Introduction: DIY Heating Pad - (small Electrical Blanket)

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Make yourself a cheap heating pad!

  • Ecological and financial benefit : you don't get to heat your room, just yourself. This pad cost me about 15-20$ to make (I already had the AC adapter and the mosquito net), way cheaper than the one you find in the market. Electrical consumption isn't much (like a laptop): less than 3 cents/night (for this price, you will get a sauna in you bed!)
  • I check it, there is no health issues with the magnetic field: http://www.cancer.gov/cancertopics/factsheet/Risk/...
  • Easy to fix on a chair, an arm chair or/and in a bed (put it under the bed sheet, if you move a lot, you can tie it with a small string)

!!! WARNING !!!

  • It's very important that the heating cable doesn't touch itself, in no point: this could create a short circuit and the cable could become cherry red and start to burn. Some cable have a coating but the coating of the one I bought is very symbolic: a simple scratch and it is off.
  • The length of the cable should be long enough to generate enough resistance (otherwise your AC adapter will burn down) but at the same time, the shorter the cable is, the warmer it will be... but it should not be too short because of the AC charger , but if the cable is too long, the cable won't be warm enough... Well, the tricky part of this project is to find the right length according to your adapter: cf Step 5

Advantage over an electric blanket:

  • Much cheaper
  • hygienic: there is no real need to clean it but you could immerse it in water if you want (since the pad is just made of plastic and metal wire).
  • Much safer and durable: the cable have less risk to touch itself or to be twisted (and then break), so technically it should be usable forever (it's not the case for electric blanket since you need to wash them and since when you move the blanket, the wire get slowly twisted (which is very bad for the cable especially hen it's hot)
  • Very light and not cumbersome: a very important feature for people who move a lot. I have a nomadic life and I can't afford to move with a lot of stuff when I move from one place to another : warm blankets are too heavy and voluminous. This pad is perfect since I can even use my computer's AC charger during the night and if for some extreme reason I cannot carry it, it's fully recyclable.

Step 1: What You Will Need

  • To make the pad:
  1. 50x40 cm pvc coated fiberglass mosquito nets for window (or you can use an old blancket to sew the wire on it)
  2. sewing thread and needle
  3. a heating wire: The difficult part is to determine the type of heating cable and the length you will need.
    • The lenght I choose: for one pad 6m for my first (hot) pad, 7 m for the second (8 or 9 would have worked). The length influences the heat generated by the cable... to make sure you use the correct lenght it all depends on you power source (for me my AC adaptator). (cf Step 5)
  • To connect the pad:
  1. an old AC adapter for a laptop
  2. a switch dimmer : DC 12-24V 8A Adjustable Brightness Light Switch Dimmer PWM (to modulate the voltage, and thus the heat) http://www.ebay.com/itm/181238963504 )
  3. some regular electrical wire (the more flexible they will be, the better it's, but they should be resistant enough)
  4. a screw terminal
  5. a 5.5x2.5 mm Female DC Power Connector Cable plug jack Adapter http://www.ebay.com/itm/A-LOT-5pcs-5-5x2-5-mm-Fema...

Step 2: Sewing the Cable on the Pad

Sew the cable on only one half of the pad, you will then fold the pad in two and sew the edges so that the cable will be protected from both side.

It took me about 1 hour to sew the cable, there may be better way of fixing the cable.

Step 3: Connecting the Pad

Connect the two ends of the heating cable to a screw terminal. You should sew this screw terminal on the pad

Connect two electrical wires from the screw terminal to the dimer. (I used the one I had at home: not very aesthetic, but efficient!)

Connect the5.5x2.5 mm Female DC Power Connector Cable plug jack Adapter to the dimmer

Connect the jack adaptater to the AC adaptator

Step 4: Another Way of Getting Warm

This one didn't work so well, especially because the cable touch itself, and it created a short-circuit. Also the top of "thing" is very fragile.

Step 5: How to Determine the Length of the Heating Cable (a Bit of Electrical Calculations )

Other 5 year old electricty skilled people (like me) might need that!

You must calculate how much resistance you need on your circuit depending on your amps (because if the resistance is not enough high your power supply will have to generate too many amps and thus will either destroy itself (burn) or shutdown)

I use an old computer charger output= 19v 3.16A my wires amps should not be above 2,85A (you must count 10% less than the limit amp for security purposes)

19/2.85 = 6.666 (V/A = it gives the minimal resistance the wire must have)

6.666/1.27 = 5.24 m (resistance total of the wire /divide by/ the wire resistance per meters (the manufacturer should give you this info) = the lengths of the wire needed)

Maximum power my generator can deliver = (amps x voltage) 2.85 x 19 = 54 Watts

with a resistance of 1,27ohms/m we need 5.24 m for 2,85A (10%)

with a resistance of 0.417ohms/m we need 15.9 m

Calcul of the power depending on the length of the wire:

Resistance for 5.24m (the shorter I can have based on the amps) = 6.65

Amps = V/R = 19/6.65 = 2.85

Power = Amps x V = 54

Resistance at 7 m =8.89

Amp = 2.13

power = 40.47W

Resistance at 10m = 12.7

amp = 1.50

power = 28.43W

For my heating cable with my AC adaptator, 6m and 7m are perfect length

when my wire is wrapped the 7m (power 40W) is quite hot but I can still hold it in my hand but when it's 6 m length (power 48W) it's too hot.
when the wire is sew in the pad none of them are too hot to by hold in my hand (when the wire is wrapped, the heat generated is warmer)

Step 6: Other Heating Wires Options

A: Carbon fiber flex heater tape = 8€/m= 80€ (best because infared heat but too expensive, cancer concern, and only available in russia... )

B: silved plated copper = 1€/m (sold 30m = 30€) : http://fr.rs-online.com/web/p/fil-pour-environnem...

C: kanthal = cooper alu et iron = 0,6/m (sold10m = 6€)

D : Nichrome

E: multistranded Teflon coated wire http://fr.rs-online.com/web/p/fil-pour-environnem...

non-insulated: http://facil-ecig.com/coils/651-fil-resistif-kant...

non-insulated 0.975 Ω/m 22m = 32eur : http://facil-ecig.com/coils/651-fil-resistif-kant... http://www.conrad.fr/ce/fr/product/604622/Cble-ch...

1,8Ω/M for 25m = 52eur : http://www.tibtech.com/smartshop/product.php?id_p...

not enough: http://www.ebay.co.uk/itm/Silicone-Waterproof-Hea...

other options (not sure it would work):

http://www.zivipf.com/epages/63862298.sf/en_GB/?V... http://fr.made-in-china.com/co_citcable/product_T... http://www.zivipf.com/epages/63862298.sf/en_GB/?V...

PTFE (Polytetrafluoroethylene) = teflon

Step 7: Others GOOD TUTO:

  • trousers on motobike:

https://www.instructables.com/id/DIY-heated-clothin...

At 48W he says the wire burns him. (Amp= 2.526 / resistance of = 7.5177 / length= 5.92 m for a power of 48)

  • electric blanket (made by the seller of the carbon fiber flex...) :

https://www.instructables.com/id/DIY-carbon-heated... http://endless-sphere.com/forums/viewtopic.php?f=...

  • jacket:

http://www.westcentralmopolarbears.com/documents/...

Step 8: Feed-backs

  • Feed-backs after few months
On a chair: to fix the pad, I sew the two little red pieces of fabric, but it I found out later that it's better to sew 4 of them, one at each corner.In a bed: you can use a string to fix the pad to the spring-bed under the mattress. But I don't have a spring-bed, so I use another sheet that I fold in the widthwise direction, put the pad in between the folds and put this folded sheet under the fitted sheet and put the two extremities under the mattress.I made two pads, one 30 x 40cm and the other one 20 x 30 cm and I definitely prefer the big one more comfortable. (the big one use 7m of heating cable the small one use 6m)During the night and when I am sitting, the dimmer is set to about 10%-15%. I never need to set it up higher, as this generates more than enough heat to feel warm. The only time when I set the dimmer to more than 15% is just before going to bed: 3 to 5 minute at 100% is perfect to warm the bed up.
  • Feed_backs after a year:

I am using for the big one (7m) about 6W per meter : both are too hot, so I only use it with the dimmer at about 30%. So I was thinking about spacing more the wire BUT it's a very bad idea because the wire will get too hot if I use the dimmer at 100%. (It might burn the pad and even the mattress, cf. the orange line on he pad: the pvc/glass fiber net turn orange with the heat). So I decided to increase the length of the heating cable to increase the resistance. I will also space the wire a bit more from 4cm to 6cm.

The small pad doesn't fit for the bed, it get crumpled. It's nearly impossible to get it stay straight.

It made some loops to enroll the extra wire but it's a bad idea as the heat it burned the pad (and even the mattress) at the location. (cf the images).

Based on my experience, the heat generated by about 2 to 2.5 W per meter is enough to get warm. with this configuration