What Are the Different Methods for Measuring Current?

Electric current is one of the basic physical quantities in amperes (A). When a constant current of 1A is maintained in two infinitely long parallel straight wires with a negligible circular cross section at a distance of 1 meter in a vacuum, the force between the two wires per meter of length is 2 × 10-7 Newton . Since this theoretical definition cannot be achieved, amps are actually reproduced with a device as close as possible to the defined conditions.

Current measurement

The current is

When 1A

The main methods for measuring high-frequency currents are the thermoelectric method and the radiant calorimeter method. Pyroelectric method: It can be used for DC, low frequency and high frequency current measurement (Figure 1). When measuring AC current, send the measured current signal from the left end and write down the indicator value; then use DC input to get the DC current value when the same indication value is equal to the measured AC current value. This DC current must be calibrated to ensure high accuracy. The core of the thermoelectric circuit is a thermocouple. In order to eliminate the forward and reverse errors, the direction of the wiring at both ends of the couple should be changed when measuring DC, and then the average value is taken twice. This method has a wide measuring range, about 10-3 to 102 amps; high accuracy, up to ± 10-5, is the most commonly used method. The method of measuring the radiant heater: The characteristic of measuring the resistance of the radiant heater is only related to the added power and has nothing to do with the frequency. The bridge circuit of the radiator is used to replace the high-frequency current with a DC current to measure the high temperature. Frequency and voltage, and then use the voltage and resistance to get the current (Figure 2). In order to reduce the influence of standing waves, the resistance value of the radiant heater should be made as equal to the characteristic impedance of the transmission line as much as possible. The output port is generally connected with a resonant circuit or a 1/4 wavelength short-circuit line to reduce the impact of shunting. The accuracy of this method is about ± (10-2 10-3), and the frequency of use can reach several gigahertz.

Other methods of current measurement

For measuring high-frequency current, there are a power method for calculating a current value by measuring power applied to a known resistor, and a photoelectric method for obtaining a current value after photoelectric conversion. In order to expand the current range, resistance shunt method (suitable for low frequency), inductor and capacitor shunt method (suitable for high frequency), and transformer method (also called current comparator method, suitable for low frequency and high frequency) can be used.

Insufficient current measurement

In the current measurement system, since the DUT and the measuring instrument are connected in series, the load effect is large, which increases the difficulty of measurement. ^[1]