3.3 V or 5 V Direct from the Mains

The SR03x range of voltage regulator chips from Supertex (http://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

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