A method and configuration from maintaining a constant of air to a protective helmet (4) from a fan (3) in a battery powered fresh air unit. The fan (3) is a radial fan with a filter (5) positioned on the suction side of the fan and is operated by a brushless continuous current motor (2). With the aid of signals from position sensors (30) in the motor (2), a control unit (12) can determine the speed of the fan (3). The control unit also detects the motor's power consumption and compares this and the speed with a characteristic curve for the motor (2) for a given flow from the fan, which is stored in a memory (14). In the event of any deviation, the control unit (12) regulates a change of the voltage supply to the motor (2) so that the deviation is reduced.