chinalie: Shunt (electrical)

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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.

Contents

1 Applications

1.1 Defective device bypass

1.2 Lightning arrestor

1.3 Electrical noise bypass

1.4 Use in electronic filter circuits

1.5 Diodes as shunts

1.6 Shunts as circuit protection

1.7 Use in current measuring

1.7.1 Current measurement techniques

1.7.2 Low-side versus high-side current shunt insertion

1.7.3 Safe high-side current shunt measurements

2 See also

3 References

4 External links

//

Applications

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 [1]

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.

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).

Use in current measuring

50A shunt resistor

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 500A/75mV 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 50mV, 75mV or 100mV when operating at their full rated current and most "ammeters" are actually voltmeters with full-scale deflections of...(and so on)