3

I'm new to electricity but have a basic understanding of circuit theory.

I came across this video using what appears to be a steel wire (looks like >0.5mm) as the heating element of a homemade hot glue gun.

youtube video

The key point is the steel wire is connected directly to a 12V 5A power. But I don't understand why it works:

1) There is no resistor to limit the current, shouldn't it blow up some fuse?

2) If a steel wire of that thickness is used as the heating element, what current is required?

3) Is it possible to rely on the contact resistance as a way to limit the current?

JavaMan
  • 243
  • 1
  • 3
  • 8
  • The wire should have enough resistance by itself, nothing else. In other case power on it is zero –  Mar 06 '17 at 08:03
  • Google Nichrome and Kanthal, two common heating wire materials. The former has a positive TCR, the latter has almost zero TCR. In both wires, the resistivity of the material is mainly what limits the current, but with a positive TCR you also get a certain degree of temperature limiting. – Dampmaskin Mar 06 '17 at 08:04
  • how much resistance do we expect from a steel wire? I tried measuring one of 0.1mm using a multimeter. It showed <1ohm only – JavaMan Mar 06 '17 at 08:05
  • Go here: https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity and learn about conductivity. Use this knowledge to estimate the resistance you can expect from your piece of wire. – Bimpelrekkie Mar 06 '17 at 08:32
  • 6
    The current limiting you want so much is something you actually do not want. If a resistor is used to limit the current, that resistor will drop a voltage. Voltage x current = Power so that resistor will become hot instead of the wire !!! You want the wire to become hot so that wire should be the resistor. The trick is to use a wire with enough resistance so that the wire will limit the current. – Bimpelrekkie Mar 06 '17 at 08:36
  • 3
    Ordinary steels oxidise, heat them up and you speed up the process. Just because a wire looks like steel is no guide to the actual material used. – JIm Dearden Mar 06 '17 at 11:43
  • 1
    The wire IS the resistor. If it's too low resistance, use a longer (or thinner) one - or a higher resistance material. Steel is an odd choice - apart from its low resistance, it tends to oxidise ( forming a hot spot, which burns in air) - nichrome is a better choice. –  Mar 06 '17 at 12:44
  • The caption at https://youtu.be/lXHBh5-LK7U?t=29s says that is is nichrome not steel. – Pete Kirkham Mar 06 '17 at 13:24
  • "But I don't understand why it works:" There are resistive heating wires specifically designed for this. people who make foam airplanes know them well. Normal steel wires would have too little resistance for this to work. – dannyf Mar 06 '17 at 11:06
  • The steel wool is both a resistor and a fuse. – user253751 Mar 06 '17 at 23:09

3 Answers3

3

1) There is no resistor to limit the current, shouldn't it blow up some fuse?

Heating elements are resistors. By choosing an appropriate wire diameter, wire material and wire length, the resistance (R) can be tuned so that the heater will draw a given amount of amps (I) at a given voltage (V): R = V/I. To calculate how much current (I) you need at a given voltage (V) to reach a certain amount of heat output (P) in Watts, you divide the target heat output by the voltage: I = P/V.

For example, if you need 30 W of heat at a voltage of 12 V, your heater has to draw 30 W /12 V = 2.5 A in order to do so. Your heater should thus have a resistance of 12 V / 2.5 A = 4.8 Ω.

2) If a steel wire of that thickness is used as the heating element, what current is required?

There isn't any "required" amount of current like a LED would have for maximum brightness. A heating element (resistor) will simply draw as much current as its resistance allows at a given voltage.

The resistance of the heating element (and consequently, the current drawn at a given voltage) doesn't just depend on the wire diameter, it's also dependent on the wire lenght, wire temperature and the wire material. For this answer I'll neglect taking the temperature into account in order to keep the answer as simple as possible. The lenght (l) required of a given wire is l = D^2 * π * R / (ρ * 4), where D is the diameter of the wire, R is the desired resistance and ρ is the resistivity of the material (dependant on temperature).

Continuing with the previous example, a 4.8 Ω heating element made of 0.5 mm diameter mild steel has a length of (0.0005 m)^2 * π * (4.8 Ω) / ((15*10^-8 Ωm) * 4) = 6.28 m. The heating element in the video is clearly much shorter than 6.28 m long; it is not made of steel.
That heating element was almost certainly made of a high resistivity metal alloy designed specifically for heating applications; nichrome wire. Plugging the same numbers into the above equation, but changing the resistivity to that of nichrome, gives us a much more plausible length of 0.75 m.

3) Is it possible to rely on the contact resistance as a way to limit the current?

Again, a simple resistor doesn't need current limiting. No, you could not use contact resistance as a way to limit current, you can't set a switch to "half off".

If you need to vary the heat output with e.g. a knob, use pulse-width modulation

jms
  • 8,631
  • 3
  • 22
  • 45
2

Besides "Genuine" 80/20 nichrome, there are some common versions of stainless steel (which, have curiously high levels of nickel and chromium ... ☺ ) which are available in wire form and have very attractive parameters for use as heating elements.

Lawrence NK1G
  • 501
  • 2
  • 5
0

I have used aircraft grade stainless steel safety wire as a heating element. I think it's about 20 gauge. Using Ohms Law and experimenting this dissipates about 1A at 12 volts per about 25 ft. 12 watts over a 25 ft length is just barely warm to the touch. Used for RV plumbing freeze proofing.

user271324
  • 1