You either spend half an hour putting on 7 layers of clothing to go outside or you spend an hour freezing because the bus was late. The cheapest commercially available electrically heated jacket that I know of costs like $300, which I think is just plain stupid for a bunch of wire and a battery. In part 1 of this project, I will show you how to mod your existing clothes so that they can be electrically heated. If you have a laptop, you can actually pull this off completely for free! :)
So some theory first: An electric heater is basically a lot of electricity running through a wire to produce heat. To make a wire heat up, just apply a voltage across it. So to make a portable heater, you just need a battery, and some wire wrapped around whatever you want to heat.
But how will you ensure you don' t make the wire so damn hot it melts? How will you ensure you get enough heat generated for the mod to be useful? How will you ensure that your battery doesn't explode because you are drawing too much current? For EE n00bs, the two equations that are your friends in this mod are Volts = Current * Resistance, and Power = (Volts²) / Resistance. (V=IR and P=V²/R from now on).
Let's say you have a 12v laptop battery that normally delivers 40W of power to your laptop. To make a 40W heater using this battery, you must choose wire of a certain resistance. From P=V²/R: 40=12²/R. Solving the equation tells you that you want a wire with 3.6 ohms of resistance to do the job. To get a wire of that much resistance, you've gotta find some pretty thin stuff. Forget about the good quality thick wires you connect your speaker system up with- they have almost 0 resistance. If you connect that stuff across your battery its going to blow up. Find a roll of the thinnest wire you can that is reasonably strong and isn't so stupidly thin its gonna melt just by farting on it. Now whip out your multimeter and cut the wire at whatever length gives 3.6ohms resistance. Now just connect this wire across your battery terminals, and you should instantly feel the wire heating up quite a lot.
Read More
