Introduction: Working With Carbon Heat Rope (Do Not Use, See Note)
I've experienced intermittent issues with the wire wrapping technique discussed in this Instructable and strongly recommend against using it. If solder is not perfectly applied the connection can act as a heating coil and get as hot as 240F. A few connections I made that initially tested fine, later developed problems.
A revised method which is easier and won't have that issue can be found here: 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
- 22 AWG silicone parallel conductor wire (optional for extension cables)
- 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
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 complete splice covering all wrapped wire, a clamp is very useful for this step.
Be sure there are no wrapped wire strands exposed otherwise the connection may get extremely hot!
- 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
It is helpful to keep a male and female connector around with wire leads attached to them. This will allow you to measure current draw or resistance for items where you have already have installed the connector
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.
Multimeters with thermocouple probes provide more accurate temperature readings than IR thermometers which require hunting for the hot spot blindly.
Step 3: Temperature Control
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. The on / off cycles are somewhat long so the temperature can spike towards the end of the on cycle, which is undesirable if you heat loops are over powered or you switch to a higher voltage power source.
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.
The thermal switches I ordered are bulky so I haven't incorporated them into any of my projects.
Pulse Width Modulation (PWM) Controllers
PWM motor controller / LED dimmers provide the widest range of temperature control. These controllers pulse the power on / off at rates varying between 0% and 99% (clicks to off in far left position). The heat is more even than the push button controllers as the power is rapidly being switched on / off without any long pauses. The main draw back is that these controllers are inherently bulkier and need to be built into a container.
I wired one along with an LED (with resistor) into a small project box (I also added a push button on/off switch). Even using a voltage that would make one item unusably hot, I was able to dial the heat right back to barely noticeable.
There is a PWM controller rated for 5-16V that seems ideal for this usage. It lacks the large capacitors that the controllers rated up to 35V contain.
Step 4: USB Power Source Options
USB Power Banks, particular USB Type-C ones that support Power Delivery (PD) offer a convenient way to power heated clothing.
Standard USB is 5V and I believe most power banks will supply 2 amps for a maximum power of 10 watts. At 5 volts heating loop lengths can't be as long as at higher voltages. USB 2.0 to female DC power jack adapters can be found online.
USB Type-C PD supports 5V, 9V, 12V, 15V and 20V at 3 amps for a maximum power between 10 and 60 watts. So you may want to tailor your heating projects to 9V or 12V to support longer heat loop lengths. In order to use an alternate voltage you will need a small USB Type-C PD trigger / decoy module for the desired voltage. These modules come in male and female versions. The male ones work best as they can be plugged directly into a power banks, a female board will require either a USB-C cable or coupler.
One issue with USB power banks is that most will automatically go into sleep mode if very little power is being drawn. The one I'm using will do this in 10-15 seconds. Controllers with an electronic power switch likely won't work without first manually waking the power bank. USB Type-C PD can aggravate this issue as the voltage can be interrupted during the switch from 5V to a higher voltage triggering a reset of the controller, followed by the power bank going back to sleep if the switch isn't turned back on before the timeout is exceeded. In my testing the PWM dial controllers didn't have any issues with the power bank going to sleep.
Step 5: Component Sources
Several of the electronic components only seem to be available through AliExpress, Banggood or ebay. I'll provide a few sample links below as there are often many similar variations so it can be tricky to find the exact item through search terms alone. Most components have multiple sellers. Keep an eye on shipping price when you use these sites, some sellers multiply the shipping for each individual component even if they are tiny.
- Female DC Power Jack Plug Socket Connector 3.5 x 1.35mm - For one end of extension cables. These were very hard to find, most are pre-molded onto a wire requiring splicing.
- Female DC Power Jack Socket Panel 3.5 x 1.35mm - Useful if building a controller in a small project box. Compact and nut secures it in place.
- Male DC Power Connector 3.5 x 1.35mm - These are a lot more common.
- USB 2.0 Male to DC 3.5 x 1.35mm Female Socket - Connecting to 5V USB power bank.
- DC Motor Speed Control PWM LED Dimming 5-16V - Many sellers for this one "5-16V pwm" is enough to bring them up. This appears to be the most compact one available. Has screw terminals, no soldering required. The 4.5-35V ones have large capacitors on the board.
- USB Type C PD Decoy - These listings usually sell various voltages. Sometimes the different voltages are listed as alternate "colors".
- USB Type C Male to Male Coupler - USB Type C power banks and PD decoys both have female sockets so a male coupler is needed.