title_body: Reduce power of popcorn popper for roasting coffee
upvoted_answer: Rather than playing with electricity and wiring you could actually just move the roasting bed/area away from your heat source and accomplish close to the same thing.  
  
  I started roasting with a popper as well.  The hole in the popper was roughly 3 inches so I got a 3 to 4 inch duct adapter, a 4 inch flour sifter and mushed it all together, similar to this.  The result was that I moved the coffee away from the heat source.  The sifter also allows you to stir and get more even results.  I think I went from 4 to 5 minute roast time to 7 or 8 minutes with this method and the parts were very cheap. 
  
  I believe the forum post for that picture actually has additional mods close to what you may be looking for if you actually want to take your popper apart and try rewiring it.