Why do I need a Shunt Resistor?

It's all about energy and how to measure the flow of electric current. How much money can I save? But first, let's talk a bit about just what a Shunt Resistor is and what does it have to do with measuring the flow of electric current.

    Definition shunt resistor

A shunt resistor is used to measure electric current, alternating or direct. This is done by measuring the voltage drop across the resistor.

Shunt resistor for current measuring

A device to measure electric current is called an ammeter. Most modern ammeters measure the voltage drop over a precision resistor with a known resistance. The current flow is calculated by using Ohm’s law:

Most ammeters have an inbuilt resistor to measure the current. However, when the current is too high for the ammeter, a different setup is required. The solution is to place the ammeter in parallel with an accurate shunt resistor.  Another term that is sometimes used for this type of resistor is ammeter shunt.

Usually this is a high precision manganin resistor with a low resistance value. The current is divided over the shunt and the ammeter, such that only a small (known) percentage flows through the ammeter.  In this way, large currents can still be measured. By correctly scaling the ammeter, the actual amperage can be directly measured. Using this configuration, in theory the maximum amperage that can be measured is endless. However, the voltage rating of the measurement device must not be exceeded. This means that the maximum current multiplied by the resistance value, cannot be higher than the voltage rating. Also, the resistance value should be as low as possible to limit the interference with the circuit. On the contrary, the resolution gets smaller the smaller the resistance and thus the voltage drop is.

Position of the shunt in the circuit for current measuring

A. Often the shunt is placed in the grounded side to eliminate the common mode voltage. However, other disadvantages exist.
B. In this configuration, the common mode voltage could be too high for the ammeter.

It is important to carefully choose the position of the shunt in the circuit. When the circuit shares a common ground with the measurement device, often is chosen to place the shunt as close to the ground as possible. The reason is to protect the ammeter from the common mode voltage that might be too high and damage the device or give erroneous results. A disadvantage from this set up is that leakages that bypass the shunt might not be detected. In case the shunt is placed in the ungrounded leg, it must be isolated from the ground or include a voltage divider or an isolation amplifier to protect the instrument. Other ways are possible to not connect the measurement instrument directly with the high voltage circuit, such as using the Hall Effect. However, current shunts are commonly more affordable and cheaper.

Specifying a shunt resistor

Several parameters are important to specify a shunt resistor. Shunt resistors have a maximum current rating. The resistance value is given by the voltage drop at the maximum current rating. For example, a shunt resistor rated with 100A and 50mV has a resistance of 50 / 100 = 0.5 mOhm. The voltage drop at maximum current is typically rated 50, 75 or 100 mV.

Other important parameters include the resistance tolerance, the temperature coefficient of resistance and the power rating. The power rating indicates the amount of electric power that the resistor can dissipate at a given ambient temperature without damaging or changing the resistor parameters. The produced power can be calculated with Joules law. Shunt resistors have usually a derating factor of 66 percent for continuous operation. This is defined for a run time longer than two minutes. High temperatures negatively influence the accuracy of the shunt. From 80 degrees Celsius thermal drift starts. This gets worse with rising temperature, and from 140 degrees the resistor will damage and the resistance value be permanently changed.

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Reference: ResistorGuide.com