What are a multimeter's RMS and true RMS values?
The magnitude of AC power varies over time, and the instantaneous value (at a certain moment) varies between zero and positive/negative peak values. The maximum value is only a momentary value and cannot reflect the working ability of AC power.
So the concept of effective value was introduced, which is defined as:
Effective value: defined by heat generation (power), where a certain alternating current generates heat through a resistor and another continuous current passes through the resistor. If the heat generated within the same time is equal, then the DC voltage value is the effective value of this alternating current voltage.
True RMS: The definition of RMS is based on heat generation, but it is difficult to measure RMS voltage using this method in measuring instruments. Therefore, in most voltage measuring instruments, such as a multimeter, the measurement method is not based on the "heat generation" defined by RMS. One type of multimeter uses a sine wave as a reference and obtains the RMS (or deduces it through the average value) based on the relationship between the peak value of the sine wave and the RMS value, which is twice the root sign. The RMS obtained by this method is only correct for the AC voltage of the sine wave type and will cause deviation for other waveforms. The voltage value of another type of multimeter is calculated by taking the square of the effective values of the DC component, fundamental wave, and various higher-order harmonics. This value is similar to the definition of effective value, and there is no requirement for the shape of the waveform. To distinguish this type of effective value from measuring instruments that use sine waves to obtain effective value, it is commonly referred to as "true effective value" in measuring instruments.
Root mean square value: Another term for effective value (which should be true effective value on a measuring instrument).
The effective value of a multimeter usually refers to one of the following three situations:
1. The calibration average method, also known as the corrected average or the rectified average calibrated to the effective value, is based on the principle of converting AC signals into DC signals through rectification and integration circuits, and then multiplying them by a coefficient according to the characteristics of a sine wave. For a sine wave, the result of multiplying by this coefficient is equal to the effective value of the sine wave. Therefore, this method is limited to sine wave testing only.
2. The peak detection method obtains the peak value of the AC signal through a peak detection circuit, and then multiplies it by a coefficient based on the characteristics of the sine wave. For a sine wave, the result of multiplying by this coefficient is equal to the effective value of the sine wave. Therefore, this method is limited to sine wave testing only.
3. The true effective value method uses a true effective value circuit to convert AC signals into DC signals for measurement. This method is suitable for testing the true effective value of any waveform.
Most multimeters use the first two methods. And there are significant limitations on the frequency of the signal.
For AC power, its voltage is a changing waveform, and we usually describe its voltage value as its effective value. As we call it 220V power supply, its peak voltage is over 310V, and the peak to peak voltage is twice the peak value, which is over 600V.
