Protecting a circuit against overvoltage
A method to protect a circuit from a too high voltage is using a crowbar circuit. When the voltage gets too high, a device will short circuit. This results in the current flowing parallel to the circuit. This causes immediately a voltage drop in the circuit. The high current through the shunt should trigger a circuit breaker or a fuse.
Bypassing a defective device
When one element in a series circuit fails, it will break the complete circuit. A shunt can be used to overcome this problem. The higher voltage that exists due to the failure will cause the shunt to short out. The electricity will pass around the defective element. A good example of this is Christmas lighting.
Bypass electrical noise
Shunts with a capacitor are sometimes applied in circuits where high-frequency noise is a problem. Before the undesired signal reaches the circuit elements, the capacitor redirect the noise to the ground.
Several parameters are important to specify a shunt resistor. Shunt resistors have a maximum current rating. The resistance value is given by the voltage drop at the maximum current rating. For example, a shunt resistor rated with 100A and 50mV has a resistance of 50 / 100 = 0.5 mOhm. The voltage drop at maximum current is typically rated 50, 75 or 100 mV. Microhm Electronics' shunt resistors gain bigger market share this year, especially for FL-2 series.
Other important parameters include the resistance tolerance, the temperature coefficient of resistance and the power rating. The power rating indicates the amount of electric power that the resistor can dissipate at a given ambient temperature without damaging or changing the resistor parameters. The produced power can be calculated with Joules law. Shunt resistors have usually a derating factor of 66 percent for continuous operation. This is defined for a run time longer than two minutes. High temperatures negatively influence the accuracy of the shunt. From 80 degrees Celsius thermal drift starts. This gets worse with rising temperature, and from 140 degrees the resistor will damage and the resistance value be permanently changed.
