Development of Piezoelectric Transformer -Coupled Solid State Relay for Electrical Circuit Control in Railway Systems

A solid state relay (SSR) is an electrical ON-OFF control device through which electrical power to the load is switched either ON or OFF by a small control signal. The device serves the same function as a classical electromechanical relay but carries no moving parts so that it has faster switching time, longer lifetime, higher reliability, no sparking, lower electrical noise, no mechanical noise, stronger tolerance to mechanical shock and vibration, smaller size, lighter weight, etc. Photo-coupled SSR (PcSSR) is regarded as the most popular type of SSR nowadays. However, the susceptibility of PcSSR to lightening or power surge often requires specific protection to ensure its normal function and performance, especially for railway systems. Recently, we have developed a novel type of SSR, called piezoelectric transformer-coupled SSR (PTcSSR), by using the high input-output isolation property of piezoelectric transformer for coupling between the input small-signal control circuit and the output load circuit. In this paper, we report the design, fabrication and performance of a 12 V dc input, 220 V, 2 A, 50 Hz ac output PTcSSR, and compare the measured performance data with those of a commercial PcSSR used for electrical circuit control in modern railway systems.