Thanks for your answers.
it's better to use PWM (and a MOSFET instead of a TRIAC) with DC.
What I really want to do now is a home brewed low voltage power supply for led lighting.
So, half wave rectify AC line ( wont use a transformer ), filter use PWM to lower voltage
and maybe a VR, which should be viable.
[quote certain LED modules (those that come with constant current driver rather than current limiting resistors).][/quote]
I ll be using discrete LEDs; will give your advice a try and let you know. Thanks.