The core working principle of an AC voltmeter is to convert AC signals into measurable DC signals or directly use the characteristics of AC for measurement, and ultimately reflect the voltage value through pointer deflection or digital display. The key lies in solving the problem of "instantaneous value changes" in AC power - the magnitude and direction of AC power change periodically over time and cannot be directly measured by a DC meter. Therefore, it needs to be processed through specific circuits to convert it into a stable signal corresponding to the effective value (or average value).
Rectified AC voltmeter: Rectify first, then measure DC
This is the most common type of AC voltmeter, especially widely used in pointer instruments, where the core is to convert AC power to DC power through a rectifier circuit.
Half wave rectification: utilizing the unidirectional conductivity of diodes to allow only the positive (or negative) half cycle of alternating current to pass through. For example, during the positive half cycle, the diode conducts and current flows through the meter head; At the negative half cycle, the diode is turned off and there is no current in the meter. The actual measurement of the meter head is the average value after half wave rectification, which is then converted into the effective value of AC power through the scale (there is a fixed ratio between the effective value of sine wave and the average value: effective value=average value x 1.11).
Full wave rectification: By using a bridge rectifier circuit (consisting of 4 diodes), the positive and negative half cycles of alternating current are converted into direct current in the same direction, allowing current to flow through the meter throughout the entire cycle. Compared to half wave rectification, full wave rectification has a more stable output and higher measurement accuracy, and also requires the display of effective values through scale conversion.
Electromagnetic or electric instruments: directly utilizing the magnetic field effect of alternating current
This type of instrument does not require rectification and can directly measure alternating current. Its principle is based on the force of the magnetic field generated by alternating current on movable components.
Electromagnetic voltmeter: It has fixed coils and movable iron plates inside. When alternating current passes through a fixed coil, an alternating magnetic field is generated, and the movable iron piece is magnetized and deflected by the magnetic field force. Due to the fact that the direction of the magnetic field of alternating current changes with the direction of the current, but the magnetization direction of the iron sheet also changes synchronously, the deflection direction remains unchanged. The deflection angle of the pointer is proportional to the square of the effective value of the alternating current, thus indicating the voltage value through the scale.
Electric voltage meter: It consists of a fixed coil and a movable coil, which are connected in series to the tested circuit. When alternating current passes through a coil, an alternating magnetic field is generated, and the interaction force between the two coils (determined by the direction of the current) drives the movable coil to deflect. Due to the synchronous change of the current direction of alternating current, the direction of the applied force remains unchanged, and the deflection angle is related to the square of the effective voltage value. After calibration, the effective value can be directly displayed.
Digital AC voltmeter: measured through digital processing
The working principle of a digital meter is more complex, with the core being the conversion of analog AC signals into digital signals.
Rectification and filtering: First, AC power is converted into pulsating DC power through a rectification circuit, and then smoothed into a stable DC signal (reflecting the average or effective value of AC power) through a filtering circuit.
A/D conversion: Using an analog-to-digital converter (ADC) to convert a DC analog signal into a digital signal, this process samples and quantizes the signal.
Calculation and display: The microprocessor processes digital signals (such as converting them into effective values based on waveform characteristics), and finally outputs voltage values through the display screen. The digital meter has high accuracy and can also compensate for measurement errors of different waveforms (such as non sinusoidal waves) through algorithms.
Key: Measurement logic centered around "effective value"
Regardless of the type, the final displayed value of an AC voltmeter is usually the effective value of the AC power (the effective value of a sine wave is equal to its peak value divided by √ 2), because the effective value can truly reflect the work capacity of the AC power (such as power calculation). For example, what we often refer to as "220V mains power" refers to its effective value, corresponding to a peak value of approximately 311V.
In summary, the essence of an AC voltmeter is to convert changing AC power into a stable and measurable signal through rectification, electromagnetic effects, or digital processing, and ultimately present it in the form of effective values, achieving precise quantification of AC voltage.