Current measurement is required in many applications. For example, applications like over current protection, 4-20mA systems, battery chargers, high-brightness LED control, H-bridge motor control, or metrology need to monitor current.
How to Calculate Shunt Resistance
The technical constraints of shunt resistors differ from those of standard resistors. They are high-precision resistors with a low ohmic value (they can be expressed in microOhm when several hundreds of Amper currents must be measured). As accuracy is critical, current sensing is best achieved with a Kelvin connection (or four-terminal connection that removes the unwanted influences of lead resistance and lead sensitivity to temperature. MVR2321-4, MVR3825-4, MVR4618-4 of Microhm Electronics are four-terminal Kelvin resistors widely used in current sense applications.
Several factors can change the value of a shunt resistor, and they are split between reversible and irreversible effects. Long term stability is an irreversible change in resistance due to mechanical, electrical, and thermal loads. Reversible effects consist of mainly 2 components:
- Temperature Coefficient of Resistance (TCR): it is expressed in ppm/oC and characterizes the drift when the resistor is cooled or heated by a changing ambient temperature.
- Power Coefficient of Resistance (PCR): it is expressed in ppm/W and characterizes the power a resistor must dissipate.
Shunt Resistor Parameters
An important parameter for shunt resistors that is not significant for standard resistors is the thermal EMF. At the junction of 2 different conductive materials, a voltage variable with temperature (explaining why it’s called thermal EMF or thermocouple effect and expressed in µV/oC) shows up. The rate of change of voltage with temperature from an intermetallic junction is a function of the metallic combination. The sense of the voltage produced is either positive or negative on whichever side of the combination is being considered the input. It is presumed that all resistors are finally soldered to copper, and copper is then the reference metal. The following table shows some Thermal EMF values.
Based on thermal EMF, TCR, and cost trade off, manganin is the favorite choice for shunts resistors. For some resistance value, evanohm and Zeranin are used.
