I bought the wrong SCR today at work. If you know what an SCR is, read on. Otherwise, this post will probably bore you, so skip ahead to the word "Wikipedia." So I got a Crydom D24125 for an old Desisti 20K dimmer, which is 240V/125A w/ DC control 3-32V. Zero-crossing (Which I still don't quite understand. If you do, let me know what it's about.). So I took the old one out, it was working, but being weird. And I put the new one in with thermal compound. There were a lot of screws, some with torx heads. A real pain for sure. And the damn thing doesn't work, so I'm thinking it's the control board for the dimmer. So I poke around on that, change the op amp (it's in a socket). Switch out the slider pot for a more appropriate value. Play around with a little trimmer pot. Won't light the lightbulb though. Try switching the polarity going into the SCR (later I found out the Crydom works down to -32VDC). Testing diodes while eating lunch. This thing is killing me. It's not that complicated. So maybe I bought the wrong SCR. It wasn't a straight replacement, the old one was an obsolete part number. I couldn't find much online about it, but I'm pretty sure I need the non-zero crossing or "random" version, which is D24125-10 and on order from Jameco for the ultra-reasonable price of 68.54 shipped. Thanks Jameco! Wikipedia is proving to be quite helpful in these sorts of technical issues though. I never would have thought. It's really becoming a great source of information. I'm working on my own article, actually. It involves a mathematical proof. Here's the basic idea: