When it is necessary to supply power to the network any household device with a small supply current (150 mA), which operates autonomously (clock, nightlight electronic thermometer, etc.) and has a plastic case (not danger of electric shock), it is advisable to use transformerless power supplies. In the proposed source in the stabilizer is applied to the chip KREN.
The source provides an output voltage of 12 V, which depends on type used integrated stabilizer. At load currents up to 150 mA DA1 chip has low dropout voltage, i.e. the difference between the input and output voltage DA1 is no more than 2.5 V. If you want to adjust the output voltage, the output 2 DA1 is connected to GND through a variable resistor or a Zener diode. Then the output the voltage can vary from 12 V to 22 V.
As can DA1 to apply any other integral stabilizer with similar electrical characteristics, for example, KREN, CREA, KREN, KREN, KREN, etc.
The capacitor C1 performs the role of ballast resistance, which is "extinguished" excess voltage. Resistor R1уменьшает the initial inrush current when turn on the device, R2 - C1 discharges when turned off. The Zener diode VD2 with voltage stabilization 22...27 protects the chip from throwing voltage at the moment of inclusion of the source. Capacitor C3 reduces noise at high frequency.
Operating voltage S1 must be at least 300 V. it is Possible to take the types of capacitors. K-3, K73-17 or similar. Resistors R1, R2 - MLT-0.5 Instead of the diode bridge VD1 suitable separate diodes types KD105B...CDG, CDA, CDB, CDE.
Attention! When the operation of the source cannot be touched not only the items but also to uninsulated parts of the device that is connected to it.
Author: A. Kashkarov, St.-Petersburg