The technical constraints of shunts are different from standard resistors. Precision shunts are hi...
The technical constraints of shunts are different from standard resistors. Precision shunts are high-precision resistors with low ohmic values. When hundreds of amperes of current must be measured, they can be expressed in micro-ohms. Since accuracy is very important, current sensing is best achieved through Kelvin connection or four-terminal connection. This connection eliminates the undesired effects of lead resistance and lead on temperature.
There are several factors that can change the value of the shunt. The precision shunt is divided into two types: reversible and irreversible. Long-term stability is the irreversible resistance change caused by mechanical, electrical and thermal loads. The reversible effect is mainly composed of two parts: the temperature coefficient of resistance TCR: its unit is ppm/oC, which represents the drift characteristics of the resistance when it is cooled or heated under changes in the ambient temperature. Resistance power coefficient PCR: expressed in ppm/W, it represents the power that the resistance must dissipate.
An important parameter of the shunt is the thermoelectromotive force, and for the standard resistance, the thermoelectromotive force is not important. At the junction of two different conductive materials, the voltage varies with temperature to explain why it is called thermal EMF or thermocouple effect and expresses µV/oC. The rate of change of the voltage of the intermetal junction with temperature is a function of the metal bonding. On either side of the combination that is considered to be the input, the meaning of the generated voltage is either positive or negative. Assume that all resistors are finally soldered to copper, and then copper is the reference metal. The following table shows some thermo-EMF values.
The splitter is based on thermo-EMF, TCR and cost trade-offs, and manganese alloy materials are the favorite splitter and exposed blades. The shunt with exposed parallel wires is made of a metal alloy material, which is a manganese-copper alloy material with a lower temperature coefficient. The shunt encapsulated in the heat sink is usually manufactured by the resistance company. The temperature coefficient of the E precision shunt is close to zero, and it is very sensitive to strain. MICROHM often uses its own technical metal foil to produce precision shunts.