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.
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 bulbs, 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.
Lightning arrestor
A
gas-filled tube can also be used as a shunt, particularly in a
lightning arrestor.
Neon and other
noble gases 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
transmitters 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
Capacitors 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
filter 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
Diodes as shunts
Where devices are especially sensitive to reverse polarity of signal or power supply, a
diode 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 circuit. 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).
===Use in current measuring===
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 milliohms, a maximum allowable current of 500 amps 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 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 transformers (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 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
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:
* The resistors 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 potentiometers and careful tweaking.
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 amplifier be designed to provide substantial gain. 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 amplifier.
Divider resistors 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 amplifiers 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.
References
External links
Java applet of using shunts or multipliers in an electric meter
Category:electrical engineering
