This device is a Fully Automatic AC Voltage Regulator of the "Auto Stac" type and is made in Japan. Model and Capacity: This model is numbered ST500W4 and has a maximum capacity of approximately 500 Volt-Amperes (VA). Function: It works as a voltage regulator to protect devices from sudden current fluctuations, and it can also convert voltage from 110V to 220V and vice versa. Technical Features: It features a Servo-Motor Control System and includes a Fuse for protection.