Due to the forward voltage drop of the output SCRs, solid state relays generate an internal power loss. The amount of power generated is a function of the load current. The manufacturer provides power loss curves, as shown in Fig 1. At normal load currents the power loss can be estimated at 1 Watt for every 1 Arms of load current. Obviously the junction temperature Tj can be calculated if the power dissipation is known. The normal maximum allowable junction temperature is 125 degrees Centigrade. Most designs are based on providing a 10 degree Centigrade safety margin and use a heat sink to keep the junction temperature to 115 degrees Centigrade. In order to maintain an acceptable power switch junction temperature, some form of heatsink must dissipate the heat generated by the power loss. For most printed circuit board types, the relay current rating is established by measuring the thermal impedance, from the dissipating elements to air, using the relay package as the heat sink. Some printed circuit board types are available with an integral heatsink; their ratings reflect the additional effects of the integral heatsink.