electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...

, a shunt is a device that creates a low-

resistance Resistance may refer to: Arts, entertainment, and media Comics * Either of two similarly named but otherwise unrelated comic book series, both published by Wildstorm: ** ''Resistance'' (comics), based on the video game of the same title ** ''T ...

path for

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movin ...

, to allow it to pass around another point in the

circuit Circuit may refer to: Science and technology Electrical engineering * Electrical circuit, a complete electrical network with a closed-loop giving a return path for current ** Analog circuit, uses continuous signal levels ** Balanced circu ...

.Rudolf F. Graf, ''Modern dictionary of Electronics'', Mc-Graw Hill, 1968 Library of Congress 68-13873 ''Shunt'' page 454. The origin of the term is in the verb 'to shunt' meaning to turn away or follow a different path.

Defective device bypass

One example is in miniature Christmas lights which are wired in series. When the

filament The word filament, which is descended from Latin ''filum'' meaning " thread", is used in English for a variety of thread-like structures, including: Astronomy * Galaxy filament, the largest known cosmic structures in the universe * Solar filament ...

burns out in one of the incandescent light bulbs, the full line

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge t ...

appears across the burnt out bulb. A shunt

resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias activ ...

, which has been connected

in parallel Two-terminal components and electrical networks can be connected in series or parallel. The resulting electrical network will have two terminals, and itself can participate in a series or parallel topology. Whether a two-terminal "object" is an ...

across the filament before it burnt out, will then short out to bypass the burnt filament and allow the rest of the string 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.

Photovoltaics

photovoltaics Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercial ...

, the term is widely used to describe an ''unwanted'' short circuit between the front and back surface contacts of a

solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.wafer damage.

Lightning arrester

A gas-filled tube can also be used as a shunt, particularly in a lightning arrester. Neon and other

noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low che ...

es have a high breakdown voltage, so that normally current will not flow across it. However, a direct

lightning Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions, both in the atmosphere or with one on the ground, temporarily neutralize themselves, causing the instantaneous release of an average ...

strike (such as on a radio tower antenna) will cause the shunt to arc and conduct the massive amount of electricity to

ground Ground may refer to: Geology * Land, the surface of the Earth not covered by water * Soil, a mixture of clay, sand and organic matter present on the surface of the Earth Electricity * Ground (electricity), the reference point in an electrical c ...

, protecting

transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to ...

s and other equipment. Another older form of lightning arrester 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

Capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of a ...

s are 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 components connected between the signal and return lines from the components connected in series along the signal 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 vulnerable to reverse polarity of a 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. All semiconductor diodes have a threshold voltage – typically between volt and 1 volt – that must be exceeded before significant current will flow through the diode in the normally allowed direction. Two anti-parallel shunt diodes (one to conduct current in each direction) can be used to limit the signal flowing past them to no more than their threshold voltages, in order to protect later components from overload.

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 A crowbar, also called a wrecking bar, pry bar or prybar, pinch-bar, or occasionally a prise bar or prisebar, colloquially, in Britain and Australia sometimes called a jemmy or jimmy (also called jemmy bar), gooseneck, or pig foot, is a tool ...

across a set of

bus bar In electric power distribution, a busbar (also bus bar) is a metallic strip or bar, typically housed inside switchgear, panel boards, and busway enclosures for local high current power distribution. They are also used to connect high volt ...

s (exposed electrical conductors).

Battle short

On warships, it is common to install battle short shunts across fuses for essential equipment before entering combat. This disables overcurrent protection at a time when removing power to the equipment is not a safe reaction.

Shunting an instrument but series connected in circuit

Ac src meters with meter-shunts speaker with load-shunt rev2

As an introduction to the next chapter, this figure shows that the term "shunt resistor" should be understood in the context of what it shunts. In this example the resistor R_L would be understood as "the shunt resistor" (to the load L), because this resistor would pass current around the load L. R_L is connected in parallel with the load L. However, the series resistors R_M1 and R_M2 are low Ohmic resistors (like in the photo) meant to pass current around the instruments M1 and M2, and function as shunt resistors to those instruments. R_M1 and R_M2 are connected in parallel with M1 and M2. If seen without the instruments these two resistors would be considered series resistors in this circuit.

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 separate shunt, a

of very low but accurately known

, is placed in parallel with a voltmeter, so that virtually all of the current to be measured will flow through the shunt (provided the internal resistance of the voltmeter takes such a low portion of the current that it is negligible). The resistance is chosen so that the resultant voltage drop is measurable but low enough not to disrupt the

. The voltage across the shunt is proportional to the current flowing through it, and so the measured voltage can be scaled to directly display the current value.''Manual of Electric Instruments'', General Electric, 1949, pages 8–9Terrell Croft, ''American Electricians' Handbook'', McGraw-Hill, 1948 p. 70 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 , 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 (more than 2 minutes) use, 66% being the most common, so the example shunt should not be operated above 330 A (and 50 mV drop) longer than that. This limitation is due to thermal limits at which a shunt will no longer operate correctly. For manganin, a common shunt material, 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 too and in the reading instrument itself. Sometimes, the shunt is inserted in the return leg (

grounded Grounding or grounded may refer to: Science and philosophy * Grounding (metaphysics), a topic of wide philosophical interest * Grounding (psychology), a strategy for coping with stress or other negative emotions * Grounding in communication, th ...

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 specifications of such devices are ±0.1%, ±0.25% or ±0.5%. The Thomas-type double manganin walled shunt and MI type (improved Thomas-type design) were used by NIST and other standards laboratories as the legal reference of an ohm until superseded in 1990 by the quantum Hall effect. Thomas-type shunts are still used as secondary standards 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. Where the circuit is grounded (earthed) on one side, a current measuring shunt can be inserted either in the ungrounded conductor or in the grounded conductor. A shunt in the ungrounded conductor must be insulated for the full circuit voltage to ground; the measuring instrument must be inherently isolated from ground or must include a resistive voltage divider or an isolation amplifier between the relatively high common-mode voltage and lower voltages inside the instrument. A shunt in the grounded conductor may not detect leakage current that bypasses the shunt, but it will not experience high common-mode voltage to ground. The load is removed from a direct path to ground, which may create problems for control circuitry, result in unwanted emissions, or both. Image:Low side current shunt.svg, Low-side insertion can eliminate common-mode voltage, but not without drawbacks. Image:High side current shunt diagram.svg, High-side insertion resolves low-side drawbacks but guarantees common-mode voltage. Image:isolation amp current shunt.gif, Isolated amplifiers resolve all the difficulties and limitations with high- or low-side current shunt measurements.

References

External links

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