The current sense resistor’s thermal EMF is another important characteristic. Current shunt resistors must operate over a wide range of currents. When the current is low, for example in a battery application during sleep or standby mode, the shunt’s thermal EMF adds a measurable error voltage to the voltage generated by the current flowing through the resistor. This error voltage should be significantly lower than the lowest expected voltage generated by the current of interest flowing through the shunt resistor, minimizing measurement error.
Shunt resistors for current sensing applications are available with two or four terminals. A shunt resistor with two terminals is the simplest to understand because it works the same way that any two-terminal resistor works. Passing a current through the two-terminal shunt resistor develops a voltage across its terminals that’s proportional to the current passing through it.
Examples of two-terminal shunt resistors include the Microhm Electronics MMS2512 shunt resistor series and the MMS5930 shunt resistor series.
The MMS2512 shunt resistors with power ratings of 2.5 to 6 watts, resistances of 0.3 to 5 mΩ, and maximum current ratings of 22 to 141 amps. A typical device in the series, the MMS2512L3EF, comes in a 2512 surface mount package and has a resistance of 0.3 mΩ and a power rating of 6 watts.
Microhm Electronics MMS series shunt resistor family includes devices in several surface mount package styles ranging in footprint size from 0603 to 8420 with power ratings of 2 to 36 watts, resistances from 0.05 mΩ to 20 mΩ, and resistance tolerances of 0.5 or 1%. A typical Microhm current shunt resistor is the MMS8420, which comes in a 8420 package with a resistance of 0.05 to 2 mΩ, a 0.5 or 1% tolerance, and a power rating of 36 watt.
Note that these surface mount technology (SMT) current shunt resistors are small and require very little board space, but because they can dissipate significant amounts of heat they should be placed well away from heat sensitive components.
