A shunt resistor has a very small but precise resistance value and can be used to determine the current through a load. The shunt resistor is placed in series with the load. The resistance is very small, so most of the current is passed on to the load. The current can be determined by measuring the voltage drop across the shunt resistor then using Ohm's law to calculate the current through the shunt.
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Find the maximum current that will pass through the resistor. In most cases this will be determined by the current requirements of the load.
Find the shunt resistor value once you know the maximum current. This is not quite as simple as it seems. If the resistance is too high, the voltage drop across the resistor will be too high, taking power away from the load. If the resistance is too low, the voltage across the shunt resistor will be too low, causing the measurement to be less accurate. As a general rule, avoid dropping more than 1 per cent of voltage across the shunt resistor. For example, if the maximum load current is 1A at 10 volts, no more than .1V should be dropped across the shunt resistor at maximum current. The shunt resistor value should be R = Vs/I = .1V/1A = .1 ohm, where Vs is the voltage drop across the shunt resistor.
Calculate the required power dissipation for the resistor. The shunt resistor must be able to pass the maximum current at the rated voltage without overheating or burning up. To calculate the power in watts, multiply the maximum current squared by the shunt resistance P = I x I x R. For the example in step 2, the power dissipation will be P = 1 x 1 x .1 = .1 Watt.
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