Portable dimmer cord for Incandescent/Halogen lighting

The halogen lights for my trade show booth are way too bright and hot at full power, so I need to dim them.

Now, there are already affordable UL-listed "dimmer switch plug" devices on the market, but they all use 2-prong plug/outlets only.  In certain applications it is desirable (or even required) to use grounded 3-prong plugs and outlets for all electrical connections.  For example, some trade show regulations mandate that all wiring used in booth displays must use this type of grounded plug.

There IS one kind of device designed for this, called a Variac, or auto-transformer.  You can buy one that is all pre-wired with plug, outlet, fuse, and volt meter.  In fact, I used one just like that for my booth lights in the past.  Variacs are expensive and heavy, but very awesome.  Unfortunately, due to their combination of mass and fragile wires, variacs are susceptible to damage from being dropped and landing hard, which is guaranteed to happen at least three hundred times during freight shipping to the trade show.  So, rather than purchasing another expensive, heavy, fragile variac, I will build a dimmer control cord that should survive years of use and (alas) punishment in my show crates.

So ironically, the 3-prong rule (which has been designed to encourage safe electrical connections) has led to the need for me to hack together cables and switches in a most un-UL-listed fashion!  Amusement aside, I shall endeavor here to make proper, safe connections that will result in a safe device.  Also I will try to point out things you should check to ensure safety along the path of constructing a similar device.

WARNING #1!  This kind of dimmer control can ONLY be used for incandescent/halogen bulbs, or other equivalent simple resistance loads.  Just because you see a 3-prong outlet doesn't mean you can plug in your kitchen appliances or fans or something.  If you do, Bad Things can happen.

WARNING #2!  Pay attention to ratings!  In my project I am using a dimmer control rated for 600 watts, which (for 120 volts) means 5 amperes.  This is sufficient for the lights I want to plug in.  Also, the sacrificial extension cord I use is rated to 13 amperes at this voltage, which is plenty.

WARNING #3!  YOUR OWN RISK.  Cutting up a cord and wiring your own switch like this definitely voids all warranties and makes you liable for whatever Horrible Accidents may occur based on your work.  ELECTRICITY CAN KILL YOU and start fires and other terrible consequences. 

Step 1 What you need

Parts used in this project:
* USA 3-prong 120v 13 ampere rated extension cord. Flat ribbon type optional but neat looking.
* 120v 5 ampere single pole dimmer switch control. You better know how many amperes your lights will draw!
* Plastic switch/outlet gang box.  Metal ones exist, but plastic is nicer looking for this exposed project, and safer.
* Switch cover plate, for that professional touch.

Tools used in this project:
* Knife to help split apart the plastic between the 3 wires. Maybe your fingernails are that sharp, but not me.
* Wire strippers+cutter.  Or are you the kind of person who uses scissors and teeth?
* Flat head screwdriver for switch plate screws
* Philips head screwdriver for dimmer switch screws
* Electrical tape, used to mark the neutral wire.  You could use a permanent marker.
* Multimeter, continuity tester, or other means of determining which is the neutral wire. IMPORTANT for safety.

Comments

[doctormerlin]

Nicely done. I did exactly the same thing with a rotary dimmer. I didn't put an inline fuse, but that is an excellent idea.