carbon fiber fabric as heating element

[Matt H]

Hello,
I have seen a couple projects on this website posted by folks trying to use carbon fiber fabric as an heating element, hopefully one more won't be too much of a bother...
My project would consist of using carbon fiber tape 1m (l) x 51mm (w). One provider of a similar tape has posted that 44mm width tape has a reistance of 8 ohms per meter. All other things being equal, would anyone be able to provide a system for determining the resistance of 51mm tape?
The jist of the project would consist of heating the tape, to 45°c, and prolonging that temp. for as close to 8 hours as possible. Would this be possible with a small battery pack, perhaps two 3.7v batteries in line? Perhaps a regulator of some sort would be necessary? And, for style's sake, an on/off switch?

Any help would be appreciated!

 

[Veraxis]

Assuming the material and thickness of the 51mm tape is the same as the 44mm tape, it would be approximately 8*44/51=6.9 Ohms/m. Beyond that you would have to just measure it with a multimeter or something. For measuring low resistances like that you may need to do a 4-wire resistance measurement for accurate results.

As for holding it at 45°c, it depends hugely on how quickly the heat is being drawn out of the tape: a length of tape wrapped in insulation with no airflow probably doesn't need much power to keep it at 45 degrees, but a length of tape bonded to a heatsink or with a lot of airflow will probably require a lot of power to keep it hot.

A 6.9 Ohm length of tape connected to a 7.4V battery pack would dissipate about 7.9W-- or about 1.07A, hence it would require a pretty sizeable 8560mAh battery pack if it were running continuously.

What is the application? What are you heating and will it be running continuously for the full 8 hours?

 

[Matt H]

Thanks for the quick response. The tape would be insulated inside of a flexible material, like bamboo fabric or nylon and worn around a person's waist, for heating the back, so it would be insulated. I should have made that more clear. It would, ideally, be able to run for up to 8 hours at a go, but would likely be turned on and off during that time period, as need and/or comfort would dictate.

 

[Veraxis]

I see. In this case, the user's back will be acting as a pretty noticeable heat sink, but it will depend on the thickness and thermal conductivity of the material between the tape and the person's back. Body temperature is about 37C, but the skin surface temperature may be several degrees less.

It may make sense in terms of efficiency to have slightly thinner, more thermally conductive material on the side that will be facing the user and thicker, more insulating material facing away, so that more of the heat is transferred to the user instead of into the air.

The device will probably need some kind of temperature-controlled cutoff so that it doesn't overheat or get uncomfortably hot if left on in open air, such as a high power MOSFET for switching the heating element on or off. There are a variety of ways that one could read a temperature sensor and switch the MOSFET on and off, ranging from a simple analog comparator (possibly with hysteresis) to a programmable microcontroller. I would recommend using binary switching or PWM-control rather than linear control for the MOSFET to reduce the amount of heat it dissipates.

A lot of this is predicated on the operating characteristics of the tape. Until you know what its actual resistance is, it is hard to be sure what voltage/current it will need to run at and how you will need to configure the battery. It may make sense to get a sample of the tape and measure it-- or even do some experiments with a bench power supply-- before designing the system to control it.

 

[Matt H]

The carbon tape has been ordered and the power supply for testing is already here, I am looking forward to testing the materials. Your suggestions for controlling the temp. are much appreciated.