Abstract: The invention relates to a control system for direct current actuators in power electronics, based on pulse width modulation, comprising a H-bridge of four switches, each switch incorporating a bipolar transistor (T1,T2,T3,T4) and a diode (D1,D2,D3,D4) connected in reverse parallel between the emitter and collector thereof, and a control circuit (11) for said transistors. The control circuit (11) supplies four signals (V1,V2,V3,V4) permitting the time shifting of the switching of these four transistors (T1,T2,T3,T4) in such a way that during a switching of one of them no pair of transistors is simultaneously switched on, both with respect to transistors belonging to two lateral branches of the bridge and transistors belonging to two opposite branches of said bridge.

Abstract: A cut-off amplifier regulating the supply voltage of a load. This cut off amplifier comprises n bidirectional switches receiving input voltages which are alternatively positive and negative. These switches are connected to the load through a choke and to control means which place only one of the switches in the "on" state when the others are in the "off" state. The phase of the input voltage which is off (selected by the switches) depends on the comparison between the voltage applied to the load and a reference voltage.

Abstract: A process and apparatus for the automatic correction of the focusing of a microscope in an image plane utilizes two microscope outputs forming two images of the same object in two different planes surrounding the image plane and two matrices with at least one line of N photodetectors receiving the two images. A contrast measuring circuit of each image is associated with each matrix and a comparator of the signals is used for the two contrast measuring circuits. Connected to the output of the comparator is a circuit for the formation of a correction signal and the microscope is focused by the correction signal.