A comparison between mercury wetted relays, ruthenium relays, and solid state switches in a thermoelectric Z meter

Relays with mercury-wetted contacts have commonly been employed in thermoelectric Z meters for switching current to the thermoelectric material on and off. These relays have been chosen because they provide a consistent noise-free switch signal with a high slew rate, absent of contact bounce. This clean switch signal is the root of the Z meter´s test. However, new governmental restrictions on using heavy metals in electronic components are being introduced throughout the world, and mercury-wetted relays are becoming more difficult to obtain and use. The potential exists for the complete discontinuance of these relays, jeopardizing future manufacture of the test systems. Solid-state devices can be used to replace the mercury-wetted relays, but the quality of the switch signal depends on the topology of the switching circuit and the components used. If the switch signal is inferior as compared to a mercury-wetted relay, the accuracy of the test is degraded. Thus, various switching circuits were constructed and tested to determine their viability as replacements for the mercury-wetted relay. The switch signals from these circuits were evaluated and compared to the switch signal of a mercury-wetted relay, which was used as a reference.