Patent ID: 7924538
Filing Date: 2011-04-12
Classification: H02H

Abstract:
1. Protective circuit for an electrical device operated in particular in an explosion-hazard area, with a limiting circuit connected downstream of a pair of input terminals for limiting an output voltage present at a pair of output terminals and/or an output current delivered at the output terminals below a threshold value and a shorting circuit, which shorts the output or input terminals when the threshold value is exceeded, a control signal that controls the limitation originating from the same place as a signal that triggers the shorting circuit, and with a measuring circuit, which uses the output voltage or the output current to provide the control signal, which is fed on the one hand to a final control element for reducing the output voltage or the output current and on the other hand to the shorting circuit, the measuring circuit comprising a ZENER diode and/or a measuring resistor and the control signal being a voltage that is in particular a transistor-amplified or impedance-converted voltage, characterized in that the final control element is a self-conducting field-effect transistor, at the gate of which the control signal is present, and the measuring circuit is connected downstream of the field-effect transistor in the direction of current flow; a trigger voltage generator for supplying the shorting circuit with a trigger voltage if the control signal present at the input of the trigger voltage generator exceeds a predetermined threshold value; wherein the threshold value of the trigger voltage generator is defined by the sum of the voltage drops across the Zener diode and a diode of the trigger voltage generator subjected to the control signal in a conducting direction; and wherein the dimensioning of the electronic components of the measuring circuit and the trigger voltage generator is chosen such that, when the measuring circuit and the field-effect transistor are working properly, the control voltage present at the trigger voltage generator is always less than the threshold value of the trigger voltage generator.