Uninterruptible power supply

An uninterruptible power supply device uses a transformer with a first primary winding and secondary winding tightly coupled to one another to transmit AC input electrical power to a voltage sensitive load. A series regulator switches taps in the first primary to keep the load output voltage within certain limits during minor variations in the AC input voltage. A standby inverter supplies standby power to the load after a loss or large variation in the AC input voltage by driving PWM pulses into a secondary primary winding of the transformer. The transformer loosely couples the second primary winding to the secondary winding to aid in synthesizing the output sinusoidal wave from the PWM pulses. The regulator opens all of the taps to prevent the inverter from supplying power to the AC input. A voltage monitor continuously senses the AC waveform of an AC sample signal produced from the output load voltage in the normal mode and from the AC input voltage in the standby mode to determine forward and reverse transfers between modes. A clipped AC sample signal detector inhibits transfer in high peak current conditions, and a digital phase-locked loop circuit operating at a fraction of the AC input frequency synchronizes the phase of a precision sine wave signal with the phase of the AC input voltage.