This is another Transformerless power supply. IC voltage regulators for SR03x made by Supertex is connect directly to main voltage and get the output voltage to 3.3V and 5V chip for SR036 SR037 to 30mA output current. To use this chip is not necessary to use step-down transformer or coil. Rectified input voltage to the diode bridge diodes D1 – D4 (figure point a) is input circuits IC1 SR03x.
Built-in chip comparator controls the power transistor T1 (Figure b): at a voltage above 18 V transistor is turned off and capacitor C1 smooths the output voltage ripple. This voltage is applied to pin 7, where the internal circuit chip performs up to its stabilization or 3.3V to 5V.