Selecting a Solid State Relay This note covers "power" rated AC output solid state relays with currentratings from 1 Arms to 150 Arms. Nominal AC line voltages are 120 Vrms, 240 Vrms, 380 Vrms, 480 Vrms and 600Vrms. These nominal voltage ratings allow a relatively wide range of operating voltage ratings, enabling designers to select a relay with a higher voltage rating than would normally be required. The use of a solid state relay with a rated voltage higher than the operating line voltage should be considered in harsh electrical environments where high voltage transients can be generated. Normally the maximum transient withstand voltage for a 240 Vrms rated solid state relay is 600 volts peak; for a 380/480Vrms rated unit, it is 1,200 volts peak. So, a 380/480 volt rated relay used on a 120 or 240 Vrms line allows a significant voltage safety factor. Printed circuit board mount solid state relays can operate at currents from 1Arms to 10/25 Arms. The 10/25 Arms types can be operated at 10 Arms with natural convection cooling and up to 25Arms with forced air cooling. Above these current ratings, a panel mount relay should be used with adequateheat sinking (discussed later). Solid state relays have either a DC or AC control. For DC control the normal voltage range is 3-32 Vdc, but for some families of relays, the DC control voltage is separated into two ranges,3-15 Vdc and 15-32 Vdc. AC control ranges are 18-36 Vrms, 90-140 Vrms and 90-280 Vrms. All AC control