DC output solid state relays are being used more and more widely. One of the major reasons for the use of solid state relays for switching dc is the fact that as there are no moving parts there is no contact arcing or wear out mechanism. However, there are some precautions which have to be taken when using solid state relays with inductive dc loads. Relay Connections Because the input and output terminals of dc solid state relays are electrically isolated by up to 3,750 Vrms the relative positions of the input signal and load connections are irrelevant. As shown in Fig 1, the input can be supplied either from a source or sink configuration and the load can be connected to either the relay positive or negative output terminals. The solid state relay can be used as a level shifter because no electrical relationship is required between the control and power output sections of the relay. Inductive Load Considerations When a DC load is inductive, special precautions have to be taken to protect the solid state relay at turn off. Energy is stored in the magnetic flux created by the current flowing through the inductive load. When the solid state relay is turned off the collapse of themagnetic flux creates an electro-magnetic force with a polarity that tries to maintain the pre-existing current flow.This is shown schematically in Fig 2.