An auxiliary supply for a current supply with at least one transformer with a primary side and a secondary side, wherein the auxiliary supply includes at least one frequency generator arranged on the primary side for generating an alternating voltage with predetermined frequency, a rectifier unit arranged on the secondary side for a secondary-side supply voltage and first and second potential separation units arranged between the primary side and the secondary side, where the first potential separation unit is connected on the primary side via, for example, a high-value impedance to the frequency generator, where on the primary side, the second potential separation unit has a high-frequency coupling to a reference potential assigned to the frequency generator and provides a low-ohmic capacitive connection between a secondary-side reference potential and the primary side, where it is possible to use a voltage drop at the impedance to evaluate the secondary-side supply voltage.