Shunt Resistor - Working & Its Applications

Shunts are commonly used to measure the current output of a circuit. The voltage drop across the extremely lower resistance value can be used to calculate current flow in high current applications.
By measuring the voltage across the shunt and knowing the shunt’s resistance value, we then can calculate the current I = V / R.
A resistor having a very low value of resistance such type of resistor is called shunt resistance.
Shunt resistors are commonly used in current measuring devices called ammeters. In an ammeter, the shunt resistance is connected in parallel. An ammeter is connected in series with a device or circuit.
Like Current Transformer ( CT) is used to measure AC similarly Shunts are used for measuring DC as high currents cannot be measured with meters or other devices directly. So we use shunts along with meters or other measuring devices to measure DC in the circuit.
It’s a low resistance which is made up of two conductor metals on the sides generally brass or copper and a Magnin strip which acts as resistance in between.
Shunts come in different shapes and sizes but the most common are as per IS1248 or DIN 43703 standards. Also, it comes in different ranges as per current requirement from 1Amp to 15000Amp and different milli volts outputs which are generally 60mV, 75mV, and 150mV.

Using a Shunt Resistor to Measure Current

If we measure the voltage across a shunt resistance, we can measure the current passing through the device by the equation of ohm’s law.
That is: I = V/R

In the above fig. Consider an ammeter that has resistance Rb and measures a very small current Ib. For exceeding the range of an ammeter, a shunt resistor Rs is placed parallel with Rm.
The total current supplied by the source is I. It is divided into two paths. According to Kirchhoff’s current law (KCL),
I= Is + Ia
Where
Is = current passes through resistance Rs (shunt current)
Ib = current passes through resistance Rb
Is= I – Ib
The shunt resister Rs is connected parallel with resistor Rb. Hence the voltage drop across both resistors is equal.

Applications of Shunt Resistor

A shunt resistor is used in an electrical circuit to protect against overvoltage.
It is used in a current measuring device.
It is used to bypass defective devices. (When several components are connected in series, one component is failed to operate, and it will break the entire circuit. In this condition, the shunt resistor bypasses the failed component and continues the circuit path.)
A shunt resistor with a capacitor is used in a circuit to avoid the problem of high-frequency noise.
It is used in overload protection control circuits, including power supplies.