost always done by one or more semiconductor diodes.Third, the pulsating dc is filtered, or smoothed out, so that it becomes a continu-ous voltage having either positive or negative polarity with respect to ground.Finally, the dc voltage might need to be regulated. Some equipment is finicky, in-sisting on just the right amount of voltage all the time. Other devices can put up withsome voltage changes.Power supplies that provide more than a few volts must have features that protect theuser (that’s you!) from receiving a dangerous electrical shock. All power supplies needfuses and/or circuit breakers to minimize the fire hazard in case the equipment shorts out.The power transformerPower transformers can be categorized as step-down or step-up. As you remember, theoutput, or secondary, voltage of a step-down unit is lower than the input, or primary,voltage. The reverse is true for a step-up transformer.Step-downMost solid-state electronic devices, such as radios, need only a few volts. The powersupplies for such equipment use step-down power transformers. The physical size ofthe transformer depends on the current.Some devices need only a small current and a low voltage. The transformer in a ra-dio receiver, for example, can be quite small physically. A ham radio transmitter or hi-fiamplifier needs much more current. This means that the secondary winding of thetransformer must be of heavy-gauge wire, and the core must be bulky to contain themagnetic flux. Such a transformer is massive.Step-upSome circuits need high voltage. The picture tube in a TV set needs several hundredvolts. Some ham radio power amplifiers use vacuum tubes working at kilovolts dc. Thetransformers in these appliances are step-up types. They are moderate to large in sizebecause of the number of turns in the secondary, and also because high voltages canspark, or arc, between wire turns if the windings are too tight.If a step-up transformer needs to supply only a small amount of current, it need notbe big. But for ham radio transmitters and radio/TV broadcast amplifiers, the transform-ers are large and heavy—and expensive.Transformer ratingsTransformers are rated according to output voltage and current. For a given unit, thevolt-ampere (VA) capacity is often specified. This is the product of the voltage and cur-rent. A transformer with a 12-V output, capable of delivering 10 A, would have 12 V 10 A 120 VA of capacity.The nature of power-supply filtering, to be discussed a bit later in this chapter,makes it necessary for the power-transformer VA rating to be greater than just thewattage needed by the load.384 Power supplies