Guys,

whats the difference between the two??

20 - 500Hz AC input... capacitor value about 500nF



Ralph

 

the first is passing positive half cycle of the ac signal input, dropping the voltage by 2 diode voltage drops (depending on the type of diodes and the output load, around 0.5v to 2 volts in total).
the second is a typical voltage doubler. the output will get near twice the peak voltage of the ac input signal. (v peak minus the voltage drops).

 

forgot to mention, in the first circuit, diode d2 is useless except that you are using it's reverse breakdown voltage (i.e. as a zener), but without any resistors it's not applicable.
in the second circuit, the polarity of the cap c1 should be the other way around (if you are using a polar one)

 

hello rwmoekoe,

the cap is not polar... so you say the 2nd schema is a voltage doubler... that explains the higher voltage output(with simulation)...

thanks man!! now that is clear now