of dissipating less energy inasmuch as the relay r in its normal operating condition is for the greater part of the time fed at voltages which are higher than the threshold voltages that is in the off zone as illustrated in fig6 in fig5 the components of the circuit are indicated with reference signs similar to those used for the circuit of fig3 therefore with reference to fig6 the non essential or optional loads are inserted in the range vs vm and vm vs 1 for instance between 13 5 and 15 6 volt while in the range vo vs and vo 1 and vs 1 only the essential loads are inserted for instance between 11 8 and 13 5 volt the circuit illustrated in fig5 performs the control of the voltage by determining the change of state of the final relay r each time the applied voltage exceed the threshold voltage vs which can be pre established by the potentiometer p while the voltage vs 1 is the threshold voltage in the discharge phase corresponding to the voltage vs during the charging stage the circuit has been constructed in such a manner that for applied voltages below vs the base current of the transistor t 1 is sufficient to keep it in the conduction phase on when the threshold voltage vs is reached the transistor t 1 blocks since the base current is shortcircuited by the.