Electrowave Ultrasonics Corporation


POWER MEASUREMENTS




PEAK AMPLITUDE (Voltage & Current) One of the most frequently measured characteristics of a sine wave is its amplitude. Unlike DC measurement, the amount of alternating current or voltage present in a circuit can be measured in various ways. In one method of measurement, the maximum amplitude of either the positive or negative alternation is measured, an oscilloscope or peak reading meter is used. The value of current or voltage obtained is called the PEAK CURRENT or the PEAK VOLTAGE, and will be found to be equal to the square root of 2 multiplied by the RMS Effective Value that is read on most multimeters. EFFECTIVE OR RMS VALUE (Voltage & Current) As the use of alternating current gained popularity, it became increasingly apparent that some common basis was needed on which AC and DC could be compared. A 100watt fight bulb, for example should work just as well on 120 volts AC as it does on 120 volts DC. It can be seen, however, that a sine wave of voltage having a peak value of 120 volts would not supply the lamp with as much power as a steady value of 120 volts DC. Since the power dissipated by the lamp is a result of current flow through the lamp, the problem resolves to one of finding a MEAN alternating current ampere which is equivalent to a steady ampere of direct current. A DC current having a value equal to the square root of the mean of the current squared (RMS) would produce the same average power as the original sine wave of current. One RMS ampere of alternating current is as effective in producing current as one steady ampere of direct current, for this reason an RMS ampere is also called an EFFECTIVE ampere. AVERAGE VALUE (Voltage & Current) The Average Value of a complete cycle of a sine wave is zero, since the positive alternation is identical to the negative alternation. However, the Average Value of a single alternation could be computed by adding together a series of instantaneous values of the sine wave between 0, and 180 degrees, and then dividing the sum by the number of instantaneous values used. Such a computation would show one alternation of a sine wave to have an Average Value equal to 2 divided by , multiplied by the Peak Value (Approx. 0.637 X Peak Value). POWER (WATTS) The measurement of power takes two forms. There is the Average power dissipated by a circuit (RMS volts X RMS amperes), and then there is the Peak power value as read on an oscilloscope (I squared) which is taken to be the Peak volts multiplied by the Peak amperes, or simply the Average power multiplied by 2. 


