Current Shunt Resistors
Current shunt resistors are low resistance precision resistors used to measure AC or DC electrical currents by the voltage drop those currents create across the resistance. Sometimes called an ammeter shunt, it is a type of current sensor.
The "Watt's Up" and Doc Wattson digital Watt and Amp Hour meters contain built-in precision current shunts to measure DC current, Watts, Amphours, etc.
Other ammeters use an external DC current shunt (e.g., see photo), but the principal of measurement remains the same.
V = I × R
states that the Voltage (V in Volts) across a resistance (R in Ohms) is the product of the resistance and the current (I in Amps) flowing through the resistance.
For example. A current shunt whose resistance is 0.001 Ohms having a current of 50 Amps flowing through it will produce a voltage of 0.001 ×50 = 0.05 Volts or 50 mV (milliVolts).
So by inserting a current shunt into a circuit whose current you want to measure your can find the current by measuring the voltage drop across the shunt. Then knowing the resistance of the current shunt you can calculate the current using Ohm's law arranged as I = V ÷ R.
Conversely, if you know the current and the voltage produced across a current shunt you can use Ohm's law to calibrate the current shunt resistance.
Some Ammeter & Current Shunt History
In the past, an ammeter shunt meant a current shunt resistor put in parallel with the coil of an analog, galvanometer type meter. Automobiles did this. The idea was that only a tiny fraction of the current to be measured passed through the meter, the rest being "shunted" through the current shunt. The resistance of the current shunt used was chosen so the meter would read full scale for the desired maximum current in the measured circuit. Today the current shunt often carries all the current and the voltage drop across it is measured with an analog to digital converter (ADC). The measured value is then displayed as numbers on a digital display.
Current Shunt Characteristics & Specifications
Current shunt resistors are usually specified with a variety of electrical and mechanical specifications. The electrical specifications indicate how close to perfect the shunt is as a current sensor.
A perfect precision current shunt has exactly the resistance claimed. That resistance doesn't change with temperature, age or current. It's inductance (resistance to AC current change) is zero. Precision calibrated current shunts approach those ideals, but are large and very expensive.
Practical current shunts are specified in terms of:
Current Rating, for example 100 Amps,
Output Voltage which indicates the resistance as the voltage produced for the rated current. V = I × R. For example an output rating of 50 mV at 100 Amps which implies 0.0005 Ohms (0.5 mOhms) resistance.
Resistance Accuracy, for example their actual resistance is within ± 0.25% of the claimed value,
Resistance Drift, for example the shunt's resistance changes less than 0.002% (or 20 ppm = parts per million) per °C of temperature change.
The sum of the accuracy and drift errors indicate by how much the current shunt output will be incorrect compared to the specified ideal output (resistance). For our example and using a 30 °C temperature change a 0.25% + 0.002% ×30 = 0.31% error could occur. Or a 50 × 0.0031 = 0.155 mV error.
Power Rating/Derating. Because current shunts are resistors and dissipate heat from the current flowing through them, they get hot. Since that heat can change their resistance and even permanently damage the shunt, current shunts are often given a power rating or a derating factor. The heat produced is power measured in Watts (W)
W = I² × R
This says the heat produced increases with the "square" of the current. So doubling the current increases the heat power dissipated by 2² = 2 × 2 = 4 times. So small changes in current produce big changed in heating. Dealing with this heating gets complicated fast. In practice current shunts are often rated to be used continuously at only 66% of their "rated current". So if you need a continuous 80 Amp measurement you can't do it wth a typical 100 Amp ( thus 66 A continuous) rated current shunt. Alternately, a shunt may have a graph that shows how you derate its continuous current as a function of the surrounding air temperature. You might find you can use it at full current if it's 0 °C around the shunt, but at only 40% of full rating with a 90 °C ambient temperature.