Abstract: A power supply for an electronic device such as the control circuitry of a thermostat. The power supply has a current transformer with a primary winding in series with the power line that is connected to the heating source, and a secondary winding coupled to the primary winding through a saturable magnetic core. A thyristor connected across the output terminals of the secondary winding establishes a momentary short-circuit condition when the voltage developed across the secondary winding exceeds a preset level in order to prevent the magnet core of the transformer from saturating.