The Meaning and Application of True RMS (Root Mean Square) of a Multimeter
For alternating current (AC), its voltage is a changing waveform. Usually, the voltage value we describe refers to its root mean square (RMS) value. For example, for the 220V power supply we mention, its peak voltage is over 310 volts, and the peak-to-peak voltage is over 600 volts.
RMS Value: It is defined based on heat generation (power). When an alternating current passes through a resistor and generates heat, and if another direct current (DC) passes through the same resistor and generates the same amount of heat within the same period of time, then the voltage value of this direct current is the RMS value of the voltage of this alternating current.
True RMS Value: The definition of the RMS value is based on heat generation. However, it is very difficult to measure the RMS voltage according to this method in measuring instruments. So, in most voltage measuring instruments, such as when a multimeter measures voltage, the measurement method is not carried out according to the "heat generation" definition of the RMS value. One type of multimeter takes the sine wave as a reference and obtains the RMS value through the relationship that the peak value of the sine wave is √2 times the RMS value (or infers it through the average value). The RMS value obtained by this method is only correct for the AC voltage of the sine wave type, and there will be deviations for other waveforms. Another type of multimeter calculates the voltage value by taking the square root of the sum of the squares of the DC component, the fundamental wave, and the RMS values of various higher harmonics. This value is similar to the definition of the RMS value and has no requirements for the shape of the waveform. To distinguish this type of RMS value from the RMS value obtained by the instrument based on the sine wave, this value is called the "true RMS value" in the measuring instrument.
Root Mean Square (RMS) Value: It is another name for the RMS value (in measuring instruments, it should be the true RMS value).
Calibrated Average Value Method: The calibrated average value is also called the corrected average value or the rectified average value calibrated to the RMS value. Its principle is to convert the AC signal into a DC signal through a rectifier and an integration circuit, and then multiply by a coefficient according to the characteristics of the sine wave. For a sine wave, after multiplying by this coefficient, the result is equal to the RMS value of the sine wave. Therefore, this method is only applicable to the testing of sine waves.
Peak Detection Method: Through the peak detection circuit, the peak value of the AC signal is obtained, and then multiplied by a coefficient according to the characteristics of the sine wave. For a sine wave, after multiplying by this coefficient, the result is equal to the RMS value of the sine wave. Therefore, this method is only applicable to the testing of sine waves.
True RMS Method: Adopt a true RMS circuit to convert the AC signal into a DC signal and then measure it. This method is applicable to the true RMS testing of any waveform.
Most multimeters adopt the first two methods and have relatively large limitations on the frequency of the signal.
