Design guidelines for 120VAC

Discussion in 'General Electronics Chat' started by 1010101, Nov 1, 2009.

  1. 1010101

    Thread Starter New Member

    Oct 25, 2009
    I'd like to do a small project (xmas light controller) using 120VAC. I've tried searching for guidelines / rules / do's and don'ts but searching on "ac design" doesn't return what I'm looking for.

    From reading here and looking at other similar designs, it seems like best practice is:

    * On/off switch on "hot" side of AC.
    * Inline fuses on hot side (I was thinking one per each outlet / string).
    * Optical isolators between triacs and digital part of controller.

    Can I use a plastic project box, or should I do metal, and ground the case? I was thinking about using terminal strips inside the box to distribute the AC power to the outlets, is that safe? Stuff like this.

    Any hints / tips / suggestions for safe design much appreciated.
  2. SgtWookie


    Jul 17, 2007
    We generally like for our "n00bs" to stick with strictly low voltage stuff (<50v) for awhile. Mains power can kill you in a big hurry if you goof up. Safety is the primary concern on these forums.
  3. 1010101

    Thread Starter New Member

    Oct 25, 2009
    Not only is that of no help, but it's also kind of patronizing. Not sure what I did to warrant that.

    The project's already in progress. But now, without the benefit of the experience you guys seem to share with others so freely.

  4. SgtWookie


    Jul 17, 2007
    You haven't even described your project fully.

    The first thing your "hot" wire should connect to inside your enclosure is a fuse holder. Then a power switch. Then your load, whatever it is. The fuse in the fuse holder should have a current rating low enough so that it will melt before anything gets damaged.

    If your enclosure is metal, it must be grounded. If it is nonmetallic, it must have the proper NEMA rating for the service conditions. If there are outlets that look like wall outlets, they must be rated the same or higher than the fuse that is protecting them.

    There is just far too much to cover when we don't even know what you're trying to build.

    How about posting your schematic, links to the specifications of the enclosure, things like that?
  5. ifixit

    Distinguished Member

    Nov 20, 2008
    * Use only UL approved parts and wire for the 120V portion of the project.
    * Have an electrical safety inspector approve your project design before you start.
    * Have an electrical safety inspector approve your project when it is finished.
  6. t06afre

    AAC Fanatic!

    May 11, 2009
    I must agree with the Wooki. All equipment connected to the mains shall be approved. It is for your own safety. If a fire is started and the investigation points to your hobbyist equipment as the reason. You will be in deep trouble. If someone is hurt or even worse. I would rather not think about it.
  7. Duane P Wetick

    Senior Member

    Apr 23, 2009
    Many light dimmer control circuits are listed in the hobbiest manuals. You can also buy a commercial unit for much less than building it yourself. A typical unit mounts in a household switch junction box. Just add a powercord, strain relief and a cover and you're in business. You can also purchase a waterproof junction box (Nema 4) for outdoor use.

  8. someonesdad

    Senior Member

    Jul 7, 2009
    SgtWookie's response was right on, but I can see how you could interpret it as patronizing -- however, it certainly wasn't.

    There are just too many things to worry about to explain fully to a neophyte. The bad side is that a single mistake you put you in danger or, even worse, your work could endanger an innocent person. And a mistake might not become hazardous for a number of years.

    There's nothing wrong with proceeding with what you're doing, but as someone else mentioned, have it checked by an expert before "releasing it to the wild".