A circuit is disclosed which has an input terminal for connection to a source of direct current (18) and has an output terminal for connection to a load (10). An electromechanical relay (16) has its contacts connected between the input and output terminals. A fast acting field effect transistor (22) has its source connected to the input terminal and its drain connected to the relay contacts. A voltage source comprising a pair of timers (24, 26) is connected to the gate of the transistor. The voltage of the source varies cyclically whereby, in use, the channel from the source to the drain is cyclically blocked to interrupt direct current flow from the input terminal to the relay's contacts.