What is Shunt Resistor : Working & Its Applications

In many electronics and power applications, detecting the flow of current is an important thing. So current detecting is frequently necessary for any application to monitor as well as control. Based on the application type, there are different types of current sensors available with special current sensing methods that are used to detect or measure current flow. The most frequently used solution for detecting current flow in any application is possible with a current sense resistor namely a shunt resistor. This article elaborates on shunt resistors with working and applications.

What is Shunt Resistor?

Shunt resistor definition: A device that is used to create a less resistance path to allow most of the current to flow throughout the circuit. This resistor is simply designed with a material that has less TCR which is also called the low-temperature coefficient of resistance.

These resistors are normally used in ammeters which are also known as current measuring devices. In these devices, the connection of shunt resistance can be done in parallel whereas an ammeter is simply connected to a circuit or a device in series. The shunt resistor symbol is shown below.

How to Build a Shunt Resistor?

The designing of this resistor can be done by using very thin copper wire but the size and length of this resistor mainly depend on the required resistance. The resistance of this resistor will decide the ammeter range.

If we use 2.59 mm diameter or 10 AWG-based copper wire, then the resistance of the copper wire is 0.9987 ohms for every 1000 feet. So, this resistance may change based on the quality of the copper wire. Therefore before using it, check the resistance.

To measure the required wire length for a specific shunt resistor value, the following shunt resistor formula can be used.

Length of wire = Required shunt resistance/Resistance per 1000 feet

For instance, if you need a 0.5 mΩ of shunt and use 10 AWG copper wire then substitute the above values in the above equation.

Length of wire -= 0.5/0.9987 = 0.5 feet

How Does a Shunt Resistor Work?

The working of this resistor can be done by providing a less resistant path to the flow of current. This resistor has less resistance & is connected in parallel to a current measuring device like an ammeter. This resistor measures the current by using ohm’s law when the resistance and voltage are known.

Therefore, if we measure the voltage across a resistor, we can measure the current flowing throughout the device by using the following ohm’s law equation.

I = V/R

How to Measure Current through a Shunt Resistor?

Consider an ammeter which has ‘Rm’ resistance & it can measured a small current ‘Im’ only. A shunt resistor like ‘Rs’ is connected with the ammeter in parallel for expanding their range.

In the above circuit diagram, where ‘Rs’ is shunt resistance, ‘Is’ is shunt current and ‘I’ is current within the circuit to be measured or total load.

The whole current supplied through the source is denoted with ‘I’ and is separated into two lanes. According to KCL or Kirchhoff’s current law.

I = Is + Ia

Is = I-Ia

Where,

‘Is’ is the flow of current throughout Rs resistance.

‘Ia’ is the flow of current throughout Rs resistance.

The shunt resistor like ‘Rs’ is simply connected in parallel with the ‘Rm’ resistor. So the voltage drop across these two resistors is equivalent.

Vs = Va

Is Rs = IaRa

We know that Is = I-Ia

(I-Ia)Rs = Ia Ra

IRs-IaRs = IaRa

IRs = IaRa+ IaRs

IRs = Ia (Ra+ Rs)

I/Ia = (Ra+ Rs)/ Rs

N = 1+Ra/Rs

Where ‘N’ is the ratio of the whole current (I) to be measured to the full-scale current deflection ‘Im’ is called the multiplying power of the shunt.

This resistor is used within the ammeter to measure the large current, so it is simply connected to the ammeter circuit in parallel. This ammeter is the current sensing device. So the flow of current direction within the circuit can be determined through the pointer of the ammeter.

Types of Shunt Resistor

Shunt resistors are classified into four types based on the material used carbon, ceramic, metal alloy & wire-wound shunt resistor.

• A carbon shunt resistor is simply designed with a combination of finely ground insulating and carbon material through a resin binder.
• A ceramic shunt resistor is designed with high-temperature, solid, resistive, ceramic materials through bonded metal contacts.
• Metal alloy shunt resistors include a minimum of two or above metallic elements.
• Wire wound shunt resistors are designed with thin winding wire onto a ceramic rod.

Other types of shunt resistor mainly include DC current, high power low ohmic, high power type, and isolated.

DC Current Shunt Resistor

A DC current shunt resistor is a specialized type of resistor which is mainly used to measure high currents. This resistor is connected in series with different heavy loads like a heater, DC motor, plating bath, battery charger, and smelter pot line.

This type of resistor is mainly designed to transmit a millivolt output to an instrument like a meter or other instruments that are in proportion to the flow of current throughout the shunt.

High Power Shunt Resistor

High power shunt resistors are mainly designed with a metal alloy element that is soldered to tinned copper terminals. These resistors are suitable for different applications like battery management systems, current sensing on hybrid & electric vehicles, power modules, industrial, frequency converters, voltage division, current sensing within welding equipment, and bus bars.

High Power Low Ohmic Shunt Resistor

A high-power low-ohmic shunt resistor is used for current detection in different applications through improved power requirements like industrial equipment & automotive systems. Shunt resistors within current detection-based applications are normally used to detect overcurrent conditions otherwise leftover battery levels.

Shunt Resistor Parameters

The parameters of this resistor mainly include resistance and power rating, tolerance, and temperature coefficient.

Resistance and Power Rating

For a shunt resistor, the resistor value is an essential parameter because this value will decide how much voltage will fall throughout the flow of current. So, this value can be simply determined from the perspective of the current sense amplifier. Based on the highest flow of current & the highest input voltage of this amplifier, the resistor value can be selected. But, the power rating of this resistor will effect by a larger value & enhances the heat dissipation.

In the above shown current sense resistor circuit, we can observe how a resistor is connected in a current-carrying lane & how the drop of voltage across the resistor can be measured through a current sense amplifier.

The shunt resistor’s power dissipation can be simply determined through the following formula

Power dissipation (Pd) = V x I

In the above equation, ‘V’ is the voltage & ‘I’ is the current

Since the voltage drop across a resistor can be determined through Ohm’s law., then the power dissipation of this resistor can be determined through the following formula.

Power dissipation (Pd) = I^2R

Therefore, the resistor with a smaller resistance value can generate low heat & overrule the use of an extra heat sink.

Typically the values of this resistor in multi ohm rating are normally appropriate & generate low power dissipation in the maximum flow of current.

The power dissipation of this resistor can also be recognized through the resistor’s PCR(Power Coefficient of Resistance) value & differentiates the different power dissipation values for different resistor values.

Tolerance

The tolerance of the shunt resistor is another essential parameter for the detecting accuracy of the resistor. Generally, normal resistors include different tolerances like 5%, 10%, or even 1%. So, a 10 Ohms resistor including a 10% tolerance will include the resistance value from 9 to 11 Ohms.

However, the resistor tolerances in most applications will not consider a lot but it is considered only where the output of the application is extremely dependent on the detected value; tolerance will become an essential parameter to consider.

The tolerance values of this resistor range from 1 to 0.1%, but, how much accuracy is necessary is completely dependent on the requirement of application & the resistor needs to select depending on this parameter.

Temperature Coefficient

The Temperature Coefficient or TCR of the shunt resistor is one more parameter, used to determine how much resistance will modify by changing the temperature of the resistor.

It is a very important parameter in any application of the resistor. It is not hard to discover 1000s of ppm/o Celsius rating for a normal 1% resistor, however, it needs to be extremely less for the resistor because of the maximum flow of current, the power dissipation for a resistor could be larger & that will enhance the temperature of resistor.

Because of the increased temperature of the resistor, the value of resistor value could be changed & probably provide reading including errors. The temperature coefficient value mainly depends on the resistor elements like tolerance, power & the physical size of the resistor. There are different types of shunt resistors available that use very low TCR values because in these resistors, metal-foil elements are used as the materials of the resistor.

Advantages

The advantages of a shunt resistor include the following.

• This resistor protects the circuit from overvoltage because when high current supplies through a circuit it gets damaged. Hence the maximum current throughout the resistor will activate the fuse.
• It is used to prevent noise.
• The current sensing method of this resistor is very simple to implement.
• It can be used for both AC & DC measurement.

Disadvantages

The disadvantages of a shunt resistor include the following.

• It does not need any galvanic isolation.
• A voltage drop can occur.
• Power dissipation can occur within this resistor.

Applications

The applications of a shunt resistor include the following.

• This resistor is used to measure AC or DC current by measuring the voltage drop across the resistor.
• These resistors are normally utilized in current measuring devices like ammeters.
• This resistor is always used whenever the measured current beats the measuring device range then it is connected in parallel with the measuring device
• It is used to avoid defective components in devices.
• A shunt resistor & a capacitor are used within a circuit to avoid the high-frequency noise problem.
• These resistors are mainly used in overload protection control circuits.

Thus, this is all about an overview of the shunt resistor, working, parameters, and its applications. Here is a question for you, what are the different types of shunt resistors?