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Shunts And Panel Meters For Protecting Electronic Equipment

By: Jeremy P Stanfords

In medicine, the word shunt refers to a mechanism that diverts a process like blood flow or food passage from its usual path to another path. In electronics, shunts work in a similar manner. They are basically devices that allow current to bypass one point and travel to another point in a circuit.

Examples in Electronics
Some Christmas lights are set in a series, which is why once a bulb fails, the entire circuit goes out. To prevent this, every bulb contains a shunt that is made of a tiny wire turned several times. The purpose of this device is to conduct currents in the event that the filament fails. By producing an area of low resistance, it is able to augment the voltage the rest of the bulbs get, resulting to brighter lighting. Unfortunately, this has the effect of making the lights have a shorter shelf life.

In a lightning arrestor, gas-filled types are used. Direct lightning usually strikes tall buildings, like radio towers. If a shunt is present, it will cause the lightning to arc to the ground, thereby protecting vulnerable equipment such as transmitters and other electronic devices.

Uses in Electronic Testing
During electronic testing, circuits are tested for current, voltage and resistance. There are devices that individually measure these parameters, such as the ammeter, voltmeter and ohmmeter, but there are also devices called multimeters. Panel meters, the devices that measure, record and display electronic parameters, can be prone to damage because of being constantly subjected to the parameters they measure. When these are used in conjunction with shunts, the risk of damage is minimized.

Shunts and panel meters are combined to make shunt-resistive circuits. These circuits are used to bypass predetermined levels of current around an electronic instrument. When a shunt and a meter are not used together, excessive current flow through the instrument may cause damage to the instrument. By using a shunt resistor, this problem is avoided.

In addition, a shunt and panel meter setup can be used to extend the range of the latter. This is referred to as a meter shunt. The principle behind this is that when current is divided between two parallel shunt resistors, the range of an ammeter is increased by adding an additional parallel resistor.

A situation that exemplifies this principle is when an available meter can only read 0-1 milliampere (1 mA) but a 100mA full scale detection (FSD) is needed during electronic testing. This can be addressed by adapting a shunt. The shunt has to be able to take on the difference between the intrinsic FSD of the meter and the desired FSD. If the desired FSD is 100 mA and the meter's own FSD is only 1mA, the shunt has to be able to take the remaining 99mA of current.

Shunts have varied uses in electronics. Their main utility lies in their ability to protect equipment and enhance the capabilities of panel meters. They are indispensable for anyone who is involved in prudent electronic testing.

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