Multimeter Shunt

Description: The current range in multimeters, especially the more affordable models, is limited by the load capacity of the internal shunts to 1 to 2 A. A precision heavy-duty resistor from manufacturers such as Dale or RCL (0.1 Ω; 20 W; 1%) can be utilized as an external shunt. Although these resistors were not specifically designed for this application, they are significantly less expensive than custom-made shunt resistors. It is important to note that the 20 W rating is applicable only when a heatsink is employed; without a heatsink, the rating is reduced to 8 W. The maximum current that can pass through the device with a heatsink is approximately 14 A, while larger versions can handle up to 17.5 A. When installing the shunt, ensure that the test terminals, as well as the device terminals, are soldered properly; otherwise, the resistance of the terminals will be added to the shunt resistance.

The use of external shunt resistors in multimeters enhances the current measurement capabilities beyond the limitations of internal shunts. The specified resistors from Dale or RCL, with a resistance of 0.1 Ω and a power rating of 20 W, are suitable for this purpose, provided that they are used with an appropriate heatsink to dissipate heat effectively. The heatsink allows the resistor to handle higher currents, making it feasible to measure up to 14 A safely, with larger resistors capable of handling up to 17.5 A under controlled conditions.

When integrating an external shunt into a multimeter circuit, it is essential to consider the configuration of the connections. Proper soldering of the test terminals is critical to minimize additional resistance that could skew measurement accuracy. The layout should ensure that the shunt is positioned in series with the load, allowing the multimeter to measure the voltage drop across the shunt accurately. The voltage drop can then be converted into a current reading using Ohm's Law (I = V/R), where V is the measured voltage across the shunt and R is the known resistance value.

Additionally, care should be taken to select a shunt resistor with a tolerance level that meets the requirements of the application, as variations in resistance can impact measurement precision. It is advisable to use a resistor with a tolerance of 1% or better for applications requiring high accuracy. Furthermore, the physical size of the shunt resistor should be considered, as larger resistors may provide better thermal management and lower temperature coefficients, which can further enhance measurement reliability.

Overall, the implementation of an external shunt resistor allows for greater flexibility and capability in measuring higher currents in multimeter applications, while also providing a cost-effective solution compared to custom shunt designs. The current range in multimeters, particularly the more inexpensive ones, is restricted by the load limits of the internal shunts to 1 to 2 A. The figure shows how easily a precision heavy-duty resistor from Dale or RCL (0.1 ; 20 W; 1%) can be used as an external shunt. These resistors were not designed for this purpose, but they are much cheaper than custom-made shunt resistors.

The 20-W rating applies only, by the way, if a heatsink is used: without that its rating is only 8 W. The maximum current through the device on a heatsink is about 14 A; the larger versions draw up to 17.5 A.

When mounting the shunt, make sure that the test terminals (as well as the device terminals) are soldered properly, otherwise the resistance of the terminals is added to the shunt.