Transformerless mains power supply

              Cheap & dirty              

Introduction

This power supply does not really excel in power efficiency, but it is the cheapest
and most compact solution for a small power supply

Note that the low voltage output is directly connected to the mains;
it can not be used when the low voltage part can be touched (e.g. chargers)

With the component values of the schematic below, the circuit can supply 12V /15mA max.

Circuit description

High voltage capacitor C1 acts as a current limiter; R1 limits switch on currents
In the positive mains cycle the current will flow through zener diode D1 resulting in a voltage
of 12V across D1. Consequently C2 will be charged through D2 to almost 12 V.
In the negative mains cycle zener diode D1 will conduct in forward; the voltage on C2
remains unaffected because D2 is switched in reverse.

Mains supply schematic:

Some rule-of-thumb formulas:

The maximum DC current which can be supplied by this circuit depends on the value of capacitor C1:

                    Imax,out = pi× C × f × Vmains [A]

The RMS mains current is mainly capacitive and almost independent from the DC load of the power supply:

                    Imains = 2 × pi× C × f × Vmains [A]

The power consumption is set by the mains current and the zener diode voltage of D1
Note: Without load, almost all power will be absorbed by zener diode D1!

                    P = ½ Imains × Vzener

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