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What is a Shunt Resistor?

Author: Apogeeweb
Date: 6 Sep 2021
 2919
How Does a Shunt Resistor Work

Ⅰ Introduction

A shunt is an electrical device that creates a low-resistance route for a current to flow through. This allows the current to flow to a different part of the circuit. Ammeter shunts and current shunt resistors are two terms for shunts. A shunt resistor is used to measure alternating or direct electric current. The voltage drop across the resistor is used to determine this. Shunt resistors were once used to describe a resistor connected in parallel to an ammeter as a shunt to increase the current measurement range, but in recent years, all resistors used to detect circuit current have been referred to as shunt resistors (current sense shunt resistor).

 

This vedio shows a shunt resistor

 

Catalog

Ⅰ Introduction

Ⅱ What Does a Shunt Resistor Do?

Ⅲ How Does a Shunt Resistor Work?

Ⅳ How to Measure Current by a Shunt Resistor?

Ⅴ Position of the Shunt Resistor in the Circuit When Measuring Current

Ⅵ How to Select a Shunt Resistor?

6.1 How to Calculate Shunt Resistance?

6.2 Shunt Resistor Parameters

Ⅶ How to Wire a Shunt Resistor?

Ⅷ Frequently Asked Questions about Shunt Resistor

 

Ⅱ What Does a Shunt Resistor Do?

The electrical shunt is a device that creates a low-resistance route that allows electricity to travel through or be redirected past a defined point in a circuit. Some meters have built-in precision current shunts that allow measurements in terms of DC and Watts to be taken. Electrical shunts can also be used to measure the flow of DC.

 

The formula for Ohm's law is as follows: V = I × R

 

This equation applies to the voltage (V) created across the resistance (R in ohms) as a function of the resistance and the current (I in amps) flowing through it. A current shunt with a resistance of 0.002 ohms and a current of 30 amps, for example, will generate 0.002 x 30 = 0.06 volts or 60 millivolts (milliVolts).

 

By including a current shunt into a measurement circuit, you can determine the voltage drop across the shunt. The calculation of current measurement using Ohm's law will be possible thanks to the assessment of current shunt resistance. The current shunt resistance can also be calibrated using Ohm's law.

 

Shunt resistors are commonly used in the following applications:

  • Current circulating through a battery is measured, and power output is monitored.
  • Before the signal reaches the circuit elements, high-frequency noise is redistributed (this requires a shunt with a capacitator).
  • Installation in a DC connects the container with a negative conductor connecting the batteries to the inverter.
  • Control equipment, such as battery chargers and power sources, provides overload protection.

 

Ⅲ How Does a Shunt Resistor Work?

The technological limits of a shunt resistor differ from those of a conventional resistor. Shunt resistors allow for high precision while maintaining a low ohmic value. To reach such great precision, a Kelvin connection is recommended. This connection eliminates difficulties like lead sensitivity and resistance.

 

The value of a shunt resistor can be influenced by several reversible and irreversible causes. Long-term stability and irreversible change in resistance are ensured by the accompanying mechanical, electrical, and thermal stresses. The Temperature Coefficient of Resistance (TCR) is measured in ppm/ and represents the drift caused by the transistor cooling or heating due to changes in ambient temperature. The Power Coefficient of Resistance (PCR) or ppm/W is used to express the amount of power that the resistor must dissipate.

 

Electrical shunts are commonly used to safeguard the speed controller from a load that consumes too much current or to limit the motor's speed. By disconnecting the shunt from the sense line, the controller's speed can be increased. After that, the sense line must be linked to the ground. Because there will be no voltage drop, the speed controller will generate the maximum amount of power feasible. However, if the load on the controller transistors is too great, this could be dangerous.

 

A high-precision current shunt can also be utilized for equipment bench testing. This current shunt can be used in conjunction with a voltmeter to determine the amount of current flowing through the circuit. The use of a sensitive voltmeter ensures a high level of safety in the measurement of greater currents than can be achieved with a regular multimeter.

 

Ⅳ How to Measure Current by a Shunt Resistor?

An ammeter is a device that measures electric current. The voltage drop across a precision resistor with a known resistance is measured by most modern ammeters. Ohm's law is used to calculate current flow: 

 

To measure current, most ammeters feature a built-in resistor. When the current is too high for the ammeter, however, a different configuration is required. The solution is to connect the ammeter to a precise shunt resistor in parallel. Ammeter shunt is a name that is sometimes used to describe this sort of resistor.

 

This is usually a low resistance manganin resistor with great accuracy. Only a small (known) amount of the current travels through the ammeter after it is divided between the shunt resistor and the ammeter. The remaining current travels through the shunt resistor, bypassing the ammeter. Large currents can still be measured this way. The actual amperage can be measured by accurately scaling the ammeter.

 

The greatest amperage that can be measured using this arrangement is theoretically limitless. However, the measurement device's voltage rating must not be exceeded. As a result, the maximum current multiplied by the ammeter resistance value cannot exceed the voltage rating. To minimize circuit interference, the ammeter resistance should be as low as feasible. A smaller ammeter, on the other hand, creates a smaller voltage drop, which results in a lesser resolution.

 

Example of calculation

A series resistor in an ammeter, for example, is a shunt resistor with a resistance of 1 mΩ. A voltage drop of 30 mV is observed across the resistor after it is inserted in a circuit. The current is equal to the voltage divided by the resistance in this case, or:I=V/R=0.030/0.001=30A. With the resistance value unknown and the voltage and current known, the same calculation might be performed. This is how shunt resistance is measured.

 

Ⅴ Position of the Shunt Resistor in the Circuit When Measuring Current

A.To eliminate the common-mode voltage, the shunt is frequently put on the grounded side. However, there are certain drawbacks.

B.The common-mode voltage may be too high for the ammeter in this arrangement.

 

Position of the Shunt Resistor in the Circuit

 

The placement of the shunt resistor in the circuit must be carefully considered. When the circuit and the measurement instrument share a common ground, the shunt is frequently put as close to the ground as practicable. The rationale for this is to safeguard the ammeter from excessive common-mode voltage, which could harm the instrument or cause incorrect results. One downside of this configuration is that leakage currents through the shunt may go undetected. To protect the instrument, the shunt must be isolated from the ground or incorporate a voltage divider or an isolation amplifier if it is put in the ungrounded leg. Other options include employing a Hall Effect sensor instead of directly attaching the measurement instrument to the high voltage circuit. Current shunts, on the other hand, are frequently cheaper.

 

Ⅵ How to Select a Shunt Resistor?

Shunt resistors are a type of resistor that creates a low resistance route. Because of their low resistance, they are commonly employed to detect high currents.

 

Many applications necessitate current measuring. Overcurrent protection, 4-20mA systems, battery chargers, high-brightness LED control, H-bridge motor control, and metrology, for example, all require current monitoring.

 

Shunt sensors are simpler to develop and less expensive than magnetic current sensors. They do not, however, afford any seclusion. A Rogowski coil, also known as a Hall effect sensor, is a noninvasive measurement in which the detecting circuitry is not electrically coupled to the monitored system and subsequently isolated.

 

6.1 How to Calculate Shunt Resistance?

Shunt resistors have different technological limits than normal resistors. They have a low ohmic value and are high-precision resistors (they can be expressed in microOhm when several hundreds of Amper currents must be measured). Because accuracy is crucial, current sensing is best accomplished via a Kelvin connection (or four-terminal connection), which eliminates the undesired effects of lead resistance and temperature sensitivity.

 

Four-terminal connection equation

 

A shunt resistor's value can be changed by a variety of causes, which are divided into reversible and irreversible effects. A change in resistance that is irreversible owing to mechanical, electrical, or thermal stresses is referred to as long-term stability. There are two fundamental components to reversible effects:

  • Temperature Coefficient of Resistance (TCR): TCR is measured in parts per million and describes how the resistor drifts as the ambient temperature changes.
  • The Power Coefficient of Resistance (PCR) is a unit of measurement for the amount of power a resistor must dissipate. It is given in ppm/W.

 

6.2 Shunt Resistor Parameters

The thermal EMF is an important metric for shunt resistors that isn't as critical for ordinary resistors. A voltage changeable with temperature appears at the junction of two different conducting materials (explaining why it's termed thermal EMF or thermocouple effect and expressed in µV/). An intermetallic junction's rate of change of voltage with temperature is a function of the metallic combination. Depending on whether side of the combination is regarded as the input, the voltage produced is either positive or negative. All resistors are assumed to be soldered to copper at some point, and copper becomes the reference metal. Some Thermal EMF values are shown in the table below.

 

Table 1: Thermal EMF of the Metal vs. Copper

Metal / Alloy Thermal EMF vs copper in μV/°C
Evanohm 2
Cupron -45
Manganin -3
Zeranin -1.3
Nickel -22
Gold 0.2
Silver -0.2
Aluminum -4

 

Table 2. TCR, ppm/  of various Resistor Element Materials

Temperature range -55°C to +25°C 0°C to +25°C +25°C to +60°C +25°C to +125°C
Manganin 50 10 -5 -80
Zeranin 20 ±2.5 ±5 10
Evanohm 5 2.5 -2.5 -5
Foil (Vishay proprietary) -1 -0.3 0.3 1
Thin Film -10 -5 5 10
Thick Film -100 -25 50 100

 

Manganin is the preferred material for shunts with exposed blades based on thermal EMF, TCR, and cost. Zeranin, a cousin of Manganin with a lower temperature coefficient, is used to make shunts with exposed parallel wires. Evanohm, which has a near-zero temperature coefficient and a high sensitivity to strain, is commonly used to make shunts contained in heat sinks.

 

Ⅶ How to Wire a Shunt Resistor?

First, read and follow any manufacturer's instructions. It will be required to make sure that the ammeter and the shunt can handle the same mV levels. The shunt must then be connected to the negative connection that runs from the battery bank to the electrical circuits. Following the negative lead from the battery to the circuits or fuse box will reveal this.

 

Adjust the negative connections on the battery to the corresponding side of the battery and shunt if you want to measure the current consumed by the connected device and supplied by the alternator. The other side of the shunt should be linked to the battery's negative terminal with a sufficiently thick cable.

 

The shunt resistor must be installed in a location where there is no possibility of shorting cables. The negative cables can be shortened to make the installation process easier. It is also necessary to drill a suitable hole for the ammeter to mount on the panel. The hole must be large enough to connect the meter firmly. The plus and minus pins on the connection between the leads and the DC  or voltage should be properly fitted. You must also confirm that the meter is correctly set (the current can be measured in AC, DC, ohms etc).

 

The wiring procedure should start with a simple check to confirm that the shunt is connected to the load in series. You'll also need to hook up a battery pack and make sure it's linked to the right side of the shunt. The wiring from the shunt should then be fed to the load. The ammeter and the ground should not be connected in any way. The ammeter, on the other hand, should be wired in parallel with the shunt, with the shunt connected to the load in series.

 

The powering of the circuit should be the first step in measuring the current or voltage. After that, you can take the meter reading. When measuring the level of resistance, however, you should not turn on the electricity.

 

Ⅷ Frequently Asked Questions about Shunt Resistor

1.What is the meaning of shunt resistor?

A resistor having a very low value of resistance such type of resistor is called shunt resistance. The shunt is used in the galvanometer for measuring the large current. It is connected in parallel to the circuit of the galvanometer.

 

2.Why is it called a shunt resistor?

In electronics, a shunt is a device that creates a low-resistance path for electric current, to allow it to pass around another point in the circuit. The origin of the term is in the verb 'to shunt' meaning to turn away or follow a different path.

 

3.Why a shunt resistor is connected in parallel?

A shunt resistor is connected in parallel to the galvanometer so as to keep the resistance low. Such low resistance galvanometer is used in series with the circuit to measure the strength of current through the circuit.

 

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