How is current measured using shunts?

07/19/2018 Know-How

The use of shunts for measuring current is a simpler and more cost-effective alternative to the use of sensors for current measurement. However, they are precision measuring devices, and a few things need to be considered to achieve measurements of the necessary precision.

How does a shunt work?

A shunt is a low-ohm resistor that can be used to measure current. Shunts are 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 entire current flows through the shunt and generates a voltage drop, which is then is measured. Using Ohm's law and the known resistance, this measurement can then be used to calculate the current (I = V/R). To keep power loss - and thus heat generation - to a minimum, shunts must have a very low resistance value measurable in milliohms.

Shunts are basically suitable for any type of current measurement - be it with a direct or alternating current.

Advantages of shunts for current measurement:

  • Faults can be quickly detected and eliminated, making shunts particularly interesting for safety-related applications where faults need to be detected.
  • They also provide precise measuring results enabling for instance drives to be efficiently controlled or battery management systems to be monitored.
  • Shunts offer excellent value for money.

Which shunts are there and which are suitable for current measurement?

Shunts are available as metal film and full metal versions.

Advantages and disadvantages of metal film resistors:

Pro: They are noticeably cheaper

Contra: Their temperature coefficient is inferior to full-metal shunts

Contra: Current measurements are distorted slightly by the nature of the construction, which is why they are only an option when induction is not a factor. With metal film resistors (shunts), a paste is applied to a ceramic substrate and adjusted to the desired value using laser trimming. This creates a non-homogeneous structure that causes serial inductance in addition to the existing parasitic inductance. As a result, Ohm's law in its basic form no longer applies, which distorts the result of the current measurement. The formula for the voltage drop at the shunt in this case is: U = I x R - L(di/dt).

Advantages and disadvantages of full-metal shunts:

Contra: They are more expensive than metal film shunts.

Pro: They provide consistent, undistorted measurements. Because full-metal shunts are made of a homogeneous resistor element, there is no additional inductance, making them ideal for high-precision applications such as medical engineering or precision measurement equipment.

Pro: They offer high measurement precision and resistance to heat shock.

Pro: They can be operated at a power of up to 7W at maximum temperatures of 275°C.

Pro: They are available in various design forms, including forms that are much larger than standard chip resistors, with TCs of well below 100ppm/K and resistance values so low as to be measurable in single-digit milliohms.

Which resistance value is ideal for current measurement?

The ideal resistance value for full-metal shunts can be determined relatively easily: The lowest measuring voltage that still achieves sufficiently accurate results is divided by the lowest current value of the measuring range.

Four-wire shunts

A variant of the full-metal shunt is the four-wire shunt, in which the current flows through two of the terminals while the voltage is measured at the other two. The voltage drop at the resistors can be determined using the internal Kelvin terminals, enabling the resultant measuring errors to be eliminated.

Four-wire shunts are used in two scenarios:

1. Where the line and contact resistance are relatively high and, relative to the measured resistance, are not negligible.

2. Where the resistance value is less than 10mR, as the resistance values of the conductors are also measurable in milliohms and must thus be incorporated.

There is a trend towards smaller sizes with higher power levels; customized versions in terms of terminal geometry and shunt form are also increasingly sought after. Whether these are preferable to the standard shunts is dependent on the application.

Tip: Conduct tests to see which shunt best fits the application! As shunt resistors are relatively expensive compared to other resistor technologies, they are already available in small batch sizes and test samples.

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