On Thu, 01 Jul 2010 18:16:14 -0500, Ignoramus14602
wrote:

I have this rectifier: GPBC 50A

http://tinyurl.com/25fmjfu

It has four terminals, but they are not marked. The datasheet is pretty
sparse, too. Which of them are AC and which are DC?

Use your ohm meter to check the polarity of the 4 diodes. The AC
goes to the points where the positive and negative diodes join.
You get DC+ where the 2 positive diodes joinn, and DC- whre the
negative diodes join. GENERALLY the cut off corner, if present, is one
of the DC terminals.