Abstract: An uninterruptible power supply which is down-sized and lightened is provided. A utility line 1 is connected with a switch 8 and the switch 8 is connected with a rectifying circuit 2 and DC power from the rectifying circuit 2 inputs to a first inverter 3-1 and charges a battery 6 by a charger 7 connected with a node between the utility line 1 and the switch 8 and the DC power from the battery 6 is converted to AC power by a second inverter 3-2 to input to the rectifying circuit 2. When the utility line 1 is interrupted, AC power from the second inverter 3-2 is supplied to the rectifying circuit 2 to continuously supply power to a load 4. As a result, the number of cells in a battery may be reduced, the cost and man-day of the unit may be reduced, the reliability of the unit may be improved and the supply may be down-sized and lightened.

Abstract: An upper limit value is set up which is larger by a prescribed value than a current value of an ideal sine wave, a lower limit value is set up which is smaller by a prescribed value than the current value of the ideal sine wave. Plural semiconductor switches are actuated according to a clock pulse of specified frequency supplied from a clock pulse generator so that a boosting mode is maintained until an alternate input current increases to reach the upper limit setting value during the boosting mode and a transfer mode is maintained until the alternate input current decreases to reach the lower limit setting value during the transfer mode, thus the semiconductor switches are actuated according to the frequency of the clock pulse at the time of this transfer mode.