Abstract: Protection circuitry incorporates detection circuitry and first and second timer circuits to protect power supply apparatus and associated circuitry from damage due to excessive voltages and/or currents. The detection circuitry initiates the first timer upon detecting potentially damaging voltages and/or currents, caused by, for example, a source or load impedance far different from that for which the power supply apparatus is designed. If the abnormal voltages and/or currents persist for longer than a first period of time, the first timer initiates a second timer that reduces the power output of the power supply apparatus for a second period of time. After expiration of the second period of time, the power supply apparatus returns to normal operation. The cycle described above an repeat indefinitely if necessary to protect against, for example, a sustained short circuit.