Electromechanical relays use magnetic fields generated by a coil to switch between OPEN and CLOSED states. When the coil is energized the relay contacts meet thus completing the circuit. In a switch module, relays are usually laid out on a PCB. Signals enter the switch module through a front connector. From here they are routed to the relay via a carved trace on the PCB. After passing through the relay, the signals make their way to the switch module output via another PCB trace. The input terminal, PCB traces, and output all form a part of the signal path. In the case of all switch modules, this signal path consists of several junctions where two dissimilar metals meet and thus form thermocouples. An example of such a junction would be where the leads of a relay come in contact with the PCB trace which is more often than not made of a different metal than the leads themselves (Refer to Figure 2).
Figure 1. Thermocouple Created at Junction of Relay Lead and PCB Trace
The voltage created by these thermocouples depends on several factors such as ambient temperature, the number of relays and air flow inside the module. The total voltage generated by all thermocouples in a signal path is referred to as thermal emf.
Figure 2. Total Thermal EMF of a Switch Module
The effects of thermal emf can be minimized by using a differential measurement setup in which the high and low terminals of the source signal both pass through a channel on the switch module. Since both channels on the switch module have close to equivalent thermal emfs, their voltage offsets cancel each other out and thus reduce the net thermal emf induced in the system. When making differential measurements it is often advantageous to use switch modules that have double-pole single-throw (DPST) relays. Such relays consist of two SPST (single pole single throw) relays that open and close at the same time (refer to Figure 3). These SPST relays are contained in a single housing package which ties them together thermally. Thus any external factor (for example, change in ambient temperature) that affects the thermal condition of one relay will do the same to the other. In addition to having the stated benefits, SPST relays used in a DPST relay come from the same manufacturing batch which helps reduce differences in their material properties. All of these reasons combined make it more plausible for SPST switches in DPST form to have minimal differences in their thermal emf.
Figure 3. Double-Pole Single-Throw (DPST) Relays
It is impossible for two relays to have the exact same thermal emf. However, DPST switches can help minimize this difference when used in differential measurements. In the circuit below, let us assume that the thermal emf of the first relay is Vemf1 and the thermal emf of the second relay is Vemf2 , the source voltage is VS, and the voltage measured is VM. Then:
Figure 4. Differential Measurement Using Two Relays
It is important to note that absolute cancellation of thermal emf never takes place. Even in the case of DPST relays, there is a slight difference in thermal emf between the two relays. However differential measurements with DPST relays can help significantly reduce voltage offsets in measurements caused by thermal emf.
To reduce the consequences of thermal emf when using nonlatching relays, it is often wise to take measurements within a few seconds after the relay is closed because these relays need a constant current to be supplied to them in order to maintain a given state or position. This constant current supply causes power to be dissipated by the relay coil which in turn causes the temperature to rise creating a thermal gradient for several minutes after the relay is closed. The higher the gradient, the higher the voltage associated with thermal emf. This phenomenon is much less prevalent in the case of latching relays which stay in a particular position without the need for continuous current supply.
Lastly, it is always advisable to use wiring made of the same metal as the PCB connectors and traces. Using a uniform metal across the signal path reduces offsets caused due the creation of thermocouples and thus improves accuracy. In general, most PCB traces are made of copper in which case using copper cabling is a good option. However, in the case that a PCB is plated with a metal other than copper, the process of choosing appropriate wiring may not be pragmatic.