I loved the idea of electric handwarmers for ages, but I could never figure out how to build a safe one, until now. This is a good little project for people who live in cold regions and don't like to microwave or boil handwarmers. You could turn a few parts from an electrical store and from home into a custom heater. This is a relatively safe handwarmer compared to ones made out of dangerous light globe filaments.
DISCLAIMER: I do not take any resbonsibility for any burns you may receive. DO NOT touch the resistor when plugged in, it gets hot so use a cloth. You do this instructable at your own risk.
10 W 10 Ohm wire wound resistor
Power Adapter (I used 7.5VDC 500mA, but you could use more or less as long as it's below 10W)
Step 1: The Resistor
Find a 10W 10Ohm wire wound resistor. It should say something like pw10 10R. These numbers are important. If the resistor says something like pw1 than it's going to have a meltdown as soon as it's plugged in. The resistor may look different because they come in all different shapes and sizes.
Step 2: The Adapter
The adapter output should be below 10W. The higher the wattage the higher the temperature. BE WARNED: This adapter made the resistor burning hot, I suggest to use well below 10W otherwise the resistor could burn up everything around it! If you are use anywhere near 10W you should use a insulator or something.
I will be using a 7.5VDC 500mA power adapter. This will heat up the resistor enough to melt hot glue, (and burn like a hot glue gun). But don't worry the resistor is designed to take almost three times that heat.
It is better to use a transformer adapter. It should be heavier than a normal adapter.
Step 3: Strip & Solder
Chop off the plug on the end of the adapter and then strip the red and black wires. Now solder these to each end of the resistor, the polarity doesn't matter as long as the two wires don't touch.
Step 4: Test
To not burn yourself during the test you will need some fabric. I used a random cloth thing. Now cover the resistor with the fabric. Plug in the adapter and the resistor should heat up within a few seconds. You can touch the fabric but not the resistor itself. Don't worry the resistor does not keep getting hotter, it reaches the max temperature (depending on the power) and stays. Remember to let the resistor cool down afterwards.
Step 5: Upgrading
This instructable is just for the basic heater, it can be modified in many ways. Here's some ideas:
Line the resistors up in parallel to create a bigger heater. Each 10W resistor lined up adds 10W to the max power so you'll need a bigger adapter.
Install some of these into a teddy or a wheatbag.
Put a fan behind some and create a fan forced heater.
You can do anything as long as you don't exceed the wattage or use near a highly flammable substance.