Batteries and other sources of direct current (dc) produce constant voltage. This can be represented by a straight, horizontal line on a graph of voltage versus time following figure :
For pure dc, the peak and effective values are identical. But sometimes the value of dc voltage fluctuates rapidly with time. This happens, for example, if the waveform in above figure is passed through a rectifier circuit.
Rectification is a process in which ac is changed to dc. The most common method of doing this uses a device called the diode. Right now, you need not be concerned with how the rectifier circuit is put together. The point is that part of the ac wave is either cut off, or turned around upside down, so the output is pulsating dc.
Following figure illustrates two different waveforms of pulsating dc. In the waveform at A, the negative (bottom) part has been cut off. At B, the negative portion of the wave has been inverted and made positive. The situation at A is known as half-wave rectification, because it involves only half the waveform. At B, the ac has been subjected to full-wave rectification, because all of the original current still flows, even though the alternating nature has been changed so that the current never reverses.
The effective value, compared with the peak value, for pulsating dc depends on whether half wave or full-wave rectification is applied to an ac wave. In below Fig. A and B, effective voltage is shown as dashed lines, and the instantaneous voltage is shown as solid curves. The instantaneous voltage changes all the time, from instant to instant. (That’s how it gets this name!) The peak voltage is the maximum instantaneous voltage. Instantaneous voltage is never any greater than the peak voltage
for any wave.
At A, half-wave rectification of common utility ac. At B, full-wave rectification of common utility ac. Effective voltages are shown by the dashed lines.
In above fig. B, the effective voltage is 0.707 times the peak voltage, just as is the case with ordinary ac. The direction of current flow, for many kinds of devices, doesn’t make any difference. But in above Fig. A, half of the wave has been lost. This cuts the effective value in half, so that it’s only 0.354 times the peak value.
In household ac that appears in wall outlets for conventional appliances in the United States, the peak voltage is about 165 V; the effective value is 117 V. If full-wave rectification is used, the effective value is still 117 V. If half-wave rectification is used, the effective voltage is about 58.5 V.