Voltage Measurement Concepts

In this section, we just want to briefly remind you of some concepts for measuring different voltages in ac and dc which will be useful for performing the experiments during this course. We also present some approximations you can use in order to estimate these voltages.


RMS or effective voltage: Mathematically, it is the square root of the mean average of the square of the voltage within a time interval (RMS stands for Root Mean Square). Electrically, it is the voltage which, when applied across a resistance, produces the same amount of heat dissipation that a dc voltage of the same magnitude would produce. It is also known as effective voltage.

Average voltage: It is calculated by taking the average of the voltage waveform in an appropriately chosen interval. For a symmetric wave in both axes, this value is zero because the negative points cancel the positive ones when the calculation is performed.

RMS and effective voltage are calculated with integrals, so they are NOT the same for all time-variable functions. The following table summarizes the approximations for calculating the RMS voltages for different signal shapes from their peak voltage values:


The crest factor is the ratio of the peak amplitude of a waveform divided by its RMS voltage. The purpose of this factor is to provide an idea of how much impacting occurs in a waveform. In a perfect sine wave with an amplitude of 1, the RMS value is equal to 0.707 and the crest factor is equal to 1.41 A perfect sine wave contains no impacting and therefore crest values higher than 1.41 indicate that there is some degree of impact.

The crest factor becomes critical when pulses are being used. To measure them with electronic meters using real-time RMS measuring methods, the crest factor for the pulse needs to be within a certain range to protect the device from overloading and to prevent incorrect readings.


Ripple Voltage and Ripple Factor:

The variation in the output due to charging and discharging is called the ripple voltage. The smaller the ripple, the better the filtering action. The ripple factor is an indication of the effectiveness of the filter and is defined as


where Vr is the rms ripple voltage and Vdc is the dc (average) value of the filter's output voltage. The lower the ripple factor, the better the filter. The ripple factor can be lowered by increasing the value of the filter capacitor. You will experiment with ripple voltage in the following sections.


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