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I am building a hot wire foam cutter. I do not know what to do about the power though. I have a 12 volt adapter that plugs into an outlet , just like any ordinary adapter. I'm wondering though, if it shorts the adapter, it will blow the fuse, correct? I'm sure that's what would happen but other hot wire cutters, being sold online, have an adapter you plug into the wall as well. I need some clarification about this. And then, there's transformers. What are they and why are they used on hot wire cutters? Help would be appreciated.

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For designing something like this, it helps to have a number in mind for how much power P, in watts, you want the wire to dissipate.

The reason why, is because when you think about it in terms of power dissipation the math is really easy. The length of wire looks like a resistor R, and the power it dissipates is simply

P = V*I = V^2/R = I^2*R

where V is the voltage across the resistor, and I is the current that flows through it.

Moreover, I think a typical design will start with P and R determined first, so that what remains is to solve for V; i.e.

V = (P*R)^(1/2)

That's the square root of P times R. I have to write it that way, because my keyboard doesn't have a "square root" key on it.

;-P
This answer for V is typically a small number (of volts), and this explains the reason why a transformer
https://en.wikipedia.org/wiki/Transformer
is used. Because the answer for V turns out to be a number much less than mains voltage.

As a bonus, the transformer provides isolation,

https://en.wikipedia.org/wiki/Galvanic_isolation

an essential safety feature, so the user cannot get shocked just by touching the bare metal of the heating element.

By the way, in the past, a transformer was the absolute cheapest way to build a low voltage, AC voltage source. It turns out, both AC and DC are suitable for making heat. So a DC voltage source will work just as well, and today small DC power supplies, cost the same or less, as a transformer with similar power throughput.

I mean for building your own wire cutter, you could probably substitute one of these ubiquitous, cheap "black brick", or "wall wart", type DC power supplies... that is provided you could find one with output voltage V that is a good match to your R, your length of heating element wire.

So far this kind of reads like teasing, or hand waving, until I can give you some believable numbers for R, P, V, and I.

Which reminds me. Do you have a heating element in mind? Do you know its resistance (in ohms) already? Or other physical info, like its length? Guessing your wire is made of NiCrome?
https://en.wikipedia.org/wiki/Nichrome

Although it might be other metal. I mean I think you've got a lot of freedom in the choice of metal wire, if you can make the voltage, or current, be whatever.

Nichrome is nice though because it does not oxidize in air, even at glowing orange temperatures.

Orange robot (author)2016-11-23

i do already have nichrome wire and i'm thinking i should just use a rechargeable battery pack, maybe 6-8 volts. But do the amps matter or the voltage?

Downunder35m (author)2016-11-06

There is a hard way and there is an easy way LOL
Main consideration starts with the length you need for cutting.
Next is to check how fast or with how much physical force you need to cut.
Last but not least there are limitations and must have things.
As a general rule of thumb:
Use the thinnest Nichrome wire possible for delicate work and something more solid for big parts, hard foam or parts that have been previously glued together.

I always found it much easier to have a longer than required wire in use.
One side directly connected, the other with a spring to keep an even tension when the wire is heating up.
A good power supply is required, which means you need to be able to control the output voltage and it needs to be able to supply enough current to heat the wire hot enough.
A good start is to use a lab power supply rated for an output of 5A.
If you need to stick with a simple power supply at a fixed voltage you can calculate what wire and length you need.
But again I like to keep it simple and just use the wire directly from the roll and connect a really long section to the power supply - to be safe with an amperemeter between to make sure you won't go over the limit of the power supply.
Then simply move and make the length shorter until you get the right temp to get clean cuts.
Note length for future reference.

So big question: How much in terms of Amps can your adapter suply and do you already have Nichrome wire at hand?
If the adapter is strong enough you just need to find the right wire diameter and length ;)

seandogue (author)2016-11-06

You'll want a constant-current supply, rather than a constant voltage supply, as the resistance of the wire changes with temperature and thermal runaway will result in localized dissociation and failure.

transformers are used to isolate you from the grid lines, which will or at least can easily kill you.

rickharris (author)2016-11-06

More than likely your going to need several amps of current. When I have ,made foam cutters I have used a car battery as a source of power, easy to find, lots of current available and easily recharged.

Use Ohms law to work out the current of your cutting wire.

Amps=Volts / resistance