Copio e l'incollo :D
"Basically, transistor 1 takes DC current from the big caps on primary and chops it into AC by turning on and off very quickly. This AC current is then sent through the 5vsb transformer where it gets converted from high voltage to low. After that the current is rectified into DC again by a diode (you can see part of it right above the 5vsb transformer). There's also probably a capacitor or two and an inductor behind the diode (you can't see them on the picture) - those filter the rectified 5vsb DC before it goes to your computer.
In order to get a steady 5v for 5vsb, the circuit needs to regulate itself. In a 2-transistor design, this is done through a small 4-pin device, called an optocoupler (should be right next to the 5vsb transformer, though it's not visible on the picture). The optocoupler "sees" what voltage is present on 5vsb (i.e. is it close to 5v) and then "tells" transistor 2 whether the voltage needs to go up or down or stay the same. Transistor 2 in turn controls transistor 1 by "telling" it how fast to turn on and off.
Problem with this circuit is that the regulation of 5vsb relies on one critical capacitor (usually 10, 22 or 47 uF). When this cap goes bad, 5vsb voltage can go well beyond 5v and possibly kill your motherboard. Of course, depending on the circuit is built, it usually takes some time before the critical cap goes bad. If that capacitor is not abused (i.e. there's little heat where it's placed), then the PSU can survive for many years before 5vsb goes bad. But if that capacitor is next to a hot component, it could easily go bad in a year or two."
Pensa su un alimentatore che avra' ormai dai 7 ai 10 anni di eta' quante probabilita' ci sono che quel condensatore sia secco...
uno scherzo divertente da fare a un amico :D
ciao
