Superimposed direct current

Chapter Superimposed direct current

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

  • The name “root mean square” was not chosen just because it sounds interesting.It literally means that the value of a wave is mathematically operated on, by takingthe square root of the mean of the square of all its values. You don’t really have to beconcerned with this process, but it’s a good idea to remember the above numbers forthe relationships between peak, pk-pk, and rms values for sine waves and squarewaves.For 117 V rms at a utility outlet, the peak voltage is considerably greater. The pk-pkvoltage is far greater.Superimposed direct currentSometimes a wave can have components of both ac and dc. The simplest example of anac/dc combination is illustrated by the connection of a dc source, such as a battery, inseries with an ac source, like the utility mains. An example is shown in the schematic di-agram of Fig. 9-12. Imagine connecting a 12-V automotive battery in series with the walloutlet. (Do not try this experiment in real life!) Then the ac wave will be displaced ei-ther positively or negatively by 12 V, depending on the polarity of the battery. This willresult in a sine wave at the output, but one peak will be 24 V (twice the battery voltage)more than the other.Any ac wave can have dc components along with it. If the dc component exceedsthe peak value of the ac wave, then fluctuating, or pulsating, dc will result. Thiswould happen, for example, if a 200-Vdc source were connected in series with theutility output. Pulsating dc would appear, with an average value of 200 V but with in-stantaneous values much higher and lower. The waveshape in this case is illustratedby Fig. 9-13.Superimposed direct current1759-11Peak-to-peak amplitude.