What are the effective values and the true effective values of a multimeter?
The magnitude of alternating current changes with time. The magnitude of the instantaneous value (a certain moment) changes between zero and positive and negative peak values. The maximum value is only an instantaneous value and cannot reflect the workability of alternating current.
Therefore, the concept of effective value is introduced, which is defined as:
Effective value: Defined by heat (power). A certain AC current passing through a resistor generates heat and another DC current passing through the resistor. If the heat generated in the same time is equal, then the DC voltage value is the AC voltage. Valid values.
True effective value: The definition of effective value is defined by heat generation, but it is difficult to measure the effective value voltage in measuring instruments in this way. Therefore, in most voltage measuring instruments, such as a multimeter measuring voltage, its measurement The method is not based on the "heat" defined by the effective value. One type of multimeter uses the sine wave as a reference and obtains the effective value through the relationship between the effective value of the sine wave whose peak value is twice the square root (or by averaging value to derive), the effective value obtained by this method is only correct for AC voltages such as sinusoidal waveforms, and will cause deviations for waveforms of other shapes. The voltage value of another type of multimeter is calculated by the square of the effective value of the DC component, fundamental wave and each higher harmonic. This value is similar to the definition of the effective value. There is no requirement for the shape of the waveform. In order to distinguish this type of effective value from the The difference between the sine wave and the effective value of the instrument is found, and this wave is called the "true effective value" in the measuring instrument.
Root mean square value: Another name for the effective value (which should be the true effective value on the measuring instrument).
The effective value of a multimeter usually refers to one of the following three situations:
1. Calibration average method. The calibration average is also called the corrected average, or the rectified average calibrated to the effective value. Its principle is to convert the AC signal into a DC signal through the rectification and integration circuit, and then according to the characteristics of the sine wave, Multiply by a factor that, for a sine wave, equals the effective value of the sine wave. Therefore, this method is limited to sine wave testing.
2. Peak detection method, through the peak detection circuit, obtain the peak value of the AC signal, and then multiply it by a coefficient according to the characteristics of the sine wave. For the sine wave, after multiplying by the coefficient, the result is equal to the effective value of the sine wave. Therefore, this method is limited to sine wave testing.
3. True RMS method, which uses a true RMS circuit to convert the AC signal into a DC signal before measuring. This method is applicable to the true effective value test of arbitrary waveforms.
Most multimeters use the first two methods. And there are great restrictions on the frequency of the signal.
For alternating current, its voltage is a changing waveform. Usually the voltage value we describe refers to its effective value. For example, when we say 220V power supply, its peak voltage is more than 310 volts, and the peak-to-peak voltage is twice the peak value. It is more than 600 volts.
The effective values of electromotive force, voltage, and current of sinusoidal alternating current are represented by E, U, and I respectively. Generally speaking, the electromotive force, voltage, and current of alternating current are the average of their effective values. The ratings marked on AC electrical equipment and the values indicated by AC meters are also valid values.
