Newbie - How to choose and connect a heating wire

Hi all !

I'm a newbie in electronic and I've a project about creating a heating cap to protect myself during the winter since I work a lot in the cold. I need to insert a heating wire in my cap and control his power in order to give me a nice warm feeling.

Since I don't really want to burn myself (not so funny...) and the wires I'm finding are primarly used to cut foam... I'd like to have some advice about it.

And finally, how should I connect it to the Duino ? I thought through a MOSFET could be ideal though I want a regulation. Do I need some additional resistor ?

Thanks for reading me,

(Sorry if my english sucks, I'm french)

Do I need some additional resistor ?

No.

I thought through a MOSFET could be ideal though I want a regulation.

Yes it is. Use a PWM pin to drive it to achieve regulation.

the wires I’m finding are primarly used to cut foam.

Use the same sort of wire but use longer lengths so that for a given voltages there is less current. This is a nickel chrome alloy called NiChrome, and you can not solder it so you will have to use screw connectors to wire it up.

(Sorry if my English sucks, I'm French)

Your English is a damn sight better than the English of some people on here whose first language is English!

You might want to look at ductable textiles, too, as these are washable and wont break as easy as wire. Or use very thin wire spun to one thick rope instead of using one thick wire.

I have no idea what it’s going to take or how big of a battery you’ll need. You’ll probably need a fairly large battery and a battery that would last all day would probably be impractically large.

But, I can give you some “basic electronics” if you don’t already know.

[u]Ohm’s Law[/u] describes the relationship between voltage, current, and resistance. (Current = Voltage/Resistance)

Heat is related to power (Watts). You’ll probably need at least 10 Watts, but I don’t have a good feel for that. It gets complicated because the actual temperature depends on the “intensity” or the area/volume you’re trying to heat-up and the ambient temperature. For example a 25W soldering iron can melt solder and a 100W light bulb can burn your fingers, but a 100W bulb can’t heat-up a room.

Power is calculated as Watts = Voltage x Current. (Current is in Amps)
Or, using Ohm’s Law we can derive:
Watts = V2/R (probably the most useful formula)
Or, Watts = Current2 x R

Nichrome wire has a resistance specification in Ohm’s per meter (or Ohms per foot) depending on the wire gauge.

A battery’s amp-hour rating is an indication of battery life. A 1 amp-hour battery will last about 1 hour at 1 Amp or 10 hours at 100mA, etc.

Sparkfun sells some heating pads that are useful for this. Run them on 5V and they get warm but not hot.

Nichrome is a very commonly used heater wire because it has a resistance that doesn't change much with temperature (unlike many metals), and it will withstand high temperatures in air without burning up (unlike most metals).

For more moderate temperatures neither of these are so vital.

Soldering a heater wire would be a foolish thing (think about it), so solderability is not normally a desirable feature of high temperature fittings.

Silicone insulation is used in some heater wires as it handle moderate temperatures of a few hundred C. I suspect this is the case for electric blankets and similar, which might be a source of suitable wire.