Contact Potential

EMF is generated when two dissimilar metals come into contact. Because a relay contact is typically a plated alloy, even the two supposedly similar halves of the contact are not the same. There are differences due to alloy EMF is generated when two dissimilar metals come into contact. Because a relay contact is typically a plated alloy, even the two supposedly similar halves of the contact are not the same. There are differences due to alloy composition and the working of the metal as well as the thermal EMFs generated by the plating to the alloy junction.

In addition, EMF is generated when heat is conducted along the leads of the switch. The difference in the heat flow between the different leads causes a thermal EMF. A secondary source of thermal EMF is from winding inconsistencies and variations in coil wire diameter. In a reed relay, these radiate heats, this causes temperature variations in the switch.

In a test system, these thermal EMFs add a voltage error to the measurement system. Depending on the relay, this error may range from less than a microvolt to tens of millivolts. If the error is significant with respect to the sourced or measured reading, the thermal EMF must be known and compensated.

The table below gives some thermoelectric potential for common connection materials. Clean copper connections are very important, since the potential across a clean Cu - Cu junction is < 0.2µV/°C, and the potential across a Cu - CuO (copper oxide) junction is 1000µV/°C.

    Typical Thermoelectric Potentials