Thermo Electric EMF in Relays
Thermo Electric voltages are generated by relays because they contain different metallic materials, and if these materials are at a different temperature they behave as thermocouples and generate a voltage that is dependent on those temperature differences.
In a switching systems using mechanical (reed or EMR) relays thermoelectric voltages are usually introduced by the relays used, for most applications they can be ignored but for some they do matter. The voltage generated is usually caused by two factors:
- The relay coil heating the materials in the relay case
- Temperature gradients in the PCB that supports the relay.
In the first case the voltage is because the coils heats different parts of the relay at different rates or to different temperatures. If a relay is mounted on a PCB that has no temperature profile across it (isothermal conditions) the voltage generated will initially be zero and will then change to a new value after a period of time determined by the thermal time constant of the relays and its parts. This could be a simple exponential change with time, but can also be rather more complex. This is the specification number quoted on relay data sheets.
In the second case the PCB has a temperature profile, so when the relay closes a voltage immediately appears because the relay legs are at different temperatures. The act of closing the relay may change that temperature profile, again with time constants similar the the relay thermal time constant.
A temperature gradient across the PCB can be caused by a number of factors including variation in air flow, heat sources on the PCB (including other relays) and the presence of heat sinks (including front panels) and heat sources (power supplies behind the backplane for example). Because these are external influences the relay manufacturers do not test for these conditions. Consequently the use of relays which have a low thermoelectric EMF does not ensure a design has low thermoelectric EMF when a PCB has a temperature profile across it - this is particularly true in PXI because of the use of forced air cooling. Relay manufacturers use test methods for thermoelectric EMF which do not reflect the way users use the relays, they have to use methods designed to produce repeatable test results for individual relays that users can emulate in a fixture for concluding that a relay meets the specified performance.
If a relay is used in differential mode the thermoelectric EMF is usually lower because to a first approximation the voltages may cancel out in each path if they are part of the same relay, so differential thermoelectric EMF numbers are usually lower than single ended specifications. The degree of cancellation is dependent upon many factors, but the critical factor in getting good cancellation is ensuring the relay design is highly symmetric and that all parts are equally influenced by both internal and external heat sources.
It should be noted that thermoelectric sources in relays can cause measurement of the contact resistance of a mechanical relay to have an error if they are measured by injecting a current source and measuring the voltage across the contact (as performed by a DMM). If the current source has a low value the thermal EMF can alter the recorded value, make the resistance appear to be time variable and make it appear as if the resistance changes according to the direction of current flow.
In most cases reed relays crate larger thermoelectric voltages than EMR's because the materials used in reed relays have strong thermoelectric properties.
Solid State Relays
In general solid state relays do not create large thermoelectric voltages and result in switching systems which exhibit lower thermoelectric effects.
Placement of Switching Systems in PXI Chassis for best Thermoelectric Performance
As thermoelectric effects are created by temperature differences and changes in the metals used to provide a connection then applications requiring the best thermoelectric performance should avoid the switching system being placed closed to modules that have a high thermal load. For example it is often wise to avoid placing them next to high embedded controllers with high power dissipation or switching systems under heavy current loads that may have cause a significant rise in the temperature of the switching system which is being used for applications where thermoelectric EMF is important.
There may also be noticeable differences in thermoelectric voltages according to the chassis that the module is placed in.