Design Ideas: February 17, 1994
The simple off-line power supply in Fig 1 can provide 150 mA or less and uses only a handful of components. Current regulator, IC1, acts as a preregulator for any of the popular 3-terminal regulators (The figure shows a 78L05). Because IC1 drops most of the voltage in this circuit, the circuit suits only low-current applications.
To get the best possible power factor and simultaneously reduce IC1's power dissipation, choose capacitor C1, such that the lowest point of its ripple voltage is greater than voltage dropped across IC1 and IC2—even at your application's maximum load. EDN BBS /DI_SIG #1364