Multimeter AC voltage measurement method and principle
Due to the fact that the measuring device of the multimeter is a DC microampere meter head, it is necessary to introduce a rectifier element when measuring AC voltage, forming an AC voltmeter with a rectifier system. The AC voltage range of a multimeter is a series of AC voltage meters with sufficient current.
The rectification circuit of a multimeter can use half wave rectification and full wave rectification. The average current after rectification can be used to measure AC voltage using the method of measuring DC voltage. Due to people's habit of measuring AC voltage using the effective value, the system on the dial of a multimeter is also represented by the effective value. The ratio of the effective current I of sinusoidal alternating current to the uniform current I is called the waveform coefficient expressed in Kf. For full wave rectification Kf=1.11, for half wave rectification Kf=2.22.
The actual AC voltage measurement circuit of a multimeter often uses a half wave current reporting method. As can be seen from the cause, when the measured AC voltage is positive for half a cycle, the half wave rectified current passes through the positive terminal and the multiplication resistors R3, R2, R1, Z1, and flows through the current meter head to the negative terminal. When the measured AC voltage is negative for half a cycle, z1 is cut off, and there is no current flowing through the meter head. The rectified current flows out from the positive terminal through z2, R1, R2 R3. The rectifier in a multimeter generally uses a semiconductor rectifier. There are generally two methods for measuring AC voltage. One is to share a set of multiplier resistors and draw a scale line on each dial when measuring AC and DC voltages.
However, the voltage values on the two marking lines differ significantly, so both of these marking lines - k - must be marked with the voltage values. Another method is to use a set of multiplier resistors for both AC and DC voltage measurements, so that the AC and DC voltage scales can use the same scale line.
