The Functions of the Effective Value and True Effective Value of a Multimeter
The magnitude of alternating current changes with time. The magnitude of the instantaneous value (at a certain moment) varies between zero and the positive and negative peak values. The maximum value is only an instantaneous value and cannot reflect the working capacity of the alternating current.
Therefore, the concept of effective value is introduced, and its definition is as follows:
Effective value: It is defined by heat generation (power). When a certain alternating current passes through a resistor and generates heat, and another direct current passes through the same resistor, if the heat generated within the same period of time is equal, then the voltage value of this direct current is the effective value of the voltage of this alternating current.
True effective value: The definition of the effective value is based on heat generation. However, it is very difficult to measure the effective value of the 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" defined by the effective value. One type of multimeter takes the sine wave as a reference and obtains the effective value through the relationship that the peak value of the sine wave is √2 times the effective value (or by deriving from the average value). The effective value obtained by this method is only correct for the alternating voltage of the sine wave shape, and there will be deviations for other waveform shapes. Another type of multimeter calculates the voltage value from the squares of the effective values of the direct current component, the fundamental wave, and various harmonic waves. This value is similar to the definition of the effective value and has no requirements for the shape of the waveform. To distinguish this type of effective value from the effective value obtained by the instrument using the sine wave, in the measuring instrument, this value is called the "true effective value".
Root mean square value: Another name for the effective value (in the measuring instrument, it should be the true effective value).
The effective value of a multimeter usually refers to one of the following three situations:
Calibrated average value method. The calibrated average value is also called the corrected average value or the rectified average value calibrated to the effective value. Its principle is to use a rectifying and integrating circuit to convert the alternating current signal into a direct current signal, and then according to the characteristics of the sine wave, multiply it by a coefficient. For a sine wave, after multiplying by this coefficient, the result is equal to the effective value of the sine wave. Therefore, this method is limited to the testing of sine waves.
Peak value detection method. Through the peak value detection circuit, the peak value of the alternating current signal is obtained, and then according to the characteristics of the sine wave, multiply it by a coefficient. For a sine wave, after multiplying by this coefficient, the result is equal to the effective value of the sine wave. Therefore, this method is limited to the testing of sine waves.
True effective value method. Adopt a true effective value circuit to convert the alternating current signal into a direct current signal and then measure it. This method is applicable to the testing of the true effective value of any waveform.
Most multimeters use the first two methods, and there are relatively large limitations on the frequency of the signal.
