A method is disclosed to produce a supply voltage (B+) in an on-board system of a vehicle in response to an excitation current (I_ROTOR) applied to a rotor of the alternator. The method is of the type that consists in: limiting the resisting torque by determining a maximum cyclic ratio (DC_C_LIMIT) of the excitation current as a function of variables taken from a group containing a rotational speed (V_ROTOR) of the rotor, a temperature of the rotor, the excitation current and the supply voltage (B+). The maximum cyclic ratio is determined as a function of at least two of the variables in the group. According to one specific embodiment, the variables are rotational speed and temperature. According to another embodiment, the variables are rotational speed and excitation current.