Method for operating a metal detector that comprises a balanced coil system with two detection coils (L62, L63) connected to a receiver unit and inductively coupled with a drive coil, which together with at least one capacitor (C1, C2) forms a series resonant circuit that is connected to a transmitter unit, which comprises a converter with a first drive switch and a second drive switch that form a half-bridge circuit and that are driven by a drive controller ➢ in accordance with a first square wave signal that is set to a fixed or selectable operating frequency (f TX) and is applied to the first drive switch, which is connected on one side to a first voltage potential (V D) and on the other side to a centre tap of the half-bridge circuit, and ➢ in accordance with a second square wave signal that is set to the fixed or selectable operating frequency (f TX) and is applied to the second drive switch, which is connected on one side to a second voltage potential (V S) and on the other side to the same centre tap of the half-bridge circuit, via which a drive current (i D) is supplied to the drive coil; comprising the steps of: ➢ providing the first voltage potential (V D) and the second voltage potential (V S) with different magnitudes with reference to ground; ➢ providing the first square wave signal with a variable first duty factor and the second square wave signal with a variable second duty factor in such a way that the first square wave signal and the second square wave signal are never active at the same time; ➢ adjusting the first duty factor and the second duty factor to set the coil current (i L61) in the drive coil to a value suitable for operating the metal detector and for measuring contaminants by ➢ predetermining and storing or online calculating operating values for the first duty factor and the second duty factor, with which the coil current (i L61) is set to a suitable value for each selectable operating frequency (f TX); and ➢ selecting one of the predetermined operating frequencies (f TX) together with the related operating values for the first duty factor and the second duty factor. ➢ in accordance with a first square wave signal that is set to a fixed or selectable operating frequency (f TX) and is applied to the first drive switch, which is connected on one side to a first voltage potential (V D) and on the other side to a centre tap of the half-bridge circuit, and ➢ in accordance with a second square wave signal that is set to the fixed or selectable operating frequency (f TX) and is applied to the second drive switch, which is connected on one side to a second voltage potential (V S) and on the other side to the same centre tap of the half-bridge circuit, via which a drive current (i D) is supplied to the drive coil; ➢ providing the first voltage potential (V D) and the second voltage potential (V S) with different magnitudes with reference to ground; ➢ providing the first square wave signal with a variable first duty factor and the second square wave signal with a variable second duty factor in such a way that the first square wave signal and the second square wave signal are never active at the same time; ➢ adjusting the first duty factor and the second duty factor to set the coil current (i L61) in the drive coil to a value suitable for operating the metal detector and for measuring contaminants by ➢ predetermining and storing or online calculating operating values for the first duty factor and the second duty factor, with which the coil current (i L61) is set to a suitable value for each selectable operating frequency (f TX); and ➢ selecting one of the predetermined operating frequencies (f TX) together with the related operating values for the first duty factor and the second duty factor.