Introduction: Working With Carbon Heat Rope
Here I cover some of what I learned working with carbon fiber heating rope and techniques for working with it. What is contained here is informational rather than a complete project.
Carbon fiber heating rope is soft and flexible making it ideal for incorporating into heated clothing projects.
- Carbon Heat Rope
- 24 Gauge Flexible Silicone Hookup Wire
- Solder (around 0.025 - 0.032" diameter)
- Solder flux paste with small brush
- Liquid Electrical Tape
- Soldering Iron
- 3rd Hand Soldering Clamp
- Wire Snips
- Small Pliers
- Infrared Thermometer
- Multimeter with Current Measurement
- Grabber Test Leads
- Battery / Power Source
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Step 1: Connecting Wires to Carbon Rope
The wire to carbon connection needs to be both physically strong and have a sufficiently large surface area. My first attempts at doing this involved wrapping the wire around the carbon. But this turned out to be weak, bulky, a poor electrical connection, and can act as a heat coil generating localized hot spots.
A much better method is to bind the carbon and wire together with very thin wires, then apply a small amount of solder to lock them together as a single conductive unit. The resulting connection is very strong and compact.
Carbon pieces should be cut 1.5 cm longer than the desired final length as you can expect to lose a bit over half a centimeter on each end during the connection procedure.
- Cut a 20 cm piece of wire and strip off all the insulation to access the individual conductor strands which will be used to bind the carbon to the electrical wire.
The conductors may be arrange in twisted groups, so untwist the groups to make removing the individual conductors easier otherwise the wire might bind up and get knotted.
- Strip 1 cm of insulation from the wire that will be connected to the carbon and give the strands a gentle twist to keep them together.
If the wire is being connected to extend past the end of the carbon (the most typical configuration), bend the end of the wire up slightly. This will help prevent the loose end from becoming embedded in the fibers and will make trimming easier later.
- Pull out 4 of the fine conductor wires and use them to wrap around the carbon rope and power supply wire.
Using a clamp to hold the carbon and supply wire can make the wrapping step easier. Try to keep the supply wire from becoming buried within the carbon so it can be soldered to the wrapping wire.
- Wrap to a width of about 4-5 mm (for higher current increase connection width).
Make any necessary adjustments such as sliding the wrapped wire towards the end of the carbon. If there is a gap between the supply wire sheath and the wrapping wire, gently push the wire to remove the gap.
- Using pliers squeeze the wrapping wire to flatten the connection and keep everything tight.
- Apply a little paste flux to the wrapped wire (without flux it is difficult to achieve a thin smooth solder layer as the solder prefers to stay on the soldering iron).
- Solder top and bottom of the splice, a clamp is very useful for this step.
- Trim the excess wire.
keep a container nearby to contain the trimmings as the short pieces of wire are sharp
- Apply liquid electrical tape to all the solder connections after you have tested the heating loops.
In some situations you may want to have the power wire run parallel to the carbon rope. Wrapping the wires for this type of connection is easier and can often be done without clamping.
Step 2: Carbon Heat Rope Information
Carbon heat rope can generate very high temperatures so planning and testing is required to prevent burns.
The temperature a loop of carbon rope achieves is a factor of its length, and the voltage used. Near the middle of this page (Temperature vs Length) there are charts for various voltages to provide a starting point. Temperature increases rapidly as length decreases, so very short lengths should be avoided. You should always test the current draw and peak temperature generated before incorporating the heating loops into your project.
If you need long heating loops a higher voltage power source is needed.
Grabber test leads are very helpful during testing. They can be used to connect heat loops to each other, the power supply and a multi-meter.
Series vs Parallel Heat Loops
Sections of heating rope connected end to end in series by wires act as a single loop the combined length of all the individual pieces. For example, three 10 cm pieces linked together, act like a 30 cm piece. This allows you to have multiple smaller heating elements that on their own would get too hot.
Sections of heating rope connected to the power source in parallel each act as if they were connected to the power source by themselves. If you have two 30 cm loops, the total current drawn from the power source is twice what a single loop would draw. For a project with many parallel loops a thicker gauge wire to the power source may be needed.
Heat Loop Design
In general you should find a length of carbon rope that generates the peak amount of heat you desire, then work within multiples of that length by adding parallel circuits.
Carbon Rope Thickness
The carbon rope linked to in the supplies section contains 12 bundles of fibers loosely woven together which can be separated. You can reduce the heat generated for a given length by creating a heating loop made up of fewer of these bundles. Reducing the number of fiber bundles in a heating loop allows you to:
- significantly reduce loop thickness
- increase loop flexibility
- create shorter loops that remain within your desired peak temperature
- balance temperature between loops of different lengths by creating separate parallel circuits
Even with just a few bundles of fibers there will be a minimum safe length. At 7.4V with 4 bundles I was reaching 180F at 16 cm, for that voltage limit the shortest length to around 21 cm.
When working with a different number of fiber bundles take care to test the amount of heat that is being generated as the link to the temperature chart above only applies to the full 12 bundle rope.
You can get an idea of the current draw before powering the circuit by measuring the circuit resistance and divide the power supply voltage by that number.
- Set multi-meter to ohms (Ω)
- Connect one probe to each end of the circuit
For the pictured loop the resistance was 12.2 ohms, with a battery voltage of about 7.5.
7.5 ÷ 12.2 = 0.61 amps.
Measuring current is helpful while testing to verify whether and how much current is being drawn. You will need to limit the current to the capabilities of your power supply / battery. Knowing the current draw will give you an idea of how quickly the heat circuit can deplete your battery.
Measuring current requires a different meter configuration than that used for measuring resistance. The red probe needs to be moved to the A port of the meter. If there are 2 ports, use the one with the higher rated current. The meter needs to be wired into the circuit so that the current flows through the meter as well.
- Set multi-meter to DC A in expected draw range
- Deactivate power supply
- Connect one probe to power supply
- Connect other probe to the heat loop circuit
- Connect other end of heat loop circuit to the power supply
- Activate power supply when you want to begin measurement
An infrared thermometer is a useful tool for checking the temperature of your heat loop segments. The target for the thermometer is very small so slowly move your aiming position until you find the hottest reading.
The most common type of control for heated clothing is a controller that cycles the power on and off at various rates. Many of these controllers are rated for a range of voltages. On ebay / AliExpress there are also battery packs with integrated controllers.
Another option is to is to use either a manual switch or a bi-metal thermal switch. A manual switch would be suitable if your heating loops are designed to provide a reasonable and safe peak temperature and heat is needed intermittently.