Low Power 12V Transformerless Power Supply

Many circuits can be
powered directly from the mains with the aid of a series capacitor (C1).
The disadvantage of this approach is that usually only one half cycle
of the mains wave-form can be used to produce a DC voltage. An obvious
solution is to use a bridge rectifier to perform full-wave
rectification, which increases the amount of current that can be
supplied and allows the filter capacitor to be smaller. The accompanying
circuit in fact does this, but in a clever manner that uses fewer
components. Here we take advantage of the fact that a Zener diode is
also a normal diode that conducts current in the forward direction.
During one half wave, the current flows via D1 through the load and back
via D4, while during the other half wave it flows via D3 and D2. Bear in
mind that with this circuit (and with the bridge rectifier version), the
zero voltage reference of the DC voltage is not directly connected to
the neutral line of the 230-V circuit.

This means that it is usually not possible to use this sort of
supply to drive a triac, which normally needs such a connection.
However, circuits that employ relays can benefit from full-wave
rectification. The value of the supply voltage depends on the
specifications of the Zener diodes that are used, which can be freely
chosen. C2 must be able to handle at least this voltage. The amount of
current that can be delivered depends on the capacitance of C1. With the
given value of 220nF, the current is approximately 15mA. A final
warning: this sort of circuit is directly connected to mains voltage,
which can be lethal. You must never come in contact with this circuit!
It is essential to house this circuit safely in a suitable enclosure.

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