Abstract: The circuit comprises a first threshold comparator (2,3) with hysteresis which is arranged to provide a reference signal (V.sub.c) indicative of the zero-crossing of the sensor signal (V.sub.p). Also connected to the sensor (P) is an integrator circuit (4) whose output is connected to the input of a second threshold comparator (7). The latter provides an inabling output signal (V.sub.e) only when the integrated signal (V.sub.i) exceeds a reference signal (V.sub.2) whose amplitude varies in a predetermined manner in dependance on the amplitude of the integrated signal (V.sub.i). An inabling circuit (14) is connected to the two threshold comparators (2,3;7) and outputs the signal (V.sub.c) generated by the first comparator (2,3) only when the second comparator (7) emits an enabling output signal (V.sub.e).

Abstract: A control circuit which includes a circuit for regulating the time taken to desaturate the switching transistor. This regulating circuit is arranged to control the time for which the transistor is conductive, so that the desaturation time of the transistor is substantially nil when the rate of rotation of the engine is greater than a predetermined value.