Hey, I know that a few people have asked questions about hot wire foam cutters, and I have viewed them through the search.... however, I am not finding the solution to my problem yet.

For clarity, a hot wire foam cutter uses electrical current to heat up a stretched exposed wire to cleanly and quickly cut styrofoam and paper. There are many diagrams on the internet, however, it's basically creating a closed circuit to feed power from an outlet to a 12v 3a power transformer, with a dimmer switch in between, and have the secondary circuit either directly connected to the exposed wire, or connected via 3 way switch, depending if the power transformer has a variant (3 wires). When I constructed this, however, my 12v 6a transformer started to smoke almost immediately once powered.

After consulting a specialist who works on large circuits, the diagnosis was that the transformer was fine, the primary circuit was fine, but I had inadvertently created a short circuit on the secondary line. Once he used a 1.5Ω reducer on the secondary circuit, everything worked out great, however the wire no longer heated enough, witch left me with the homework of finding what reducer I would need to allow the secondary circuit to consume it's wattage properly. He mentioned it might need to be done either in circuit or in parallel, but I will admit that it was a lot of verbal information, so I forget some details of how much I need to reduce the transformer's wattage by, but...

The figures I have are:

Approximate temperature of wire needed: 200°C (according to Wikipedia)
Wire resistance: 0.5Ω (the specialist tested it)
Transformer: 12v 6a.
Wattage produced: 60w (P=IE=5a*12v)


I studied the Ohm's Law formulas to find out that what I have currently is:

R=E/I =12v/ 5a (recommended) = 2.4Ω needed, thus I am 1.9Ω short of this load.

However, when I turn the formula to find the Amperage, as I have it now, I get:

I=E/R = 12v/0.5Ω = 6a which means I am just reaching maximum, only needing to reduce it by 1a, which seems like a lot of ohms to reduce this circuit by 1a...



Then checking parallel circuits, I get:
I (of wire splicing to hot wire) = E/R =6a (when the total of my amps should be 5a to begin with...


So I am guessing that I need to add a 1.9Ω reducer in single circuit. Further studies, using this site:
http://ffden-2.phys.uaf.edu/211.fall2000.web.projects/Jeremie Smith/page5.htm
explain that if I put the hot wire first in succession, I'll be using the full 12v to convert it to 60w of heat. This means that the hot wire consumes (E=IR = 5a*0.5Ω =) 2.5v leaving 9.5v for the reducer.

So does that mean that I need to find a reducer of 1.9Ω 9.5w (or total of) to save my power transformer?

And will this still offer enough wattage to reach 200°C?

I feel lost and hope this forum can help me fill in the gaps I cannot find.... (Converting Watts or Joules to Celsius still perplexes me)

 

explain that if I put the hot wire first in succession, I'll be using the full 12v to convert it to 60w of heat.

No.
The order of resistors in a series circuit makes no difference as to the dissipation in each element.
It's always I²R (not IR as you posted).

I see no statement about how much current the wire requires to operate properly.
Without that info, you can do no meaningful calculations.
The transformer can put out a maximum of 6A.
This dissipates a maximum power of 6² x 0.5Ω = 18W in the wire.
Is that sufficient to do the cutting you want?
If not, you need a different transformer.

 

Wire resistance: 0.5Ω (the specialist tested it)
Transformer: 12v 6a.
Wattage produced: 60w (P=IE=5a*12v)

Why are you using a transformer that can only provide 6A?
\( \small I = \frac{V}{R} = \frac{12V}{0.5\Omega} = 24A \)

 

I think the question that needs to be asked is what type of wire are you using and what is the diameter of the wire?
0.5Ω appears to be somewhat low and perhaps you need finer wire or wire made of nichrome (resistance wire).

 

I think the question that needs to be asked is what type of wire are you using and what is the diameter of the wire?
0.5Ω appears to be somewhat low and perhaps you need finer wire or wire made of nichrome (resistance wire).

Many of the Youtube "experts" are using guitar strings for their wires, not a proper resistance wire. It may work fine in a long wire but not so good in a very short wire in a cutter. That is a problem for people not building the exact thing shown in the videos.

 

You would be defeating the whole purpose of a heating wire if you insert an additional (reducer) resistor in series. You want the entire heater wire to be about 3Ω resistance. This makes the wire draw about 4A and dissipate about 48W @ 12V.

 

Many foam cutters, us nichrome wire. it is a resistance wire, that's how it is not a dead-short. It is the resistance of the wire that causes it to heat. The concept is really no different than what is use for a burner on a conventional stove or oven.

You run a specific amount of current through a specific resistance to get heat.

Foam cutters are usually DC circuits by the way.

 

Foam cutters are usually DC circuits by the way.

Do you know why?

 

Foam cutters are usually DC circuits by the way.

But yet when you look at all of the reputable companies selling them they are all AC not DC. Even the industrial ones use AC. Just one of many -
https://jacobs-online.biz/power_supply_design.htm

 

Do you know why?

I beg your pardon (thank you shortbus ). The ones I've seen are DC, but that's been awhile. The key component is the dimmer-- which is actually a triac with a Z/C detector that they are using to do sort of a PWM to control duty cycle. You still need some sort of resistance in series with the wire to create enough of a load to heat the wire sufficiently. By using the PWM, you can use a single resistance in series with the wire, rather than a variable one.

 

Something I would like to mention before answering some questions: Where I live, trying to build this, or have shops where electronic repair people, electronics shops, and.... well, anyone who looks beyond their trade's common threads is.,... almost too rare for me to find. (ie. A computer tech is familiar with coding computers but has no interests in robotic programming, or even finding out how a dishwasher is programmed, or how a welder who works on piping doesn't consider artistic welding as a talent to look into). I am not trying to insult the great workers of my community, but rather express that finding information about where I might have gone wrong has been a real task, and I am thankful that I finally found this community who's willing to help.

No.
The order of resistors in a series circuit makes no difference as to the dissipation in each element.
It's always I²R (not IR as you posted).

I see no statement about how much current the wire requires to operate properly.
Without that info, you can do no meaningful calculations.
The transformer can put out a maximum of 6A.
This dissipates a maximum power of 6² x 0.5Ω = 18W in the wire.
Is that sufficient to do the cutting you want?
If not, you need a different transformer.

The reason you don't see how much current is that I have been having a hard time finding it myself. Like I mentioned, the original design was made using a 12v 3a transformer and he used hanging wire (don't know what grade that is, but he did separate the strands) in order to make the hot wire. Other than that, the only modifications I made, that blew up in my face.... sort of, was to try and attach a 12v flush LED light to the main circuit (I wasn't thinking too much at that time) to allow me to see when it was turned on for safety reasons. I added an LED light to the secondary circuit and had to remove that one as well as it seemed to create a short (I don't know if I might have burnt it... Again, I just wanted a safety guard easily visible in case my nephews came over.

On wikipedia's hot wire foam cutter page, it stated that the machines use a wire heated to 200°C to work, which I am still unable to understand how to convert that into wattage, although I know watts means heat in this case (and 1w/s which is what the wire would be producing is equal to 1J/s which can be measured in degrees... but there's all this talk about heating water, BTU and such which I haven't found a clear enough understanding to use to find an answer... which is why I came here.)


As for the correction of I²R, I admit my mistake. I was going by a couple of site I found and didn't notice what was wrong until after I posted this and walked away....
P=IE and E=IR meaning that: P=I(IR) or P=I²R (which means that Pie is Riip backwards.... cause I love when equations make words)

Why are you using a transformer that can only provide 6A?
\( \small I = \frac{V}{R} = \frac{12V}{0.5\Omega} = 24A \)

I used the following video:

However, in the example, as well as a few other posts related to this model, the recommended power transformer suggested is a 12v 3a transformer (which he suggests salvaging from an old radio.... which I discovered should not have a wall wart.... or be related to Iphones since it doesn't use a transformer, but a circuit board....). As for why I went with the 12v 6a model, well, that's because the only shop that knew what I was talking about, asking to buy a transformer and not a power bar, only had this model. However, this does produce twice the energy, so...yay?

I think the question that needs to be asked is what type of wire are you using and what is the diameter of the wire?
0.5Ω appears to be somewhat low and perhaps you need finer wire or wire made of nichrome (resistance wire).

You would be defeating the whole purpose of a heating wire if you insert an additional (reducer) resistor in series. You want the entire heater wire to be about 3Ω resistance. This makes the wire draw about 4A and dissipate about 48W @ 12V.

I don't know if this will help, but I didn't think, when I built this, that the type of wire mattered, but I used a old 24 gauge blackish wire that I found in my shed (probably bought to hold chicken wire together.) it says 24 YRDS on it, Made in Taiwan, and it says it selled for $1.79 Canadian. The Ω resistance on it (measuring from the first nickel (plated?) eye bolt to the next (including the old steel spring used as a tentionner) is only 0.5Ω, which I noticed you mentioned is a problem.

I used this because I had it around, but I was also considering some aviation wire which we bought for my dog and didn't use, but I did think of unthreading it. I just measured the distance from bolt to bolt, and the whole thing is 1' long. So am I correct to understand that changing the wire is the only issue wrong with this machine? (Also, I am not quite understanding what nichrome is... or where I would get it...)

Do you know why?

This one I do remember the specialist explaining, and it's because of the Transformer.

(sorry but I'll use history here) Thanks to Edison frying dogs to demonstrate a mute point... trying to prove that Alternating current is safer than the direct current since direct current (at an absurd amperage that would have killed the dog even using alternating current) fried the dog in a metal cage, and he stated (but did not prove) that Alternating current is much safer.

All that fun history (cause I love stating why Tesla isn't as well known as Edison) that our homes are powered by alternating current, AC, and, what the specialist said, was that Transformers are built in a way that doesn't fluctuate the current, making the secondary circuit a DC circuit (although with larger more industrial machines, as a safety precaution, the transformer is either replaced by a capacitor of some kind, or has an alternator (to change DC into AC) along the circuit.... But that's just what I heard.

 

Find someone who owns or sells or tunes guitars and get them to save you the 1st (E) steel string when they ever change strings.
I just measured the resistance of the full length of the 1st string and it is 3Ω. Perfect for what you need.

 

I use a " Power Stat " variable transformer feeding into a 75 W , 24V sec. transformer. Wire , 3in to 3ft., NiCr or other. A good find was filament from 500 W quartz lamp. Best temp. around a dull red.
Stainless is iffy, a little too hot and it delaminates ?