How to measure voltage and current?

The methods of measuring AC voltage mainly include detection method, sampling method, thermoelectric method, radiant heat method and compensation method.

The detection method uses the detection function of electron tube and transistor to convert AC voltage into DC voltage for measurement. The working frequency of detection voltmeter is generally from tens of Hz to 1000 MHz and the range is 100 microvolts ~ 1000 volts. When the frequency is below 300 MHz, the accuracy is generally about a few percent, and when the frequency is 1000 MHz, it can reach tens of percent.

Sampling method: Sampling is essentially frequency conversion, which is a process of describing a continuous variable with a series of discrete sampling pulses. Generally, the high-frequency signal to be measured is changed into a low-frequency signal of 20 kHz, and then it is detected and measured. The frequency range of this voltmeter is 1~ 1000 MHz or even higher; The voltage range is about 300 microvolts ~ 1 volt (external attenuator can measure large voltage), and the accuracy is from 1% to 10% or more.

Thermoelectric method mainly adopts thermoelectric conversion standard or micro potentiometer. Thermoelectric conversion standard consists of thermocouple and appropriate current limiting resistor or attenuator, which can measure the voltage of 0. 1~300 volts or higher, and the frequency range is generally 20 Hz ~ 100 MHz. If high frequency compensation measures are taken, 1000 MHz can be achieved, and the measurement accuracy is about 0.0 1%~ 1%. The accuracy of AC -DC conversion in low frequency band can reach 1× 10-5 or higher, and the original standards of contemporary low frequency voltage belong to this category. Potentiometers are mainly composed of thermocouples and disc resistors. The known current is multiplied by the resistance to obtain the standard output voltage, which is generally 0. 1 microvolt ~400 mv, with a frequency range of 0~ 1000 MHz and an accuracy of 0.02%~5%.

Generally, the radiation calorimeter method uses radiation thermal resistance (hereinafter referred to as thermal resistance) for measurement. Practical thermal resistors mainly include thermistors, ballast resistors and thin film thermistors. Thermistors have the highest sensitivity (up to tens of thousands of ohms/watt), but the frequency response is poor; Ballast resistor has high sensitivity (about several thousand ohms/watt) and poor frequency response. The sensitivity of thin-film thermistor is low (about1~100Ω/w), but the frequency response is good, so it can be selected according to different needs. The working principle of bolometer is to use the sensitivity of thermal resistance to electric power to convert the measured high-frequency voltage into corresponding resistance change, and then replace the high-frequency voltage with known DC or low-frequency voltage by calorimetry technology according to the principle of power substitution. This device has two types: power meter type (standard meter type) and standard source type. The former is to convert voltage by measuring power and impedance. With the continuous improvement of power and impedance measurement accuracy, it can achieve high accuracy, which is one of the methods to establish the original standard of high-frequency voltage. The latter directly gives the standard voltage value, which is more convenient and can obtain higher accuracy. The typical scheme is to measure the thermal resistance bridge. The original standard of high frequency voltage is mainly radiant heat measuring device. The measuring range is about 0. 1~ 1 volt, the frequency range is about 10~ 1000 MHz, and the accuracy is about 0.2%~ 1%.