Carbon Tape Heating Pads

My toes always get cold when I go for a bike ride in the winter, so I use electric heating pads to keep them warm. I prefer for the heating pads to be wider than my insoles so that when they are placed inside my boots the pad wraps around the outside of my foot as the outer toes get colder than the rest.

I've used carbon rope for heating in the past, but this is my first project using 15 mm carbon tape which has a larger surface area. The techniques shown can be used to make heat pads for a variety of purposes.

• 15 mm Carbon Heat Tape
• 24 Gauge Flexible Silicone Hookup Wire
• 24 AWG Silicone Parallel Conductor Wire
• Solder (around 0.025 - 0.032" diameter)
• Solder Flux
• Copper Tubing: 7mm outer diameter with 0.2mm wall thickness
• 1/4" drill bit

8mm with 0.2 wall thickness if available should better match the tape width but I only had 7mm

• Flexible Silicone Adhesive
• Non-stick parchment paper
• Heat Shrink Tubing

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

Crimped copper ferrules makes a strong physical and electrical connection between the carbon fiber and electrical wire. I couldn't find a source of suitable small crimps, so I make them from thin walled copper tubing. 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 1/4" drill bit which is roughly the size of the 7mm tubing. 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 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 4mm 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 6-7 repetitions you will have performed a half rotation and a small ring should break free.

• Slide the ring off the end of the drill bit.
• If necessary use wire snips to break off any pointy protrusions.
• Pull the drill bit out of the copper tube.
• 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.

Note: The carbon material is capable of achieving extremely high temperatures if the heat loops are very short.

The temperature of a heating loop is determined by the length of carbon tape used and the voltage of the power source. In general you need to adapt your project to work with the heat loop length that provides the desired peak temperature and not lengthen (less heat) or shorten the loop (more heat). Consult the charts for 15 mm heat tape here: Temperature vs Length. The chart also includes current draw which is helpful for selecting a power source.

To heat a larger area you would need to use multiple heat loops, each the same length. You can create 2 heat loops from a single double length piece of heat tape by having a positive wire at each end and a negative at the mid-point. Or you could have two equal length pieces sharing the same wiring connection points.

For this project I am using a 9 volt USB-C power source. A 50 cm long heat loop gives me a measured peak temperature of around 130F. Switching to 12 volt would allow for a longer loop at the same temperature. I also use a controller to dial back the output and don't usually run them at maximum continuously.

I find it easiest to template out the heating pattern with paper strips before preparing the carbon tape. In my case I made a paper strip 15 mm wide and 50 cm long (needed to tape two pieces of paper together). The heat loop must not intersect itself or another heat loop. Some folding at the bends is acceptable.

While planning the layout you should also consider the path the electrical wiring will need to take. Both wires should meet up at one spot to exit the heat pad so the power supply wire can be connected.

For additional information see my other Instructable: Working with Carbon Heat Rope

Note: Ensure you use a long enough piece of wire that it will reach the exit point of the heating pad. If not inconvenient you can wait until the heat loop is laid out before cutting the other end of the wire from the spool.

• Cut a piece of carbon tape for desired output (add an extra 1-2 cm for trimming), for this project 52 cm.
• 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 22 gauge hookup 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.

It is a good idea to test the heat output before it is sealed inside the heating pad. You can use an infrared thermometer or multimeter thermocouple.

The carbon fiber heat loop should be enclosed in a covering material for two reasons. The loop needs to be prevented form touching itself to keep it from shorting. And the carbon fibers that make up the tape are extremely fine and can snag / fray if not protected.

There are several different ways to do this. You can use double sided fabric tape or heat fused hem tape to bond two pieces of fabric around the heat loop. I usually arrange the tape to create a pocket which gives me the option of removing the heat loop in the future.

What I did for this particular project was laminate the heat loop between two pieces of polyethylene plastic using a clothes iron. I used a heavier weight zipper freezer bag, but a laminating pouch would also work. I have also used vapor barrier plastic which is even heavier than laminating pouches and ends up being more rigid.

Laminating the Heat Pad

• If using a heavy poly bag cut open the sides.
• Place the paper heat loop template under the plastic as a guide.
• Lay out the heat loop (use small pieces of tape to hold it in place on the plastic).
• Arrange the wires so they come together at the desired exit location.
• Fold over the top layer of plastic.
• Place baking parchment down on an ironing board, place the heat pad on the parchment, then place another piece of parchment over the heat pad. This will prevent the plastic from sticking to the ironing board or iron during heat bonding.
• Run an iron over parchment covered heat pad to laminate. You may need to repeat this a few times. Around the wires can be tricky, you likely won't get a perfect seal as the plastic won't stick to silicone wires but that is fine.
• Trim the excess plastic.

The power wire I'm using has 4 bonded wires, so I split it in the middle.

• Strip the ends of both heat pad wires and both power wires.
• Slide a piece of heat shrink tubing over each of the heat pad wires.
• Apply flux and solder the power wires to the heat pad wires.
• Slide the heat shrink tubes over the solder connections and shrink with heat.
• Add a connector to the other end of the power wire (be sure to slip the shroud over the wire first).
• If using plastic to seal the heating pad, add some flexible adhesive where the wires exit the plastic as the plastic will not bond to silicone wires.

I've been using USB-C power banks to supply 9V for my heated projects with good results. USB-C PD Power banks are easier to source and work with than raw batteries, and many support 9, 12, 15 and 20 volt output. The higher voltage available through the USB-C PD feature allows for longer heating loops than would be possible with a 5V power bank. The following Instructable shows how to construct a USB-C PD cable and controller for heating pads / heated clothing, Heated Clothing Controller.