Three Cases of Effective Values for Multimeters
① Effective Value: It is defined by heat generation (power). When an alternating current passes through a resistor and produces the same amount of heat within the same time period as a direct current passing through the same resistor, the DC voltage value is the effective value of the alternating‑current voltage.
② True RMS Value: The effective value is defined based on heat generation. However, it is difficult for measuring instruments to measure effective voltage strictly according to this definition. Therefore, most voltage‑measuring instruments such as multimeters do not measure based on the "heat‑generation" definition of effective value. One type of multimeter takes sine waves as reference and obtains the effective value from the relationship that the peak value of a sine wave is √2 times its effective value (or derived from the average value). The effective value obtained by this method is only accurate for sine‑wave AC voltage and will produce deviations for waveforms of other shapes.
Another type of multimeter calculates the voltage value from the squares of the effective values of DC component, fundamental wave and each higher‑order harmonic. This value is similar to the definition of effective value and imposes no requirements on waveform shapes. To distinguish it from effective values obtained by sine‑wave‑based meters, this reading is called "true RMS value" in measuring instruments.
③ RMS Value: Another name for effective value (true RMS value in measuring instruments).
Three Cases of Effective‑Value Measurement for Multimeters: ① Calibrated‑average method: Also known as corrected average value or rectified average value calibrated to effective value. Its principle is to convert AC signals into DC signals via rectifying and integrating circuits, then multiply by a coefficient according to sine‑wave characteristics. For sine waves, the result after coefficient multiplication equals the sine‑wave effective value.
Therefore, this method is only applicable to sine‑wave tests.
② Peak‑detection method: Obtain the peak value of AC signals through a peak‑detection circuit, then multiply by a coefficient according to sine‑wave characteristics. For sine waves, the result after coefficient multiplication equals the sine‑wave effective value. Therefore, this method is only applicable to sine‑wave tests.
③ True‑RMS method: Adopt true‑RMS circuits to convert AC signals into DC signals for measurement. This method applies to true‑RMS measurement of arbitrary waveforms. Most multimeters adopt the first two methods and have significant limitations on signal frequency.
