Heating Pad Failure Build Your Own How Not To Start A Fire

[John1397]

I have a little heating pad 12" X 15" runs on AC 120 volt has around 220 ohms resistance on low final got to hot burned up wire. What to replace it with using junk box parts light bulb wastes 10% for light would need metal cage to keep bulb from breaking seems like not good way. Have 300 feet coated 40 gauge copper wire only problem would be keeping that much wire all separated so does not short out almost impossible. Then I come up with what I think should work change 120 Volt AC to 12 Volt AC or similar voltage that transformer may put out. If you use 12 Volts AC and 27 gauge wire 60' feet this would be easier to manage. Just take the volts the transformer puts out X volts / divided by watts this will give you resistance then find copper wire with that much resistance and it should work will it not? I am think'n old printer cables or monitor cables have fine stranded wire which would stand up good under bending conditions.

 

[anorlunda]

The power calculation is trivial $P=\frac{V^2}{R}$ but that is not the problem.

Are you talking about a heating pad that you put against your body? If so, there are plenty of safety concerns for a DIY version at 120VAC.

12VAC is of course safer, provided that the power supply/transformer is properly housed. But 27 gauge single strand wire, I suspect might easily break due to fatigue or tensile forces when repeatedly bent and stretched.

I find it hard to imagine that this is a fruitful DIY project.

 

[jrmichler]

If you use 12 Volts AC and 27 gauge wire 60' feet this would be easier to manage. Just take the volts the transformer puts out X volts / divided by watts this will give you resistance then find copper wire with that much resistance and it should work will it not? I am think'n old printer cables or monitor cables have fine stranded wire which would stand up good under bending conditions.

That is exactly what these people do: https://www.aerostich.com/clothing/heated-gear. I seem to recall (too lazy to look for it now) that they discuss this somewhere on their web site.

You are correct about old low voltage cables having fine stranded wire. Also volts is volts and ohms is ohms, so copper wire should work just fine. And multiconductor cables have several wires than you can connect in series. That way you retain the strength of the cable and get, for example, 30 feet of wire in a four conductor cable 7.5 feet long.