Insole Electric Heating Pads

743

8

About: Year round cyclist, hot, cold and everything in between.

Keeping my toes warm while cycling in the winter has been a problem for many years. Pedaling keeps the pressure on the front of the foot continuously which seems to impede circulation causing toes to get cold more rapidly than when walking. This is further compounded for those of us who suffer from Raynaud's where our body cuts off circulation to the extremities in response to cold exposure.

Chemical warmers are bulky, often don't fit inside boots, and cease to function without oxygen for the chemical reaction.

My solution is a pair of thin vinyl pads that can be inserted into any footwear and connected to batteries to deliver heat for hours. Although this design is for keeping feet warm these pads could be useful for other purposes.

Step 1: Overview & Design

Process

This project involves sandwiching carbon fiber heating loops between 2 layers of adhesive vinyl and wiring each to a 7.4 volt battery. The temperature generated drops as the length increases which is why 2 loops are connected in parallel. Since we are creating an electronic circuit the carbon fiber needs to make a loop without crossing itself, or the other loop. A switch and power plug may also be added.

Vinyl Selection

The Oracal 651 vinyl I selected has a strong permanent adhesive and is designed for several years of outside use. Other vinyl sheets with permanent adhesive may also be suitable. Vinyl sheets such as shelving liners with non-permanent adhesive do not have a strong enough bond.

Carbon Fiber Heat Rope

The carbon heat rope linked to in the shopping list generates many times more heat than is needed in this heat pad application. Fortunately the rope is a loose weave of 12 smaller bundles of fibers which gives us the opportunity to both stretch our material further and lower the heat to a more appropriate level. The heating loops made with 3 bundles will reach a peak temperature around 50 °C when plugged directly into a 7.4v battery, drawing around 0.5 amps. Since the heat loop has low thermal mass, your feet will pull the heat out of it fast enough to keep the pads from feeling excessively hot.

Battery Life

A heating pad drawing a constant 0.5 amps (not accounting for a battery being affected by the cold) might run for more than 4.5 hours on a 2,600 mah Li-ion bike headlight battery, or 2.5 hours on a 1,500 mah LiPo drone battery.

Step 2: Shopping List

Items you may need to order along with links to sample items:

Optional items depending on design choices:

  • Power Connectors (varies with battery selection)
  • Flashlight Push Switches x 2
  • Power LED x 2
  • Multi-setting Power Controller x 2

Other items you may already have:

  • Soldering iron and solder
  • Heat source for shrink tubes (BBQ lighter works well)

Step 3: Prepare the Heating Loops

Note: The temperature the heat loops achieve depends on length, number of bundles and battery voltage. The directions listed below should not be modified without testing to determine how changes affect the heat generated and current consumed.

Separating Rope Bundles

  • Cut a piece of carbon fiber heat rope 39 cm long
  • Squeeze about an inch from one end of the rope to separate the bundles
  • Slowly pull one bundle until the rope begins to bunch up
  • Use one hand to gently propagate the bunch downward until the rope is straight again
  • Slowly pull the individual bundle again until it comes out
  • Repeat until all bundles are separated, it will be much easier once the first 2 bundles are removed

Making the Heating Loops

  • Cut 2 equal lengths of wire
    • this wire will need to go from the toe of the footwear, under the insole, and out the back of the heal to the battery so length will depend on footwear / battery placement
    • one option might be to have the wire long enough to get to the outside, then an extension cable to a battery
    • I have winter boots and attach my battery to the boot, 65 cm was sufficient for me
  • Strip about 2.5 cm of housing from on end of each wire
  • Arrange 6 bundles of rope into 2 groups of 3
    • while working try to keep the 2 groups from twisting around each other
  • Line up one end of all 6 bundles and clamp with an alligator clip
  • Tightly wrap the wire around the carbon fiber, trim excess fiber
  • Cut a piece of heat shrink tubing long enough to cover the joint with a little overlap on each side
  • Slide heat tube down the wire and over the joint, and heat to shrink
  • Repeat previous 4 steps for the other end of the carbon fiber
  • Keep the 2 groups of 3 separated and set the heating loop aside

Step 4: Prepare Vinyl Sheets

  • Take a 12" x 12" vinyl sheet and cut it into 4 6" x 6" squares
  • From the 6" squares cut off a 1" strip
  • Trace the outline of your footwear's insole onto a piece of paper and cut out
  • Use the paper insole outline to trace the upper part of the insole onto the back of one of the vinyl rectangles
    • leave a 1 cm gap from the top
    • the short ends of the vinyl rectangle should be at the top and bottom
    • two of these will be required

For the remainder of construction do one insole at a time to avoid exposed adhesive mishaps

  • Peel off the backing from the vinyl and set the vinyl sheet aside
  • Cut out the insole shape from the backing material in a single piece
    • stop 2 cm from cutting through and cut horizontally over towards the start of the cut
  • Carefully place the outer cut piece back onto the vinyl sheet leaving only the insole shape exposed
  • Use a fine tip marker to trace the border onto the vinyl (skip bottom part), to help guide trimming later

Step 5: Plan Your Heat Loop Pattern

The carbon rope frays very easily so you can't pull it back up after putting it on the vinyl adhesive. Therefore you need to template your heat loops using a different material. The loop template string is a few cm shorter than the carbon fiber to compensate for the electrical joints. If you have transparent vinyl you can layout your pattern on paper using string and tape, then trace over that directly for each of the two pads. With colored vinyl a little more effort is required as you need to draw out the template twice.

In order to get heat coverage on the small toe, the heat loop needs to be lower on that side of the foot. On my pad that was about 11 cm down from the top. You can use your paper insole cutout to determine how far down you should go.

Heat Loop Pattern

Apart from the start and end, no parts of the loop should touch each other.

Clear Vinyl

  • Trace the upper part of the insole cutout onto a piece of paper
  • Cut out the trace leaving a border
  • Tape the piece of paper to the work surface so it doesn't shift around
  • Cut two pieces of string 37 cm long
  • Draw the heating loop pattern leaving about 1 cm hanging over the border at the start and end
  • It usually takes several tries to get a good pattern

Colored Vinyl

  • Cut two 37 cm pieces of polyester sewing thread
  • Draw the heating loop pattern leaving about 1 cm hanging over the border at the start and end
  • It usually takes a few tries to get a good pattern
  • Use a fine tip marker to trace the heat loop pattern
  • Remove the sewing thread

Step 6: Attaching the Heating Loops

Again the carbon rope will fray easily if it touches the adhesive and is then pulled away, so try to avoid that as much as possible.

  • Use the insole cutout piece of vinyl backing to cover parts of the adhesive outside your immediate working location. Move it around as necessary
  • Leave about 1.25 cm of rope outside of the border, this will allow the wires / heat shrink joint to be wrapped to the under side of the insole.
  • Using your thumb and index finger try to lay down the 3 bundles of fibers roughly parallel to each other. This will minimize the thickness of the pad.
  • When you reach the end of the loop also leave 1.25 cm outside the border even if it means cutting your pattern a little short.
  • Once the second loop is in place, begin to peel off the remainder of the vinyl backing
  • After passing the first wire, lay the wire and its fiber down pointing outward
  • The fiber near the second wire might be a bit loose so just give it a half twist before laying it onto the vinyl

Step 7: Sealing, Trimming and Installation

Sealing and Trimming

  • Peel the backing off another vinyl rectangle
  • Carefully line up the edge with the bottom of the heat pad
  • Slowly spread the top sheet down towards the top of the heat pad
  • Thoroughly press the two sheets together, pressing down around the heat loops and wires
  • Even with colored vinyl you should be able to see the insole marker outline through the vinyl to use a trimming guide
  • Stay a few mm away from the heat loops and wires to ensure the pad stays sealed

Installation

  • Tape the pad to the insole using the first aid tape
  • Wrap the wires to the back of the insole and secure the wires along its length with tape

Step 8: Batteries & Controller

Battery
Each pad is powered by a 7.4v battery pack. They are available in lithium polymer or higher capacity lithium ion. Many suitable batteries can be found on AliExpress, some even include built-in multiple power level output.

Power Controller / Switch

There are several multi-setting heated clothing controllers available through AliExpress that can be connected inline with a battery pack. One source I found for batteries / power controllers is here: https://www.aliexpress.com/store/2683007?spm=2114.12010108.pcShopHead_8168168.0

Pad Connector

Attach a male DC barrel connector to the pad that matches the jack of the battery. Common sizes are 5.5 x 2.1 mm, and 3.5 x 1.35 mm.

Then you should be ready to go enjoy several hours of heated toes outdoors.

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