Shunt (electrical)

In electricity, a shunt is a device which allows electrical current to pass around another point in the circuit.

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 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 an AC detector 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 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.

Diodes as shunts

Where devices are especialy sensitive to reverse polarity of signal or power supply a 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 in current measuring

A shunt can also be used to measure current. In this case a resistor of accurately-known resistance, the shunt, is placed in series so that all the current you wish to measure will flow through it. Since the resistance is known, by measuring the voltage drop across it, you can calculate the current flowing. 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, eg a 500A/50mV shunt would have a maximum allowable current of 500 amps and at that current the voltage drop would be 50 millivolts. If the current being measured is also at a high voltage potential this voltage will be present in the enclosure containing the reading instrument, a disadvantage solved by the use of Hall effect devices for current measuring.

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 buss bars (exposed electrical conductors).

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