A boost chopper circuit converts AC power to DC voltage and supplies the DC voltage to a load. The field emission cathode may have from four to ten field emission arrays linearly arranged. It is furthermore provided with a control unit for detecting a change in the capacitance of the electrical field caused by the presence of an obstacle in the opening range of the closure element, the control unit providing a control signal for a drive powering the closure element. A support bracket is adjustably attached to another of the side walls for supporting the adjustably positioned box with respect to the back wall board.