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Calculating heating effect.?

I have been experimenting with making a hot wire foam cutter. This needs to be at least 600 mm wide. I have to hand a PSU capable of providing 14 volts at 2 amps (or slightly below) This is 28 watts.
Now the question i would like to calculate the heat the Nichrome wire will reach at any given wattage but have no idea if I can do this.

I have several wires that work to different levels, resistance of 30R, 4 R, 137R at the length I am using.

Any one any  ideas how to calculate the temp or are there too many variables?

So far I have worked empirically and arrived at the conclusion I need either around 18 volts at about 3 amps or I need a wire with a resistance of around 7R to suite the power i have available. I do know that the 20+ watts I am generating is just heating the wire to the melting point of the polystyrene foam. The melting point of the foam is apparently around 220 to 240 deg C (internet)

At present it isn't cutting fast enough for my project. which required producing 5 mm thick sheets of foam with a reasonable consistency. i am cutting them from a 50 mm thick 1000mm x 500 mm block.

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The amperage pulled by the wire will very by which wire you use. If you use the 30 ohm wire then it will only draw .46A drawing only 6.44W. The 4 Ohm wire will try to draw 3.5A which could blowing a PSU rated at only 2A.

If you want the wire to get hotter then you need to increase the voltage to it. Increasing the voltage to 18V on the 4 Ohm wire will draw 4.5A drawing a total of 81W. Making it nearly 4 times hotter than if it was only using 14V. SO if you add more voltage be sure your power supply can handle the added current draw. Then you also have to worry about applying to much power to the wire and burning out the wire.
rickharris (author)  mpilchfamily3 years ago
Yep - BUT how hot in terms of temperature.
Have you done a search for a calc yet?

http://www.jacobs-online.biz/nichrome/NichromeCalc.html
rickharris (author)  mpilchfamily3 years ago
Excellent find thanks - I didn't think to look for a calc.

Still doesn't tell me what the relationships are he is using - I guess I could analyse it - or on the other hand i could just use it ! :-)

The results for the experiments I have already done suggest the calc is fairly accurate,

thanks again.
rickharris (author)  rickharris3 years ago
I am impressed that my empirical results exactly match the calculator though :-)
Great link
iceng3 years ago
rickharris (author)  iceng3 years ago
I had seen that - thanks - I am still looking to calculate the actual temperature of the wire from the known parameters.

I have done this before with a car battery and a guitar string - current no problem.I just wanted to be more technical.
Pre-calculating  always a good thing to do.

My  concern  are the necessary two attached electrical junctions
which may easily be 5% to 30% of the resistance depending on
how you mechanically accomplish the copper to your wire connection.

It is that resistance variable that you may find giving you most prediction difficulty !

As far as temperature there may be data about fusing currents for pure metals
and knowing the melting point would give an upper bound reference point.
However, I suspect the wires you have are  alloys with dramatically altered qualitative.

Do you intend to drive it through a DC supply ?
Or a variac feeding a step down transformer AC like soldering pistols.
step-downwire heater.gif
rickharris (author)  iceng3 years ago
Thanks Iceng - This is more or less what I am going to try next although I will use a 500 watt dimmer as I don't have variac.
Works for plasma cutter
You may have to put a 5 uF or10 uF 400 VAC capacitor in series with the XFMR primary for stability....