I wanted a synchronous recitifier for a voltage regulator in an old- style dc generator on an antique truck (to block the battery from discharging into the generator). This is what I came up with. I built it on a breadboard with junkbox parts: For the battery under charge used a junk motorcycle battery, and I used a 12 volt battery in series with a 6 volt battery to serve as a simulated generator.
generator | | 1N4148 +--->|--- ,--, | \\ / | ,---+--||-+ D V___/ | | | ||-> | | | 15K ||-+ S | | | | | 1N4148 | | | '-----+-->|-- | | | | \\ | | | | >| | | | ___ |--+ | | _ /| | 27K | ___ | | | | _ '----+ | | ___ | | | _ 220K | | | | | | gnd gnd | | | \\| | |------+-------------------'