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Shunt (electrical)
Encyclopedia
In electronics
, a shunt is a device which allows electric current
to pass around another point in the circuit
. The term is also widely used in photovoltaics
to describe an unwanted short circuit between the front and back surface contacts of a solar cell
, usually caused by wafer
damage.
s, the electrical resistance
becomes very high. The much higher voltage
that this creates (equal to the full line voltage rather than the normal voltage divider level) causes the shunt to short out (becoming an antifuse
) and become part of the circuit, again allowing electricity to pass and the set to light. If too many lights burn out however, a shunt will also burn out, requiring the use of a multimeter
to find the point of failure.
-filled tube can also be used as a shunt, particularly in a lightning arrestor. Neon
and other noble gas
es have a high breakdown voltage
, so that normally current will not flow across it. However, a direct lightning
strike (such as on a radio tower
antenna
) will cause the shunt to arc and conduct the massive amount of electricity to ground
, protecting transmitter
s and other equipment.
Another, older form of lightning arrestor employs a simple narrow spark gap, over which an arc will jump when a high voltage is present. While this is a low cost solution, its high triggering voltage offers almost no protection for modern solid-state electronic devices powered by the protected circuit.
s are sometimes used as shunts to redirect high-frequency noise to ground before it can propagate to the load or other circuit components.
and similar circuits with a ladder topology to refer to the components connected between the line and common. The term is used in this context to distinguish the shunt connected components from the series connected components in series with the line. More generally, the term shunt can be used for a component connected in parallel with another. For instance, shunt m-derived half section is a common filter section from the image impedance
method of filter design
may be used to protect the circuit. If connected in series with the circuit it simply prevents reversed current, but if connected in parallel it can shunt the reversed supply, causing a fuse or other current limiting circuit to open.
. When this device detects an overvoltage it causes a short circuit between the power supply and its return. This will cause both an immediate drop in voltage (protecting the device) and an instantaneous high current which is expected to open a current sensitive device (such as a fuse
or circuit breaker
). This device is called a crowbar as it is likened to dropping an actual crowbar
across a set of bus bars (exposed electrical conductors).
An ammeter
shunt allows the measurement of current values too large to be directly measured by a particular ammeter. In this case the shunt, a manganin
resistor
of accurately known resistance
, is placed in series with the load so that all of the current to be measured will flow through it. The voltage
drop across the shunt is proportional to the current flowing through it and since its resistance is known, a millivoltmeter
connected across the shunt can be scaled to directly display the current value.
In order not to disrupt the circuit, the resistance of the shunt is normally very small. Shunts are rated by maximum current and voltage drop at that current, for example, a 500 A, 75 mV shunt would have a resistance of 0.15 milliohm
s, a maximum allowable current of 500 amp
s and at that current the voltage
drop would be 75 millivolts. By convention, most shunts are designed to drop 50 mV, 75 mV or 100 mV when operating at their full rated current and most ammeters consist of a shunt and a voltmeter with full-scale deflections of 50, 75, or 100 mV. All shunts have a derating factor for continuous use, 66% being the most common. Continuous use is a run time of 2+ minutes, at which point the derating factor must be applied. There are thermal limits where a shunt will no longer operate correctly. At 80 °C thermal drift begins to occur, at 120 °C thermal drift is a significant problem where error, depending on the design of the shunt, can be several percent and at 140 °C the manganin alloy becomes permanently damaged due to annealing resulting in the resistance value drifting up or down.
If the current being measured is also at a high voltage potential this voltage will be present in the connecting leads to and in the reading instrument itself. Sometimes, the shunt is inserted in the return leg (grounded
side) to avoid this problem. Some alternatives to shunts can provide isolation from the high voltage by not directly connecting the meter to the high voltage circuit. Examples of devices that can provide this isolation are Hall effect
current sensors and current transformer
s (see clamp meters). Current Shunts are considered more accurate and cheaper than Hall effect devices. Common Accuracy is ±0.1%, 0.25% in North America and 0.5% in the rest of the World.
The Thomas Type Double Manganin Walled Shunt and MI Type (improved Thomas Type Design) were used until the 1990s by NIST and other Government labs as the legal reference of an ohm until the advent of the Quantum Hall Effect. Thomas Type shunts are still used by Government and private labs to take very accurate current measurements, as using Quantum Hall Effect is a time consuming process. The accuracy of these types of shunts is measured in the ppm and sub-ppm scale of drift per year of set resistance.
The primary difference between low- and high-side current shunt placements is that the former can eliminate common mode voltage, which appears simultaneously and in phase on either side of the current shunt. Since the presence of common mode voltage can create complications for the instrument
used to measure shunt voltage, low-side current shunt insertion is often recommended, especially in high voltage situations. However, the low-side approach is not without drawbacks, which include the following:
A current shunt placed in the high-side of a load resolves most of these problems, but common mode voltage is virtually guaranteed to be present with the high-side approach and will complicate the instrument used to make the measurement as a result. Failure to recognize this and make appropriate instrumentation adjustments, especially in high voltage applications, can have dire consequences that include explosive destruction of the instrument, and potential injury to nearby personnel. Novice technicians who have been victimized by this fiery event often lament that they attempted to measure only a 50 mV current shunt signal. Of course, they completely overlooked that the millivolt signal was riding on top of a destructive common mode component.
Two techniques are used to safely measure high-side currents using shunts in high potential applications. The lowest cost, and least desirable option, is to apply a voltage divider to each input of a differential amplifier
. The divider is sized to reduce the magnitude of the common mode voltage to within the range of the amplifier. This is usually ±15 V to ±30 V, but the actual specification can vary widely as a function of the amplifier being used. With the common mode voltage reduced to a manageable level, the amplifier’s difference capacity can be used to extract the shunt voltage within the limits of the amplifier’s common mode rejection specification. However, the voltage divider approach suffers from several serious flaws:
These and other undesirable characteristics of the voltage divider approach to high-side current shunt measurements conspire to force its use in only the most cost-sensitive situations and where accuracy is not a consideration. The second high-side technique, isolated amplifiers, remains the best alternative for both high- and low-side current shunt measurements.
Isolation amplifier
s feature an electrically floating front end that allows it to rise or fall in response to the magnitude of the applied common mode voltage. As a result, the amplifier’s input and output ground references are free to remain at completely independent potentials. The breakdown voltage of the isolation barrier defines the common mode voltage magnitude that may be tolerated, but values as high as ±1,000 V are not unusual. Amplifiers with isolation have historically been more expensive than alternatives, but time and innovation have reduced their price to such affordable levels that they should be seriously, if not exclusively considered as an instrumentation solution for any high voltage current shunt application.
Holt Instrument
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
, a shunt is a device which allows electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
to pass around another point in the circuit
Electrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
. The term is also widely used in photovoltaics
Photovoltaics
Photovoltaics is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material...
to describe an unwanted short circuit between the front and back surface contacts of a solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
, usually caused by wafer
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
damage.
Defective device bypass
One example is in miniature Christmas lights which are wired in series. When the filament burns out in one of the incandescent light bulbIncandescent light bulb
The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...
s, the electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
becomes very high. The much higher voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
that this creates (equal to the full line voltage rather than the normal voltage divider level) causes the shunt to short out (becoming an antifuse
Antifuse
An antifuse is an electrical device that performs the opposite function to a fuse. Whereas a fuse starts with a low resistance and is designed to permanently break an electrically conductive path , an antifuse starts with a high resistance and is designed to permanently create an electrically...
) and become part of the circuit, again allowing electricity to pass and the set to light. If too many lights burn out however, a shunt will also burn out, requiring the use of a multimeter
Multimeter
A multimeter or a multitester, also known as a VOM , is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance...
to find the point of failure.
Lightning arrestor
A gasGas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
-filled tube can also be used as a shunt, particularly in a lightning arrestor. Neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
and other noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
es have a high breakdown voltage
Breakdown voltage
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse...
, so that normally current will not flow across it. However, a direct lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
strike (such as on a radio tower
Radio masts and towers
Radio masts and towers are, typically, tall structures designed to support antennas for telecommunications and broadcasting, including television. They are among the tallest man-made structures...
antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
) will cause the shunt to arc and conduct the massive amount of electricity to ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
, protecting transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
s and other equipment.
Another, older form of lightning arrestor employs a simple narrow spark gap, over which an arc will jump when a high voltage is present. While this is a low cost solution, its high triggering voltage offers almost no protection for modern solid-state electronic devices powered by the protected circuit.
Electrical noise bypass
CapacitorCapacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s are sometimes used as shunts to redirect high-frequency noise to ground before it can propagate to the load or other circuit components.
Use in electronic filter circuits
The term shunt is used in filterElectronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
and similar circuits with a ladder topology to refer to the components connected between the line and common. The term is used in this context to distinguish the shunt connected components from the series connected components in series with the line. More generally, the term shunt can be used for a component connected in parallel with another. For instance, shunt m-derived half section is a common filter section from the image impedance
Image impedance
Image impedance is a concept used in electronic network design and analysis and most especially in filter design. The term image impedance applies to the impedance seen looking in to the ports of a network. Usually a two-port network is implied but the concept is capable of being extended to...
method of filter design
Diodes as shunts
Where devices are especially sensitive to reverse polarity of signal or power supply, a diodeDiode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
may be used to protect the circuit. If connected in series with the circuit it simply prevents reversed current, but if connected in parallel it can shunt the reversed supply, causing a fuse or other current limiting circuit to open.
Shunts as circuit protection
When a circuit must be protected from overvoltage and there are failure modes in the power supply that can produce such overvoltages, the circuit may be protected by a device commonly called a crowbar circuitCrowbar (circuit)
A crowbar circuit is an electrical circuit used to prevent an overvoltage condition of a power supply unit from damaging the circuits attached to the power supply. It operates by putting a short circuit or low resistance path across the voltage source, much as if one dropped a tool of the same name...
. When this device detects an overvoltage it causes a short circuit between the power supply and its return. This will cause both an immediate drop in voltage (protecting the device) and an instantaneous high current which is expected to open a current sensitive device (such as a fuse
Fuse (electrical)
In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...
or circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
). This device is called a crowbar as it is likened to dropping an actual crowbar
Crowbar (tool)
A crowbar, a wrecking bar, pry bar, or prybar, or sometimes a prise bar or prisebar, and more informally a jimmy, jimmy bar, jemmy or gooseneck is a tool consisting of a metal bar with a single curved end and flattened points, often with a small fissure on one or both ends for removing nails...
across a set of bus bars (exposed electrical conductors).
Use in current measuring
Ammeter
An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes , hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters...
shunt allows the measurement of current values too large to be directly measured by a particular ammeter. In this case the shunt, a manganin
Manganin
Manganin is a trademarked name for an alloy of typically 86% copper, 12% manganese, and 2% nickel. It was first developed by Edward Weston in 1892, improving upon his Constantan ....
resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
of accurately known resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
, is placed in series with the load so that all of the current to be measured will flow through it. The voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
drop across the shunt is proportional to the current flowing through it and since its resistance is known, a millivoltmeter
Voltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...
connected across the shunt can be scaled to directly display the current value.
In order not to disrupt the circuit, the resistance of the shunt is normally very small. Shunts are rated by maximum current and voltage drop at that current, for example, a 500 A, 75 mV shunt would have a resistance of 0.15 milliohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
s, a maximum allowable current of 500 amp
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s and at that current the voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
drop would be 75 millivolts. By convention, most shunts are designed to drop 50 mV, 75 mV or 100 mV when operating at their full rated current and most ammeters consist of a shunt and a voltmeter with full-scale deflections of 50, 75, or 100 mV. All shunts have a derating factor for continuous use, 66% being the most common. Continuous use is a run time of 2+ minutes, at which point the derating factor must be applied. There are thermal limits where a shunt will no longer operate correctly. At 80 °C thermal drift begins to occur, at 120 °C thermal drift is a significant problem where error, depending on the design of the shunt, can be several percent and at 140 °C the manganin alloy becomes permanently damaged due to annealing resulting in the resistance value drifting up or down.
If the current being measured is also at a high voltage potential this voltage will be present in the connecting leads to and in the reading instrument itself. Sometimes, the shunt is inserted in the return leg (grounded
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
side) to avoid this problem. Some alternatives to shunts can provide isolation from the high voltage by not directly connecting the meter to the high voltage circuit. Examples of devices that can provide this isolation are Hall effect
Hall effect
The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...
current sensors and current transformer
Current transformer
In electrical engineering, a current transformer is used for measurement of electric currents. Current transformers, together with voltage transformers , are known as instrument transformers...
s (see clamp meters). Current Shunts are considered more accurate and cheaper than Hall effect devices. Common Accuracy is ±0.1%, 0.25% in North America and 0.5% in the rest of the World.
The Thomas Type Double Manganin Walled Shunt and MI Type (improved Thomas Type Design) were used until the 1990s by NIST and other Government labs as the legal reference of an ohm until the advent of the Quantum Hall Effect. Thomas Type shunts are still used by Government and private labs to take very accurate current measurements, as using Quantum Hall Effect is a time consuming process. The accuracy of these types of shunts is measured in the ppm and sub-ppm scale of drift per year of set resistance.
Current measurement techniques
Low-side versus high-side current shunt insertion
In this discussion, low-side refers to the return path from the load. High-side refers to the supply path to the load. The low-side is usually at a low voltage to ground; similarly, the high-side is usually at a high voltage to ground. The decision to place a current shunt in either position has advantages and disadvantages that must be accounted for and assessed based on the particular application.The primary difference between low- and high-side current shunt placements is that the former can eliminate common mode voltage, which appears simultaneously and in phase on either side of the current shunt. Since the presence of common mode voltage can create complications for the instrument
Measuring instrument
In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item...
used to measure shunt voltage, low-side current shunt insertion is often recommended, especially in high voltage situations. However, the low-side approach is not without drawbacks, which include the following:
- The load is removed from a direct path to ground, which may create problems for control circuitry, result in unwanted emissions, or both.
- Only current directly returned to the supply by the load is measured. Current leaking to ground through the load's chassis, control circuitry, cabling, etc. are not measured, which can lead to faulty diagnostic results.
- The only beneficial reasons to use a low-side shunt, the assumption that common mode voltages will be zero, may not prevail in applications where high in-rush currents create ground bounce. This effect is essentially a momentary potential difference (or common mode voltage) between the grounded side of the current shunt and the measuring instrument’s ground. This event may disrupt the measurement and even damage the instrument given sufficient common mode potential.
A current shunt placed in the high-side of a load resolves most of these problems, but common mode voltage is virtually guaranteed to be present with the high-side approach and will complicate the instrument used to make the measurement as a result. Failure to recognize this and make appropriate instrumentation adjustments, especially in high voltage applications, can have dire consequences that include explosive destruction of the instrument, and potential injury to nearby personnel. Novice technicians who have been victimized by this fiery event often lament that they attempted to measure only a 50 mV current shunt signal. Of course, they completely overlooked that the millivolt signal was riding on top of a destructive common mode component.
Safe high-side current shunt measurements
Differential amplifier
A differential amplifier is a type of electronic amplifier that amplifies the difference between two voltages but does not amplify the particular voltages.- Theory :Many electronic devices use differential amplifiers internally....
. The divider is sized to reduce the magnitude of the common mode voltage to within the range of the amplifier. This is usually ±15 V to ±30 V, but the actual specification can vary widely as a function of the amplifier being used. With the common mode voltage reduced to a manageable level, the amplifier’s difference capacity can be used to extract the shunt voltage within the limits of the amplifier’s common mode rejection specification. However, the voltage divider approach suffers from several serious flaws:
- The resistorResistorA linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s that make up the divider must be almost perfectly matched to avoid unbalancing the amplifier, which would result in accuracy-destroying offsets. Such tolerances are only obtained through the use of high precision resistors, or by the application of trim potentiometerPotentiometerA potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...
s and careful adjustment. - The low-level of the current shunt is divided by the same amount as the high-level common mode voltage, which requires that the differential amplifierDifferential amplifierA differential amplifier is a type of electronic amplifier that amplifies the difference between two voltages but does not amplify the particular voltages.- Theory :Many electronic devices use differential amplifiers internally....
be designed to provide substantial gainGainIn electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
. This usually leads to a noisy representation of the current signal. - The divider increases source resistance, which may complicate the design if it competes with the input resistance of the differential amplifierDifferential amplifierA differential amplifier is a type of electronic amplifier that amplifies the difference between two voltages but does not amplify the particular voltages.- Theory :Many electronic devices use differential amplifiers internally....
. - Divider resistorResistorA linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s with adequate power ratings may be difficult to locate and implement for higher common mode voltages.
These and other undesirable characteristics of the voltage divider approach to high-side current shunt measurements conspire to force its use in only the most cost-sensitive situations and where accuracy is not a consideration. The second high-side technique, isolated amplifiers, remains the best alternative for both high- and low-side current shunt measurements.
Isolation amplifier
Isolation amplifier
Isolation amplifiers provide electrical isolation and an electrical safety barrier. They protect data acquisition components from common mode voltages, which are potential differences between instrument ground and signal ground...
s feature an electrically floating front end that allows it to rise or fall in response to the magnitude of the applied common mode voltage. As a result, the amplifier’s input and output ground references are free to remain at completely independent potentials. The breakdown voltage of the isolation barrier defines the common mode voltage magnitude that may be tolerated, but values as high as ±1,000 V are not unusual. Amplifiers with isolation have historically been more expensive than alternatives, but time and innovation have reduced their price to such affordable levels that they should be seriously, if not exclusively considered as an instrumentation solution for any high voltage current shunt application.
External links
Holt Instrument