DC shunt measurement
To measure a large DC current, such as tens of amps, or even more, hundreds of amps, without a large current meter to measure the current, what should I do? This requires a shunt. The shunt is a precision resistor that can pass a large current. When the current flows through the shunt, a millivolt-level voltage appears across it, so it is measured with a millivolt voltmeter. This voltage, which is converted into a current, completes the measurement of the large current.
Ammeters come in many different sizes, but the actual meter is a standard millivolt voltmeter. For example, a voltmeter with a full scale of 75mv. Then using this voltmeter to measure a current such as 20A, you need to equip it with a shunt resistor that produces a 75mv voltage drop when flowing through 20A, also known as a 75mv shunt.
The shunt is a small resistor that can pass a large current. The commonly used 15A or 20A and 35A ammeters require a shunt. The shunt impedance = the head mark full voltage / the head full current. For example, the current limit of the 20A ammeter is 75mv/20A=0.00375Ω. After the impedance is constant, according to Ohm's law U=IR, the current is proportional to the voltage, and the current is linear, so you can use a full scale of 75mv. The voltmeter shows the current current. Therefore, the ammeter used is actually a voltmeter.
How to measure the AC large current? Use a current transformer to change the large current into a small current of 5 amps or less with a certain ratio. Therefore, a small-range AC ammeter can measure the large current, but the measured current is multiplied by that. Change ratio.