Help with a Buck Transistor

[AtomAmp]

Somewhere I saw a schematic of a Buck transistor.  I can't for the life of me find it again or remember where I saw it.

It would go in this order: 18v 1a ac to a 1a bridge rectifier to a capacitor to the transistor,(not a dip. Simple 3 pin trans.) to another capacitor and
end up with 12v to feed a fan running at 12v 0.07a

I can't remember which transistor they used or the size of the caps.
Can anyone help?

Thanks...

[Jon86]

Do you mean a voltage regulator?

[kxenos]

I think this is the buck transistor (!)you want http://www.farnell.com/datasheets/1696820.pdf 

[Dragonfly]

I agree. I sounds like you are looking for the LM7812.  Here's a reference schematic similar to the one you described.

http://schematiccircuit.com/12-volt-power-supply-lm7812/

[AtomAmp]

My apologize. Yes, I am speaking of a voltage regulator.

The transistor that I saw was a 3/4 round, flat top, 3 pin, E,B,C  I believe is a TO-92  I think it was a PNP.

[Dragonfly]

Like this?

http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/LM78L12ACZ/?qs=sGAEpiMZZMug9GoBKXZ757KFYdywgvBVGeJTtZXlf7A%3d

[AtomAmp]

Yes Dragonfly,  just like that! 
you're the best.   
Thank you, thank you all for your help!
Now I can get back to work...