Current Shunts and Calibration

Current Shunts are versatile tools used to extend the current range that can be measured by using typical ammeters or Digital Multimeters. Most Digital Multimeters on the market can only measure current up to 10 A. By using a Current Shunt, accurate measurements can typically be made to around 500 A, and by using larger specialty shunts measurements greater than 5000 A are possible.

Current Shunts are designed to be of a very low resistance, a few milliohms or less, so that the resistance will not affect the measuring circuit. When used properly a current shunt will produce a voltage output proportional to the current flowing through it.

Current Shunts are able to measure higher currents accurately and are able to be used in a wide variety of industries and applications such as in automotive systems, medical fields, energy equipment, manufacturing processes, and calibration laboratories.

Current Shunt Identification and Rating

Current Shunts are typically rated and identified by the maximum current that they can safely handle and the voltage drop at their full-scale current. For example, a 100A/100mV shunt will have the capacity to measure currents up to 100A and will have a voltage output of 100 mV at that current value. By using Ohms law where R=V/I, (the resistance is equal to the voltage divided by the current), this shunt will have a resistance of 1 mOhm. Due to the fact that their resistance is relatively stable, one of the benefits of Current Shunts is that their current/voltage relationship is very linear so that in the above example, a 25 A current will produce an output of 25 mV, a 50 A current, 50 mV and so on.

Current Shunt Connections

Most Current Shunts have connections for four wires, two larger connections for the current to flow through and two connections where the voltage drop is measured. Four wire connections are required for Current Shunts to accurately measure the voltage drop across the shunt. Because the value of the shunt resistance is extremely low, the resistance of the connected leads may be greater than that of the shunt resistance, so any measured lead resistance from the connecting wires will significantly affect the accuracy of the reading. In the four-wire configuration, the wires that carry the current and the wires that measure the voltage drop are separated, which eliminates the resulting measurement errors.

Shunt Temperature and Self Heating

One major factor to be considered when using Current Shunts is the heat generated in the shunt during the measurement. With the large current values being measured, the resistive element in the shunt can heat up quickly and will cause the output voltage to drift, causing a measurement error. During the measurement, the reading should be taken as quickly as possible to avoid this situation. Current Shunts will have a lower current threshold for operating for longer than few minutes to ensure that the heating effect will not damage the shunt. Most Current Shunts are made from a Manganin alloy which will begin to show a thermal drift after the shunt temperature reaches approximately 170°F and will become permanently damaged at around 280°F.

Current Shunt Characterization

For calibration laboratories, current measurements need to be measured with the highest accuracy possible. By using the Ohms Law and the rated values of the shunt, accuracies of approximately 0.5% of the current reading can be achieved. To attain accuracies better than this, calibration laboratories characterize the Current Shunts before each use.

Characterization involves inputting a known current value into the Current Shunt from a high precision calibrator and reading the voltage drop with a highly accurate digital multimeter. The current used is typically 10 A or 20 A depending on the range of the shunt and output of the calibrator. By doing this, the high accuracies of the calibrator and multimeter are effectively ‘transferred’ to the Current Shunt to obtain greater accuracy.

For a 100A/100mV shunt, the nominal resistance is 1 mOhm. After characterizing the Current Shunt, it may be found that the actual resistance is 0.9985 mOhms. This value is then used in the Ohms Law calculation to more accurately determine the current value. Accuracies down to 0.05% or better can be achieved depending on the tolerances of the equipment used for the characterization.

Should you be calibrating your instruments in-house or outsourced? Read our guide to find out.