Applications using Solid State Relays for Interference Suppression

The advent of the solid state relay has presented a major breakthrough in the ability to switch AC loads with a minimum of emissions. The technique employed is to activate the load at the next zero voltage crossing upon receipt of a command and deactivate the load at the next zero current crossing of AC power when the command is removed. The innovation is not without drawbacks in the areas of emissions and susceptibility due to the non-resistive parameters of the loads and the sensitivity of the triac gating. The paper briefly reviews the operation of the solid state relay (SSR) and presents test data for several types presently available. The data shows the emission reduction effectiveness and the EMC limitation of the devices. The application of the SSR is explored for aerospace applications showing the advantages in using such devices.