Abstract: A protection system is described for series regulators exposed to power sources such as permanent magnet alternators which may exhibit high open circuit voltages. The subject system includes a circuit which protects the control transistor in the regulator from a forward bias second breakdown by clamping it OFF responsive to the alternator output voltage exceeding a predetermined level. In one embodiment the clamping circuit keeps this transistor off during enough of the duty cycle to reduce the voltage across the control transistor to a level above which second breakdown occurs. In one embodiment, an auxiliary circuit shorts the alternator output terminals to ground to protect electrical apparatus connected to the alternator from burn-out due to high voltages occasioned by sulfated batteries or loose battery terminals which result in a high impedance being presented to the alternator.