Abstract: This application discloses a voltage regulator for use in conjunction with alternators to increase the life of the power transistor which is used for switching the field current to the alternator field windings. The regulator determines the relative speed between the alternator rotor and stator by means of an operational amplifier having one input connected through a voltage divider to an isolated speed dependent output, e.g. the Y connection of the stator, and the other input connected through a voltage divider with the alternator output and the battery to be charged. When the rotational speed exceeds a predetermined value, the operational amplifier produces a high output which functions as a reference voltage. A second operational amplifier has one input connected with the first operational amplifier to receive the reference voltage and a second input connected through a voltage divider to the alternator output.