What Is a Shunt?

A shunt is an instrument for measuring DC current, which is made based on the principle that a voltage is generated across the resistor when the DC current passes through the resistor ^[1] .

Shunts are generally used to extend the current range with a fixed low resistance. It is usually connected in parallel with the moving coil of the ammeter or galvanometer. There are two types of connection inside and outside the meter ^[2]

(1) According to the ammeter (or current and voltage dual-use meter) used

For motor test measurement, multiple current shunts are often configured for one ammeter to solve the problem of ensuring the required measurement accuracy over a large measurement range. At this time, it is required that the rated voltage drop of all shunts used is consistent with the ammeter provided, such as 75mV. In this way, after the shunt is selected, the full scale of the ammeter is the rated current value of the selected shunt, and the multiple of the ammeter (ie, the number of currents per dial scale) is the rated current of the shunt divided by the total number of dial scales.

Used for

The shunt is actually a resistor with a very small resistance value. When a DC current passes through, a voltage drop is generated and supplied to the DC.

To measure a large DC current, such as tens of amps, or even a few hundred amps, there is no such range of ammeters for current measurement. What should I do? This requires a shunt. It is a short conductor, which can be of various metals or alloys, and also connected to the terminal; its DC resistance is strictly adjusted; it is connected in series in a DC circuit, and the DC current passes through the shunt, and millivolt levels are generated at both ends of the shunt The DC voltage signal swings the pointer of the meter connected in parallel to the shunt, and the reading is the current value in the DC circuit. The so-called shunting is to divide a small current to push the indication of the meter. The smaller the ratio of the small current (mA) to the current in the large circuit (1A to several tens of A), the better the linearity of the reading indicated by the ammeter is and the more accurate it is. This is a common product of electrical circuits, and there are shunt measures for lightning protection.

There are many different specifications of the ammeter, but the actual meter head is a standard millivolt voltmeter. For example, it is a voltmeter with a full scale of 75mv. Then when using this voltmeter to measure a current of 20A, for example, it needs to be equipped with a shunt resistor that generates a 75mv voltage drop when a 20A current flows, also known as a 75mv shunt.

A shunt is a 15A or 20A and 35A ammeter that can pass a large current. Generally, a shunt is required. The resistance of the shunt is the full-scale voltage of the head mark / full-scale current of the head. For example, the current shunt resistance of a 20A ammeter = 75mv / 20A = 0.00375. After the impedance is constant, according to Ohm's law U = IR, the current is proportional to the voltage, and the current is linear. The voltage is also linear, so you can use a full scale of 75mv. The voltmeter shows the current. Therefore, the ammeter used is actually a voltmeter.

Power plant excitation system shunts (8 photos)

How to measure a large AC current? Use a current transformer to change the large current to a small current of less than 5 amps with a certain transformation ratio, so a small range AC ammeter can measure the large current, but the measured current must be multiplied by that. Ratio.