A shunt has a low resistive value and can be used to measure the current – it is therefore also referred to as a current sense resistor. It is always employed when the measured current exceeds the range of the measuring device. The shunt is then connected in parallel to the measuring device. The total current flows through the shunt and generates a voltage drop that is measured. Using Ohm’s law and the known resistance, this can then be used to calculate the current (I = V/R). To keep power loss – and thus heat development – to a minimum, shunts must have a very low resistive value in the milliohm range, some are even below that.
The advantage of this measuring method is that faults can be detected and eliminated quickly. These shunts are therefore particularly interesting for safety-relevant applications where faults need to be detected. Furthermore, they deliver precise measuring results and thus enable, e.g., the efficient control of drives or the monitoring of battery management systems. At the same time, shunt resistors are excellent value for money.
Shunts are basically suitable for any type of measuring application – be it with a direct or alternating current. Shunts are currently experiencing a real boom, especially due to the increasing number of condition measurements in cars, e.g. engine and battery management, airbag control unit, ABS, and further safety, locking, and infotainment systems. Current sense resistors are also becoming ever more popular in industrial applications, medical technology and for regenerative energies and smart metering.
Various technologies are on offer
Shunts are available as metal layer technology and full metal versions. Layer resistors are considerably less expensive but have a poorer comparative temperature coefficient. There is also a further, design-related disadvantage: With metal layer resistors, a paste is applied to a ceramic substrate and adjusted to the desired value using laser trimming. This results in an inhomogeneous structure that is trimmed to the rated value as a meandering shape. Along with the already existing parasitic inductances, this causes a series inductance, thus invalidating Ohm’s law, in its purest form, and falsifying the result of the current measurement. The voltage drop at the shunt then takes place according to the formula U = I x R – L(di/dt). Metal layer resistors are therefore only worth considering if inductance is unimportant.
Full-metal shunt resistors consist of a homogeneous resistive element so that no additional inductance develops and a consistent and correct measuring result is achieved. This is key to high precision applications, for instance in medical technology, or precision measuring devices. Furthermore, they are characterized by high measuring accuracy and resistance to thermal shock. They are available in various sizes – including ones that are much larger than for standard chip resistors – and TK-values way below 100ppm/K. Full-metal resistors can be operated with an output of up to 7W at maximum temperatures of 275°C. Resistive values up to the low single-digit milliohm range can be selected.
The perfect resistive value can be determined quite easily: The lowest measuring voltage that still achieves sufficiently accurate results is divided by the lowest current value of the measuring range.
There is a trend towards smaller sizes with higher outputs; and customer-specific versions in terms of connection geometry and shunt size are also increasingly sought after. Whether they are the preferred choice to standard models depends on the respective application. As shunt resistors are relatively expensive compared to other resistor technologies, they are already available in small batch sizes and test samples.
4-wire shunts
The 4-wire version is one option. Here, the current flows through two connections and the voltage is measured at the other two. The voltage drop at the resistors can be determined using the internal Kelvin connections, so that the resulting measuring errors can be eliminated.
4-wire shunts are used in two scenarios: Firstly, when the line and contact resistance are relatively high and, in contrast to the measured resistance, not negligible. Secondly, when the resistive value is smaller than 10mR. Since the resistive values of the conductors are also in the milliohm range and must thus be incorporated.
Author: Bert Weiss, Technical Support Resistors
Rutronik | www.rutronik.com
