Living in Northern Ontario means frequently using a block heater to start your car on a -35C morning. For those of you lucky enough to live in a climate where you've never heard of a block heater, here's the wikipedia entry: http://en.wikipedia.org/wiki/Block_heater
One of the fears of winter driving is getting in your car at 7:00 in the morning, turning the ignition and all you hear is "Errrrr...errrrr...click...click...click" then nothing. Maybe it was the wind or a stray dog that unplugged your cord or it could even be a bad cord or heater, either way you're calling a cab to get to work. There are quite a few manufacturers that include a little status light in the female end of an extension cord but as far as I'm concerned this is next to useless because it is just an indication of available power not a test of the actual load. What I wanted was some way to sense the current draw, kind of like a clamp-on ammeter.
After a bit of internet window shopping, I came across a device that did almost everything I wanted. For about $20 you get a little adapter that you either plug in between your cord and outlet or cord and block heater and an LED indicates current. Perfect...except that I sure don't want to venture out in the deep freeze, to check that little light, any more than I have to.
This then is my project: A current monitor that can be embedded in an outlet box inside your home so you can monitor the use of a block heater or roof heating cables etc.
Step 1: Warnings
Before you go any further here are a few caveats that you need to be aware of:
1. Although the final circuit is electrically isolated from household 120 volts, a line voltage lead needs to be disconnected from the switch, passed through the coil and reconnected.
2. You need to comply with any electrical or building codes in your area.
3. I am not responsible for any misadventure, this instructable is just to show you how I did it.
Before starting on my project I sent in a question to Ontario's ESA (Electrical Safety Authority).
Here was my question and their response:
"I am considering installing a current transformer in the interior switch outlet for my exterior outlet feeding my car's block heater. The transformer supplies 10mA to an LED installed in the switch plate. The transformer is a toroid that the switched load simply passes through so there is line voltage isolation. Are there any concerns with this?"
"To get a technical advice about your project, ESA highly recommends contacting a licensed electrical contractor. You may want to check the following link for a list of Licensed electrical contractors in your area: http://www.esasafe.com/GeneralPublic/hc_003.php?s=8"
After asking a few local contractors it was clear that they had no idea of what I was talking about.
Oh well, I tried. On with the project.