Working With Carbon Heat Rope

Carbon fiber heating rope is soft and flexible making it ideal for incorporating into heated clothing projects. 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.

One of the biggest challenges of working with carbon fiber for heating is connecting electrical wire to the fiber. The connection need to be both physically and electrically strong. My earlier attempts at doing this involved wrapping wire around the carbon. However this resulted in very high localized heating at the connection point which I measured as high as 240F. I thought I had solved the problem by encasing finely wrapped wires in solder, but the issue would reappear even on connections that tested fine during construction. The ideal solution are tiny copper ferrules / crimps, but I was never able to find any small enough. So I decided to make my own!

• 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
• Copper Tubing: 4 mm outer diameter with 0.2 mm wall thickness
• 9/64 drill bit
• Liquid Electrical Tape

Suggested Equipment

• Soldering Iron
• 3rd Hand Soldering Clamp
• Wire Snips
• Small Needle Nose Pliers
• Fine File
• Diagonal Cutters
• Infrared Thermometer
• Multimeter with Current Measurement
• Grabber Test Leads

Other

• Battery / Power Source
• Connectors

Using a 300 mm long copper tube I was able to make a little over 100 copper ferrules each a bit over 2 mm wide. One copper ferrule is needed for each fiber to wire connection.

Copper is easy to cut, however the tubing will crush unless it is supported internally during cutting. I did this by inserting a 9/64 drill bit which is roughly the size of the tubing I purchased. The drill bit also has the added benefit of deburring the inner part of the tube after each cut. The drill bit shank will get very marked up during this process, so it would be best to use a cheap or old one.

• Slide the drill bit into the copper tube with the shank end at the top.
• Position your diagonal cutters approximately 2 mm from the end of the tube.
• Squeeze the handles with moderate force.
• Ease up while maintaining contact with the tube.
• Rotate the tube a little, then squeeze again.

After rotating and squeezing 4-5 times you will have done a half rotation and a small ring should break free.

• Slide the ring off the end of the drill bit.

If I couldn't slide the ring off with the cutters, I'd grab the uncut part of the tube near the end and hit it against the desk to push the tube and cut ring to the end of the drill bit. The more marked up the drill bit becomes, the more difficult it will be to remove the small rings. If it becomes very difficult file the drill bit smooth now and then.

• Pull the drill bit out of the copper tube (use pliers if it is tight).
• Lightly file the end of the tube to remove sharp edges.
• Re-insert the drill bit for the next cut. This should remove sharp edges on the inner lip of the tube.

• Carefully slide a copper ferrule over the of the carbon rope with the rough end of the copper pointed towards the end of the rope.

If the end of the carbon is frayed you can use a strand of hookup wire conductor to temporarily cinch it together to get the ferrule in place.

• Holding the carbon rope in one hand, use needle nose pliers to hold the copper ferrule.

The carbon rope I use is flat, so I hold it in such away that the flatten ferrule aligns with the flat side of the rope.

• Slide the ring to about 2 mm from the end of the carbon rope.
• Press the carbon rope to one side of the ferrule, then gently squeeze copper ferrule to partially close it until the carbon rope is held in place. A gap should remain on one side into which a wire can be inserted.
• Strip insulation from the end of the silicone hookup wire and give it a twist to keep the strands together.
• Insert the wire into the ferrule's open gap.

Keep in mind whether you want the wire to extend past the end of the carbon rope or double back along it.

• Use the needle nose pliers to firmly clamp down on the ferrule locking the carbon rope and wire in place.
• Using the needle nose pliers, grab the side of the ferrule opposite of the wire, and bend it into an L shape. I do this by pressing the opposite side into my desk.

Whether you bend "up" or "down" depends whether you want a flat spot to the left or right of the wire.

• Reposition the pliers and squeeze to finish bending the ferrule back onto itself.
• Apply flux to the wire, then solder the wire for additional strength. There is no need to coat the entire ferrule.
• If the wire is doubling back along the carbon rope you can trim the excess wire / carbon from the end.
• If desired coat the connection with liquid electrical tape.

I've been working with carbon rope for all my projects since it is easy to route through tight bends, but the 15 mm carbon heat tape seems to be more popular. I tried a quick test and the copper ferrule technique may also be effective for the carbon tape if a larger copper tube is used. Fortunately I had ordered an assorted box of copper tubes. The largest tube in the set had a 7 mm diameter, which also appears to be the largest tube that uxcell sells with a 0.2 mm wall thickness.

A 1/4" drill bit fits inside the 7 mm tube to reinforce it during cutting. I made these rings a bit wider than the ones I made for the rope.

Preparing the Copper Ferrules

• Slide the drill bit into the copper tube.
• Position your diagonal cutters approximately 3 mm from the end of the tube.
• Squeeze the handles with moderate force.
• Ease up while maintaining contact with the tube.
• Rotate the tube a little, then squeeze again (more cuts are required than for the smaller tubes).
• Slide the ring off the end of the drill bit.
• Pull the drill bit out of the copper tube.
• Lightly file the end of the tube to remove sharp edges (the ring may also need filing).
• Re-insert the drill bit for the next cut. This should remove sharp edges on the inner lip of the tube.

Attaching Carbon Tape to Wire

• Fold the end of the carbon tape in half twice, then slide the copper ferrule over it.
• Straighten out the tape so it exits the ferrule in a U shape.
• Gently flatten the copper ferrule, but do not squeeze it tight, then straighten out the tape again.
• Push the carbon away from one end of the ferrule and insert the wire.
• Squeeze the ferrule flat against the carbon and wire.
• Using needle nose pliers bend each side of the copper ferrule upwards. Start with the side containing the wire to prevent it from coming loose.
• Continue bending both sides inward and crimp flat.
• Apply flux to the wire then solder it to the ferrule.
• Trim the excess wire end and loose carbon tape.

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.

Loop Resistance

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.

Current Draw

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

Testing Tip

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

Temperature Testing

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.

I have an Instructable for creating a heated clothing controller here: Heated Clothing Controller

Heat Controllers

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.

Manual Switches

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.

Thermal Switches

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.

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-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-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-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 unless a heating load wasn't connected.

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-C Male PD Decoy - Useful for making a power cables at desired voltage.
• USB-C Female PD Decoy - USB-C socket, would require a USB-C cable to use.
• USB-C Male to Male Coupler - Can be used to connect female PD decoy directly to a battery. Generally it is better to just use a male PD decoy instead.