A plug-in power supply for supplying a consumer device with a low voltage, the power supply comprising a power adapter unit and an interchangeable detachable mains plug unit, wherein: the power adapter unit comprises a housing; a voltage transformer module for converting a mains voltage into the required low voltage in the housing; an engaging seat for the plug unit, at an inner side of the power adapter unit to be directed towards a mains socket, the plug unit being slidably connected with the engaging seat; at least two electric contacts in the engaging seat connecting the transformer module with contacts at an inner side of the plug unit directed towards the power adapter unit; and a locking member, at a rear profile of the engaging seat which limits the sliding of the plug unit, the plug unit comprises at least two mains plug pins, extending from an outer side of the plug unit and connected to the contacts at the inner side of the plug unit; safety recesses, for said contacts at the inner side of the plug unit; and a locking structure, comprising a button, for interacting with the locking member so that pressing on the button releases the structure and allows sliding of the plug unit from the engaging seat, wherein the plug unit has a flange at an edge of the power supply device in contact with a matching profile at a front edge of the engaging seat, the flange extending to cover gaps between the plug unit and the engaging seat, and wherein said contact between the flange and the matching profile is sealed by a soft material attached to an inner surface of said flange of the plug unit, the inner surface of said flange is perpendicular to the sliding direction of the plug unit, the plug unit and the power adapter unit are locked in a state in which the inner surface of the flange is pressed to the power adapter unit, and the soft material being under pressure and thereby waterproof due to the locking between the plug unit and the power adapter unit. the power adapter unit comprises a housing; a voltage transformer module for converting a mains voltage into the required low voltage in the housing; an engaging seat for the plug unit, at an inner side of the power adapter unit to be directed towards a mains socket, the plug unit being slidably connected with the engaging seat; at least two electric contacts in the engaging seat connecting the transformer module with contacts at an inner side of the plug unit directed towards the power adapter unit; and a locking member, at a rear profile of the engaging seat which limits the sliding of the plug unit, the plug unit comprises at least two mains plug pins, extending from an outer side of the plug unit and connected to the contacts at the inner side of the plug unit; safety recesses, for said contacts at the inner side of the plug unit; and a locking structure, comprising a button, for interacting with the locking member so that pressing on the button releases the structure and allows sliding of the plug unit from the engaging seat, wherein the plug unit has a flange at an edge of the power supply device in contact with a matching profile at a front edge of the engaging seat, the flange extending to cover gaps between the plug unit and the engaging seat, and wherein said contact between the flange and the matching profile is sealed by a soft material attached to an inner surface of said flange of the plug unit, the inner surface of said flange is perpendicular to the sliding direction of the plug unit, the plug unit and the power adapter unit are locked in a state in which the inner surface of the flange is pressed to the power adapter unit, and the soft material being under pressure and thereby waterproof due to the locking between the plug unit and the power adapter unit.