# The role of the shunt

Update:2020-05-04
Summary:The shunt is made according to the principle of generating a voltage across the resistor when a DC c...

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

The shunt is widely used to expand the measuring current range of the instrument. There are fixed fixed-value shunts and precision alloy resistors, which can be used for current limiting and current-sampling detection of communication systems, electronic complete machines, and automatically controlled power supply circuits.

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

Slotted shunts have the following rated currents: 5 A, 10 A, 15 A, 20 A and 25 A; non-slotted shunts have rated currents from 30 A to 15 kA standard intervals.

Standard shunt: The shunt is mainly used for DC current detection. In fact, it is a manganese copper resistor. Before the Hall element appeared, except for the method of measuring the magnetic field strength, this method is very common. There are various structural forms, mostly in the form of a plate, or there are multiple round copper rods connected in parallel (the power is too large, consider heat dissipation), and the conductive cross section is changed (by filing or slotting) to adjust the resistance. I have used a maximum of thousands of amps (electric locomotive). Standard shunts have various current specifications, which are connected in series in the circuit, and the current flows to generate a voltage drop, usually 75 mV, and 60mV or 100mV is rarely used.

Its resistance can be calculated (1000A as an example): 75mV / 1000A = 0.000075Ω. The voltage drop of 75mV is relatively small, usually displayed with an electromagnetic ammeter (pointer). The digital display is amplified with an electronic circuit amplifier to the full range of the A / D converter.

What should I do if I want to measure a large DC current, such as tens of amps, or even larger, hundreds of amps, and not so many ammeters? This requires the use of shunts. The shunt is an accurate resistance that can pass a large current. When the current flows through the shunt, a voltage of millivolt level will appear at both ends of it, so use a millivolt voltmeter to measure this voltage, and then this voltage Converted into current. The measurement of large current is completed. Ammeters have many different specifications, but the actual meter head is a standard millivolt voltmeter. For example, a voltmeter with a full scale of 75mv. Then use this voltmeter to measure, for example, 20A current, you need to equip it with a shunt resistor that produces a 75mv voltage drop when a 20A current flows, also known as a 75mv shunt. Shunt is a resistance that can pass a large current. The commonly used 15A or 20A and 35A ammeters all require a shunt. The impedance of the shunt = meter full-scale voltage / meter full-scale current. For example, the shunt resistance of a 20A ammeter = 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. 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.