The power of fully superconducting rectifiers can be improved by increasing either the operating frequency or the transformer primary inductance . The frequency is usually limited by the recovery time of thermally controlled switches. In order to achieve a higher switching speed, magnetically controlled switches are preferable [1,2]. This paper describes a magnetically controlled switch which can be used for currents up to 500 A at 25 Hz. The switch element, consisting of several Nb1%Zr multifilamentary superconductors, is placed between two concentric solenoids which generate the necessary magnetic field. The Nb1%Zr superconductor is well suited for this purpose because of its relatively low critical field (¿ 0.75 T) and high maximum current density (about 5.109A/m2below 0.3 T).