Amplitude Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 6, 2011

Superimposed Dc

Sometimes a wave can have components of both ac and dc. The simplest example of an ac/dc combination is illustrated by the connection of a dc source, such as a battery, in series with an ac source, such as the utility main.

Any ac wave can have a dc component along with it. If the dc component exceeds the peak value of the ac wave, then fluctuating or pulsating dc will result. This would happen, for example, if a 200-V dc source were connected in series with the utility output. Pulsating dc would appear, with an average value of 200 V but with instantaneous values much higher and lower. The waveshape in this case is illustrated by Fig. 13-12.

Alternating Current Amplitude Superimposed Dc


Fig. 13-12 . Composite ac/dc wave resulting from 117-V rms ac in series with + 200-V dc.

Amplitude Practice Problems

Problem 1

An ac sine wave measures 60 V pk-pk. There is no dc component. What is the peak voltage?

Solution 1

In this case, the peak voltage is exactly half the peak-to-peak value, or 30 V pk. Half the peaks are + 30 V; half are −30 V.

Problem 2

Suppose that a dc component of +10 V is superimposed on the sine wave described in Problem 13-5. What is the peak voltage?

Solution 2

This can’t be answered simply, because the absolute values of the positive peak and negative peak voltages differ. In the case of Problem 13-5, the positive peak is +30 V and the negative peak is −30 V, so their absolute values are the same. However, when a dc component of +10 V is superimposed on the wave, both the positive peak and the negative peak voltages change by + 10 V. The positive peak voltage thus becomes +40 V, and the negative peak voltage becomes −20 V.

Practice problems of these concepts can be found at: Alternating Current Practice Test

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