Shunt (electrical)
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In electronics, a shunt is a device which allows electrical 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.
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[edit] Applications
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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 [1]
[edit] Diodes as shunts
Where devices are especially sensitive to reverse polarity of signal or power supply, a Zener diode may be used to protect the circuit. If on the power supply this may in turn cause a fuse or other current limiting circuit to open.
[edit] 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 a metal tool called a crowbar across a set of bus bars (exposed electrical conductors).
[edit] 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 a manganin resistor of accurately-known resistance, the shunt, is placed in series with the load so that nearly 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 millivolt meter connected across the shunt can be scaled to directly read 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 75 mV when operating at their full rated current and most "ammeters" are actually designed as voltmeters that reach full-scale deflection at 75 mV.
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).
[edit] Current measurement techniques
[edit] Low-side versus high-side current shunt insertion
In this discussion low-side refers to the return path of the load. High-side refers to the supply path of the load. 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:
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- The load is removed from a direct path to ground, which may create problems for control circuitry, result in unwanted emissions, or both.
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- 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.
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- 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 50mV current shunt signal. Of course, they completely overlooked that the millivolt signal was riding on top of a destructive common mode component.
[edit] 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 ±15V to ±30V, 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:
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- 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.
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- 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.
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- The divider increases source resistance, which may complicate the design if it competes with the input resistance of the differential amplifier.
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- 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 ±1,000V 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.
[edit] See also
[edit] References
- ^ Mathaei, Young, Jones Microwave Filters, Impedance-Matching Networks, and Coupling Structures, p66, McGraw-Hill 1964
