What Is a Shunt Resistor

A shunt resistor or a current shunt resistor or an ammeter shunt resistor is a high precision resistor which can be used to measure the current flowing through a circuit. The current can be measured using shunt resistor using different techniques. These techniques are: using low side current shunt insertion which refers to the return path from the load and high side current shunt insertion which refers to the supply path to the load. These are used to measure AC or DC electrical currents by the voltage drop those currents create across the resistance.

Basic shunt resistor in the simplest case is a copper wire of the calculated length which is simply soldered in series into the circuit to be measured and the positive and negative leads of the voltmeter are connected to those joints. Calibrated shunt resistor is much more accurate in the case if it is calibrated by comparing the results it gives to those given by a precise ammeter.

An ammeter should be connected into the circuit so the true value of the current flowing through it can be measured and this reading compared to that given by the voltmeter and copper wire shunt combination.

The voltage dropped across a resistor can be known by using Ohm’s Law. Thus we can say that the voltage dropped across a resistor divided by the resistance of that resistor is equal to the current, therefore by measuring the voltage across a shunt resistor in a circuit, the current can easily be calculated.

The most common application of a shunt is measuring current where a low resistance shunt is inserted in a conductor to develop a voltage that can be read by an instrument. This voltage is proportional to the current flowing through the shunt. Almost all of the current passes through the shunt and very little through the instrument because instruments are usually designed to have high input impedance.

Current shunt resistors are usually specified with a variety of electrical and mechanical specifications. The electrical specifications indicate how close to perfect the shunt is as a current sensor. A perfect precision current shunt has exactly the same resistance. This resistance does not change with temperature, time or current. Its inductance is zero. But if a shunt resistor overheats it can permanently change the resistance of the shunt.

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