A demand printer has an input structure for receiving command signals related to the operation of the printer; a control circuit coupled to the input structure, and a power supply circuit for processing command signals and generating corresponding control signals for controlling the operation of the printer. A switch circuit line is coupled to at least a first and a second point on the power supply circuit. The switch circuit line provides a first operating voltage for the operation of the printer when connected between the first and second points in the power supply circuit and provides a second operating voltage when the connection between said first and second points is opened by a severing structure. The severing structure projects into a base cavity of the case and is retained therein for maintaining an open circuit between the first and second points on the power supply circuit once said switch circuit line is severed. The severing structure includes a wall of the base cavity which has an control aperture extending therethrough, a severing body having a head end and a severing end wherein the severing end projecting through the control aperture and contacting and severing the switch circuit line; a retaining segment on the severing body operatively associated with the case when the severing body is inserted through the control aperture for retaining the severing body in contact with the switch circuit line to maintain an open circuit between the first and second points.