Q:

Can I make a rectifier to convert from AC to DC?

I have a project that calls for an 18VDC power supply. I've found one that meets specs online for about $40.00, and I've found an 18VAC power supply that meets specs (apart from being AC) at my local science & surplus store for $3.99. 
So I was thinking about this over the weekend, and it seems to me that I could run the AC through the rectifier (assuming the fwd/rev voltage is within diode parameters) and get DC out the other end without a significant voltage drop. But c'mon...is it really that simple? 
Can I really get a working, safe, and reliable 18VDC power supply for $3.99 plus four diodes? Or should I stop being cheap and get the out-of-the-box 18VDC? 

A:

You can obviously build an AC to DC converter circuit. It does involve a bit more than a few diodes. Four diodes in a bridge circuit can turn the AC into pulsating DC. It will require a bulk storage capacitor to smooth this out into a DC voltage. You would apply this capacitor downwind from the diode bridge. Do note that an 18VAC transformer is rated as an RMS voltage and when you rectify that to DC and smooth with the storage capacitor the resulting voltage will be higher than the 18V level. The actual level you would see varies depending upon how much load current the supply would be delivering. For this reason it is often necessary to add a regulator downwind from the storage capacitor to produce the constant 18V DC output that you apparently need.

A:

Assuming the 18VAC is regulated enough for you, you might want to add a smoothing capacitor on the output side but that's about it.
There's a fair chance the AC one is just a transformer so unregulated, whereas a DC one will likely be switch-mode and reasonably well controlled. Depends on your requirements.

