The role of the shunt

Photocoupler

The shunt is made according to the principle that a voltage is generated across the resistor when the DC current passes through the resistor.

The shunt is widely used to expand the meter's measuring current range. It has a fixed-valued shunt and a precision alloy resistor. It can be used for current limiting of the instrument, communication system, electronic whole machine, and automatic control power supply. . The principle is shown in the figure below, and the detected voltage is used for instruments, control circuits, etc.

Shunts for DC current measurement are available in slotted and non-slotted versions. The shunt has a manganese-nickel-copper alloy resistance bar and a copper strip and is plated with a nickel layer. Its rated voltage drop is 60mV, but can also be used as 75, 100, 120, 150 and 300mV.

Slot-type shunts are rated for the following current types: 5A, 10A, 15A, 20A and 25A

Non-slot shunts are rated for currents from 30A to 15kA.

The shunt is used to measure DC current;

The shunt is actually a resistor with a small resistance. When a DC current is passed, a voltage drop is generated for the DC current meter to display;

The DC ammeter is actually a voltmeter with a full scale value of 75mV;

The DC ammeter and the shunt are used together;

For example, the shunt resistance of the 100A ammeter is 0.00075Ω;

That is 100A × 0.00075Ω = 75mV;

The shunt resistance of the 50A ammeter is 0.0015Ω;

50A × 0.0015 Ω = 75 mV.

To measure a large DC current, such as tens of amps, or even more, hundreds of amps, we don't have a large current meter to measure the current. What should we do? This requires a shunt. The shunt is one that can pass. The high-current precision resistor, when current flows through the shunt, a millivolt-level voltage appears across it, so we measure the voltage with a millivolt voltmeter and convert this voltage into a current. Completed the measurement of 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 20A, also known as a 75mv shunt.

The shunt is a 15A or 20A and 35A ammeter that can pass the very large current. The current meter needs the shunt. The shunt impedance = the head mark full voltage / the head full current. For example, the shunt of the 20A ammeter. The resistance value is 75mv×10-3/20A=0.00375. After the impedance is constant, according to Ohm's law U=IR, the current is proportional to the voltage. The current is linear and the line voltage is also linear. So we can use a full scale of 75mv. The voltmeter shows the current. Therefore, the ammeter we use 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. Only the measured current is multiplied by that. Change ratio.

Is a short conductor, can be a variety of metals or alloys, but also connected to the terminal; its DC resistance is strictly adjusted; serially connected in the DC circuit, DC current through the shunt, the output of the shunt to the millivolt level The DC voltage signal causes the meter pointer connected to both ends of the shunt to swing, and the reading is the current value in the DC circuit. The so-called shunt, that is, a small current to push the meter indication, the smaller the ratio of the small current (mA) to the current in the large loop (1A-tens of A), the better the linearity of the current meter indicates the reading and the more accurate.

This is a common product of electrical circuits, and there are shunting measures for lightning protection.