Hot Wire Foam Cutter

A hot-wire foam cutter. Frame and table made from cardboard. Wire made from a guitar string. Powered by a transformer and controlled by a dimmer switch. Protected by a 10A fuse. Intrigued? Read on!

The hot wire foam cutter and its associated mess
The Hot Wire Foam Cutter and its Associated Construction Mess. Power Supply is the Blue Box

Ok, so you probably already knew that hot things can cut Styrofoam…   I had foam cutter “knife” from Hobby Lobby that was made of cheap plastic and couldn’t cut very accurately or fast. I wanted something more precise, because it’s really hard to freehand something accurately. So I settled on a design that looks like a scroll saw. I wanted to keep the cost to essentially Zero, so I used cardboard as the frame. I will make a nicer one out of wood or metal someday. Maybe.

I cut 15) 2″ strips of cardboard and stacked them together 5 high in a C-shape to form the bow. I used .43mm guitar string as the hot wire, and attached it to the cardboard bow with paperclips. A small spring from a Harborfreight multipack provides additional tension on the wire from one end. The table I used was a 12 x 12 x 1 honeycomb test panel that I had leftover from my boat building.  I cut several 3″ strips of cardboard and glued them to the bottom of the table to serve as legs and to support the bow in a vertical position.



Uncovered Power Box

I used an old lamp cord for the power. I then ran the hot side into a push-on/off dimmer switch. The hot wire then runs to a 120-12V 150W transformer that’s typically used in lighting applications. If we keep following the power, it then runs from one of the transformer’s secondary coil output wires into a 10A fuse, then into the hot wire, then back into the transformer’s other secondary coil lead. The power then flows out of the transformer’s primary coil and into the neutral side of the cord that runs back into the wall. Did that make sense? It’s just a loop.

Ok, so it’s actually two loops, and if you want to get a bit more technical, keep reading.

The transformer splits the system in two and takes one AC voltage and converts it to another AC voltage. There is no direct electrical connection between the input and output leads. Instead, there is a nice electromagnetic connection that allows us to step down the 120v from the wall to a safer 12v for the wire. Power in must equal power out, so the current has to go up if voltage is going down, which is what I want, Low Voltage and High Current.

The transformer also has an output rating of 150Watts. This is important, because I needed a certain current flowing through the wire to heat it up. I didn’t measure it, but research shows that about 4A is fine. Remember, it’s current that heats a wire, not voltage. So, If I know my transformer outputs 150W @ 12V, this must mean that it can give me up to 12.5A (V=IR) which is 3X what I need. Awesome!  That gives me room to use longer wires if I want to later.

Closed and Labeled Power Box. It Goes to Z!
Closed and Labeled Power Box. It Goes to Z!

The transformer is controlled by a a dimmer switch as I mentioned.  The dimmer goes WAY past Eleven!   This limits the power going into the transformer, thus also the power coming out. It gives smooth control of the wire temperature, and allows me to set it, and forget it. This is why the push on/off is critical. You can dial up the right temperature, then push it on and off without having to return the dial to zero each time and losing your setting. Now you could install a simple switch to serve the same purpose, but why do that?

The Setup
The Setup and Mess

The secondary (output) side of the transformer is what runs the wire. One of the leads first runs into a 10A fuse. Why 10A? Because it’s less that the 12.5A that my transformer can supply before dying, so I have a 2.5A safety margin. I’d much rather blow a 20 cent fuse than a transformer.

There is a salvaged extension cord with both ends cut off next. One end of the cord has alligator clips crimped on, and the other end goes into the transformer and fuse. This completes the secondary side. The alligator clips are simply clipped onto the guitar string above and below the workpiece. In this case, one clip is below the table, and the other clip is above the table near the tension spring. These clips allow me to easily replace the wire and move them as necessary.

That’s basically it. It’s actually a really simple setup. Cutting foam with this thing is insanely fun. Let me know if you have any other questions.



I have a transformer from a doorbell that gives me either 8v, 16v, or 24v… Will this work?

No. I tried this first and it didn’t work, because there’s not enough power. If you look at the output rating, it will say something like 20VA, or whatever. VA is the apparent power that the transformer produces. We can still use the Power = Volts x Amps formula, so to figure current (amps), just divide 20VA by the output voltage. (8,16,24). This gives, (2.5,1.25,0.83) each. That’s just not enough. You really need at least 6A to be sure you have enough heat. So just multiply 6 x Whatever output voltage you use. 12v output is common, so you’d need 72vA. Easy enough.

Why did you use AC for your wire instead of DC? Does it matter?

No it doesn’t really matter. Just make sure to use a voltage you won’t get shocked with. 12V or less is generally safe. Voltage is what shocks you, but current is what kills you. (stun-gun principle. very high volts, very low amps) If the voltage is too low to travel through you, then current can’t flow either = safe. I used AC all the way, because that’s what transformers do. Transformers only work on AC because of the fluctuating magnetic field inside the coils. I’d have to use a rectifier to give me DC, and that would just be one extra component and expense. AC is simpler and cheaper.

Do I really need a fuse?

Yes, unless you have an unlimited supply of free transformers. Still, even if you do, it’s quicker to replace a fuse than to replace a blown transformer. Remember to use a fuse that’s a bit lower than your maximum transformer output. If your transformer is popping and sounding like bacon, turn it off. It’s dying. They don’t die immediately, but those sounds are letting you know that it’s time to begin shopping for a new one.

My transformer is hot, and it’s humming. Is that bad?

Probably not.  Transformers can get hot and still be ok. If you’re really concerned, add a small fan next to it. Transformers hum & buzz because the AC flowing through there is vibrating things around just a bit, and that’s ok too. All transformers buzz to some degree. A buzzing sound is how I tell that my system is on.





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