Heating Pad for Cat

Cats like it warm, so kitty will love a warmed-up basket, especially in a non-heated room. Heat pads are for sale in pet shops, but since it can easily be made from mostly scrap material, we decided to make one ourselves. All it takes is an old towel, an old power supply and some NiChrome wire, plus a bit of soldering and sewing. Happy cat guaranteed!

NB: particular care should be taken about safety, in particular fire:

• Use a cotton towel, no synthetics.
• Keep the current low enough (<300mA) that the wire does not heat up too much
• Avoid short-circuits by proper layout of the heating wires and proper fixing of the top and bottom layer.
• Provide strain-relief for the cable
• Do not use power supplies with excessive voltage (>12V) or current (>2A)

DO NOT ATTEMPT TO BUILD AN ELECTRICAL HEATER UNLESS YOU ARE 100% SURE ABOUT HOW TO USE ELECTRICITY IN A SAFE WAY AND FOLLOW ALL FIVE SECURITY MEASURES MENTIONED!!!

• An old cotton towel, at least 3x larger than the size of the heating pad. 40x70cm will do.
• A 9V 1A power supply. Check your basement for any old power supply from a printer, router, phone, toy etc.
• 3,5m of 32-Gauge (0.2mm) NiChrome heating wire (10m rolls sell for circa 1.5EUR on AliExpress)
• TOOLS Soldering iron, needle and thread

The right amount of power is in the range 5-8W, spread out over a surface of 25x20cm. It will hardly feel warm at first touch, but together with the body heat of the cat, it is just right.

I used AWG32 NiChrome wire, which has a resistance of 37Ohm/m. A 1m wire connected to 9V will give a current of I=V/R=9V/37Ohm=0.24A, and develop a power P=V*V/R=9*9/37=2.2W. This means that three 1m wires in parallel will draw a total current of circa 0.72A and deliver 6.6W, just right!

With a 5V, 2A power supply you can reach similar results using 5 wires of 55cm each. With a 12V, 1A power supply, use 2 wires of 135 cm.

The important constraints are that:

• The total power does not exceed 8W (unless you spread it out over a larger surface)
• The current per wire does not exceed 300mA (otherwise the wire gets too hot)
• The total current does not exceed the current that the power supply can deliver.

Some things to watch out for when designing how the wires should be laid out are:

• The heat should be distributed in a uniform way over the heated area.
• The possibility and consequence of short-circuits should be minimized
• Local hot-spots should be avoided.

The layout in the picture satisfies all these constraints:

• A zig-zag pattern spreads out the heat well everywhere
• Points with large voltage difference are kept far away from each other
• The three parallel wires do not meet in the same point.

For the case of a 9V supply with three 1m-wires, I drew a grid of 17x15 dots, with a spacing of 1.5cm in the centre of the towel. Make sure that there is an equal amount of area both on the left and on the right, as it will later be needed to fold and sew and protect kitty from the wires. Then draw the pattern of lines connecting the dots. Note that the three lines have exactly the same length (70x1,5=105cm).

The wire is thin and strong enough that it can be ‘sewn’ into the towel without a needle or tool. Follow the pattern on the grid, where at every dot you go through the towel and change top-side for down-side or vice versa. First cut the wires to a length that is a few cm longer than what is strictly required. For each wire, it is easiest to start in the middle of the pattern then work each half up to the end.

About the choice of power-supply: make sure that the voltage matches the wires and that it can deliver sufficient current. Avoid power supplies that are too powerful, as they may create a fire hazard in case of short-circuit. Computer power supplies are too powerful and should be fitted with a fuse. An amperage that is 1-1.5 times the required current is ideal.

Remove the connector from the power supply. Make a small hole through the towel and lead the cable through it. Make a knot in the cable, this will prove strain relief and protect the electrical connections in case the wire gets pulled on.

Strip the final 4cm of the cables clean. Solder at both ends the 3 heating wires to the copper, respecting the 1.5cm distance between them (otherwise you create a hotspot).

On the picture, the towel is connected through a JST connector to the power-supply, but this is not really needed, it can be connected directly just as well.

Kitty should not be in direct contact with the wires, and it should be possible to fold the towel without making a short-circuit.

Fold the towel in three, where one empty third of the towel covers the top, and another third covers the bottom. Sew it all around, but also sew a few a bit through the middle parts, this way the whole unit stays well together, and again it helps to avoid shorts even in case the cat decided to fumble up the towel.

For occasional use, just plug in when needed and plug out when done. Adding a switch to the cable may make it easier to switch on and off. The heater can also be attached to a timer in case it is needed on a daily basis at fixed hours.