http://electronicdesign.com/site-files/electronicdesign.com/files/archive/electronicdesign.com/files/29/1823/figure_01.gif [Broken] So let me just start by saying that, I want to understand how to analyze circuits and I would like to find a good source for that. If some one could point to online tutorials, or a website it would be great. In the above circuit (link) for example, I get the part that we step-down the AC mains, we rectify it and get a 15 V pulsating DC. Now, I assume capacitor C is to keep it at a constant 15V, right? Though I don't understand how a simple capacitive filter is enough for that. When does it charge, and when does it discharge in this circuit? Next, could someone explain the role of the diode D1, and the transistor Q1. I mean like what exactly happens with them in the positive and negative half-cycles of AC mains? The diode will be likeshort if forward biased, and like open if reverse biased. But if we have a rectified DC, so when will the diode be forward biased? Then after I am done understanding that, we could go to more complicated circuits like this one: http://www.circuitstoday.com/wp-content/uploads/2008/08/battery-charger-circuit-using-l200.JPG If someone could elaborate/discuss these circuit (starting with the first) so that I can try and understand it, I would really appreciate. Thanks.