Anyone know what type of diode to make a full bridge rectifier? I have a 12VAC 18w transformer, but I have 12VDC lamps and i'm probably going to make a miniature light fixture. What type of diode to I need to make a simple full bridge rectifier? Thanks.

Ahh... Time to dig through my old boxes...

SO generally I don't use Diodes for rectification anymore, Usually I use the 4 pin type rectifiers pictured below, along with a capacitor (I usually use 1000uF or more) in parallel with the output DC to smooth it... Or sometimes I use my MR1124 (The silver thing with the screwbase) But if you really want to go the diode route what I usually use are either IN4007 or IN5402...

If your buying diodes you might as well buy a diode bridge like these but if you digging through a parts draw, just about any power diode will work and most of the common ones would be the 1N series like the 1N4007 just stay away from signal type diodes..

Also older burned out CFL's commonly have a diode bridge in them as do most switched mode power supplies and for the most part they don't fail so they are easy to reuse.. 

12V AC rectify into 17..18V DC, not into 12V DC. What sort of lamp you'd be using ?

just add a 7812 series voltage regulator, they will accept 14 to 35 volts input and maintain 12 volts out, the MC7812BD2TR4G from ON-SEMI would work and are only in the $1 range each.

12V AC rectify into 17..18V DC, not into 12V DC. What sort of lamp you'd be using ?

And even that range is very optimistic - unloaded transformers of such power rating use to have secondary voltage around the 18Vrms, that means the capacitor get charged way above 24V. And that voltage woulc be presented to the lamp for a few 10's of ms, way long enough to kill any electronic not designed for such high overvoltage instantly and in case of incandescents, kill the filaments after just few switching cycles (they get quite nasty overvoltage shock every tutn ON, beside the overvoltage during steady operation Ash wrote about.
With the 7812 you have to count with quite high losses: About 1/2 of the lamps rated power will be dissipated on that regulator, so you need good heatsink.
On the other hand the common electronic 12V LEDs use to withstand 40V abs max, so should be fine. However some cheepeese products may use just 20V rated components inside and these will definitely fail.

Andanother thing to take into account: The bridge rectifier and a capacitor form a low power factor load for the secondary, usually around 0.6 or so. That mean from a 18VA rated transformer you may draw maximum 10W of power. The reason is,the transformer is stressed the same as with 18Wresistive load. And the tranformer rating figures are ALWAYS for a clean, unity power factor resistive load.

If your buying diodes you might as well buy a diode bridge like these but if you digging through a parts draw, just about any power diode will work and most of the common ones would be the 1N series like the 1N4007 just stay away from signal type diodes..

Also older burned out CFL's commonly have a diode bridge in them as do most switched mode power supplies and for the most part they don't fail so they are easy to reuse..  

In my experience, Especially when breadboarding, the rectangular shaped ones such as these (pictured below) with the leads all lined up with each other are much easier to work with since you can line all the components up alot of times without jumpers.. especially with circuits like power supplies.. Don't get me wrong I love the little cylinder Rectifiers its just they are a tad bit harder to work with sometimes... Plus He can attach the rectangular ones to heatsinks depending on what hes powering....

Most CFL's I've come by either have IN4007 Diodes (Or something similar) or those little SMD things... What brand of CFL did you encounter the bridge Rectifier??  

They have the four pin variety as well, I just went with a 2 amp version as they are smaller and a tad cheaper, but those ones you have pictured aren't much more (a few cents a piece difference)

And the CFL was some cheap oversea kind, I don't remember the brand as it was headed to the trash and I wanted to see if they are easy to open with out busting the tube, just a few wacks with the handle of a screw driver around the plastic base and it fell apart so I scavenged the ballast but that was many years ago, I basically don't use or own any CFL's anymore, except for a few FEIT blacklight's for my parrot (yes Parrots need a UVA light to see things correctly) and the rest of his lights are 2 x PL-L 40 watt tubes..

But I do know those diode bridges are common in small switched mode supplies even in wall wort's and computer supplies and if you get them from value village you can get a good handful of parts for a buck if your willing to remove them from the board, just don't be like me and horde a 8 cubic foot box of wall wort's ( your future wife will thank you if you don't )

Speaking of wall wort's, for your project it might be easier to just rip open a small wall wort, you can get 12 volt 2 amp in both switched mode and transformer types and they will have all the part including a filtering caps inside them and if your lucky at value village or the re-store it'll only set you back like 10 cents..     

just don't be like me and horde a 8 cubic foot box of wall wort's

LOL thats alot of wall wort's  I'm honestly a bit jealous... I tend to hunt for those things, since I tend to burn them out alot when I do my little Experiments... So a hoard of them has always been my goal but I've never quite met the reality... LOL i need to be a bit more careful with them....

You need Laptop power supplies, the ones from Dell for there Alienware laptops are 240 watt units at 19 VDC, they show up from time to time at Value village, and for the most part laptop supplies have all the safety rating you can get in the world and they will just turn off if you go a little crazy on them, and the X-box power supplies are nice as well and I think they are 245 watt max and they they will accept pretty much anything you feed them between 70 to 265 VAC and they have all the safety ratings as well and are way more common then large laptop supplies and output 5 VDC & 12 VDC, you just need to connect the red 5 VDC @ 1 amp which is always on and the blue power enable with a small switch and wire the yellow+ and black- and then it will give you a nice regulated clean 12 volts at 16 amps easy...

Really the future wife won't be impressed...

12V AC rectify into 17..18V DC, not into 12V DC. What sort of lamp you'd be using ?

Well, I have 12VDC incandscent automobile bulbs, that i'll be using @Ash.

Incandescents don't generally care if its AC or DC... I think you'd be fine running it off the raw output from the transformer.

Hmm, ok I guess i'll try that.

Homebrewlamps, is spot on, the only common light that could care if you feed it DC is an LED, and in rare applications some DC only metal halides and Xenon arc tubes, otherwise AC will work just fine.

And with automotive lamps if you took ten of the same bulbs and wired them in series you could power them right off a 120 volt AC plug with out any issues..