Abstract: A switch mode power supply for electronic circuits includes a rectifier for providing direct current from an alternating current line, a transformer connected to the rectifier, and an oscillating switching circuit connected in series with the transformer and the direct current source, operative to chop the direct current source to power the transformer. Power for starting up the oscillator, which requires a substantially lower voltage than that of the rectified source, is derived from a capacitor charged by the direct current source. A comparator senses the voltage on the capacitor and a reference voltage, and controls a transistor switch to discharge the capacitor into the switching circuit, when the proper voltage is attained, initiating the oscillator. A running switch mode power supply connected to a secondary of the transformer then provides power to the oscillator and turns off the transistor switch.