A security circuit for a power supply feeding a DC system is provided. The security circuit is disposed on an outlet of the power supply. A switch element is disposed between a positive power supply clamp and a positive output clamp towards the DC system. A choke coil is disposed between the switch element and the positive output clamp. The choke coil is connected to an output capacitor on a side connected to the positive output clamp, and a side of the choke coil that is connected to the switch element is connected to a cathode side of a diode that is connected in parallel to the output capacitor. The security circuit also includes a control for the switch element, connecting the switch element in accordance with the power measured in the security circuit.