Circuit assembly for protecting a unit to be operated from a supply network against surges, comprising an input having a first and a second input connection, which are connected to the supply network, an output to which the unit to be protected can be connected, and a protective circuit which is placed between the first and the second input connections in order to limit the voltage applied thereto, said protective circuit comprising a power semiconductor device. The power semiconductor device includes at least one Zener element between the collector and the gate, or a digital-to-analog converter is placed between the collector and the gate of the power semiconductor device. The protective level of this type of protective circuit can be easily adjusted by setting the clamping voltage for the power semiconductor device.