The circuit comprises a tank capacitance and a charge circuit supplied with the same voltage as the bridge and comprising an inductance and a control transistor. There is also provided a control circuit, which comprises an oscillator controlling the periodic switching of control transistor and a comparator which controls the momentary clamping of control transistor in the condition wherein the charge circuit is interrupted when the difference between the voltage across capacitance and the power supply voltage exceeds a present maximum value and the unclamping of the same transistor when such difference falls below a preset minimum value. A further comparator similarly clamps control transistor if there is an excess current in the transistor itself.