Thread: Building electric clothing View Single Post
11-10-2006, 08:27 PM   #82
kerhonky

Joined: Jan 2006
Location: Selkirk, NY
Oddometer: 1,093
Quote:
 Originally Posted by Nesbocaj Hmmm, at 3.4 ohms that means your jacket is putting out 54 watts. So a Gerbings jacket has about 77 watts of power. Your wire is about 0.1ohm per ft, so you need about 57' of wire, no? More wire, more resistance, more heat. Unless I have this backwards??
Actually, I think you do have it backwards. Heat energy in Joules is current squared times resistance times time. So current has a much bigger effect on heat than resistance. Less wire equals less resistance, which equals more current, which equals more heat. Kind of counter intuitive.

Here's two examples assuming 12 volts for ease of computation:

30 feet of wire

3 ohms
4 amps
(4^2)*(3) = 48 Joules per second

40 feet of wire

4 ohms
3 amps
(3^2)*(4) = 36 Joules per second

All of that said, I'm a little embarassed to say that I checked my wiring with my multimeter when I got home tonight, and found that the jacket is producing infinite resistance. Hmmmmmm. Testing the various components revealed that there was no current getting through the switch I used. Making a straight connection without the switch seems to have restored the jacket to about 3.5 ohms. I went and plugged it into the bike and it definitely was producing more heat. We'll see how warm it keeps me tomorrow.
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kerhonky screwed with this post 11-10-2006 at 08:32 PM