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3.3 V or 5 V Direct from the MainsThe SR03x range of voltage regulator chips from Supertex (www.supertex.com) connects directly to the rectified mains supply and provides a low-current 3.3 V or 5.0 V output without the need for any step-down transformer or inductor. The circuit requires a full-wave rectified mains voltage input (waveform a). A built-in comparator controls a series-pass configured MOSFET. The MOSFET is only switched on whenever the input voltage is below an 18 V threshold. A 220 µF capacitor is used to smooth out fluctuations so that the resultant voltage has a sawtooth waveform (waveform b) with a peak value of 18 V.
This unregulated voltage is connected to the source input of the chip (pin 7) and an internal voltage regulator produces a regulated output (waveform c) of 3.3 V for the type SR036 or 5.0 V for the SR037. Normally you would expect to see a reservoir capacitor fitted across the output of a full wave rectifier in a power supply circuit but in this case it is important to note that one is not fitted. For correct operation it is necessary for the input voltage to fall close to zero during each half wave.
This circuit must only be used in a fully encapsulated enclosure with no direct connections to any external circuit. It is important to be aware that the circuit is connected to the mains and the chip has lethal voltages on its pins! All appropriate safety guidelines must be adhered to.
Author: Gregor Kleine - Copyright: Elektor Electronics