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Wires, wires, everywhere, and not a color change

June 18, 2013

Goal: Use thermochromic pigment in a costume to make it change colors. More specifically, an entire corset.

I assume this project is the result of a deep and never fulfilled desire for a Hypercolor shirt, because that’s essentially what we’re talking about here. But pressing a hand against it won’t do. I need to heat the whole thing to at least 93° F, which means something powered.

It turns out heating an entire corset that warm is no cheap-and-easy task. There is such a thing as a heated vest, but they’re not cheap, and they generally plug into a motorcycle’s 12v socket because that’s who they’re meant to warm. I looked at a lot of other options from conductive thread to stainless steel ribbon to carbon tape to having someone follow me around with a cordless hair dryer. OK, I didn’t consider the last one for too long, but it turns out those aren’t easy to come by either. The easiest solution seemed like it would be something USB-powered that plugged into my iCarrier power brick, but USB-powered heating things are very small, and 5v is not very much. I was at a dead end.

Then I remembered that I have an old seat warmer for the car that plugs into that aforementioned 12v socket. I talked to awesome friends like Tom and Eco, not to mention Chris Gammell, who has sent me lengthy emails about electricity even though I don’t know him just because at one point somewhere between my exhaustive USB hunt and carbon tape adventures, I ran out of Googling ideas and tweeted, “I need a nice electrical engineer friend.” And I concluded that this turkey was just the gobble gobble I needed. (Guess what I had for dinner?) This thing is pretty much perfect–it’s a maze of wires that heat up and cover an area roughly my body size, and it already has low/high/off switches in its cord, which I figure I can hide in a belt or something:

(This is where I’ll show you a picture eventually, but my phone is so dead it won’t stay on long enough to upload a picture anywhere.)

I got this AA battery holder (because 8 AA x 1.5v = 12v) and one of these female car charger bits. (Say what you will about Radio Shack, but it’s across the street from my house, and the guy was totally into making this work.) The battery holder has the 9v-style connector snaps on it, so I got the other half of that with wires out of my handy-dandy box-o-bits.

Thus the connection is:

Seat warmer power cord > female car charger dangly bit (technical terms!) > 9v snap wires > battery pack

I tested what was coming out of the snap connector wires, and it was solidly 12v. Hooray, I thought. It works! And then the electricity proved me wrong. I plugged it all in, and that bad boy did not heat. I gave it several minutes, and at best it warmed to 77°. I’m near an a/c vent, so it started around 72-74°. That’s not going to get me to 93°. I took it out to the car and plugged it in there to make sure I didn’t break something when I cut it apart, and within a minute, it was over 100°, so I know it’s still working.

Here are my readings:

  • Without the female DC connector attached, the bare wires off the 9v snaps read 12v.
  • When I attach them to the connector and test the connections (being careful not to touch the wires instead), it’s 10.6v.
  • When I plug in the seat warmer and turn it on, that measurement on the connections drops to to 6.3v. And it doesn’t heat.

Suggestions? What’s going wrong?


Thanks to the lovely people of FB and Twitter, I believe I’ve concluded that the resistance of the heater is around 6-8 Ω. Thanks to this handy-dandy Ohm’s law calculator doing the math for me, I see I should be aiming for around 1.7 A and around 20 W. Problem not solved quite yet, but information added. 🙂

3 Comments leave one →
  1. June 18, 2013 11:07 pm

    1700 mA is a pretty high current draw for AA batteries – even high end lithium batteries will be drained within a couple of hours. At that kind of current, you may be running into issues with the internal resistance of the batteries badly affecting the available output voltage.

    With a high end lithium battery the internal resistance shouldn’t be a problem (Energizer’s “ultimate” series spec sheet says ~0.1 Ohms, so you’d only lose 0.17 V even at a 1.7 A drop and one of the main advantages of lithium over other battery tech is the ability to sustain high current draws), but I can’t say the same for general alkaline batteries (sustained current draw can suppress the output voltage – the voltage may then recover if the current draw stops for a while and the internal chemistry has time to sort itself out).

  2. Jimmy Dorff permalink
    June 19, 2013 8:31 am

    20W is a fair amount of power for small AA batteries. I would use a cordless power tool battery instead. They are designed for high amp loads. I would also add a fuse, just in case something goes wrong!

    Oh, and I totally had a hypercolor shirt circa 1991.

  3. June 19, 2013 9:40 am

    I can’t wait to see just what you’re doing with this project. Sounds very interesting and very cool.

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