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cant get my foam cutter to work. Answered

ok im building a foam cutter, kind of following the instructions from https://www.instructables.com/id/EK8Q92824BEP286S6S/

aaand it doesent work. i have a 10v 5 w transformer attached to a metal frame that holds the wires. and the transformer is attached to this stainless steal wire. i was thinking that maybe because i have it attached to a metal frame it sucks all the current but technically shouldent it try to "find" the thinnest place to short circuit and heat up? When I touch the wires to the frame it sparks up, so there is a current going through. And also if I just hold a piece of wire on my hands and connect it to the transformer, it doesn’t heat up either.

sorry no pics, i might have some in a couple of days.

helps! or suggestions as to what the reason might be for it not working. i am trying to build model for school, alas i think the foam cutter itself is more of a project then what im building

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motley (author)2007-03-21

hm, you maybe right. here is a diagram of it though(hope it works, im not quite sure how the uploading thing works either. in flywoodkb s tutorial he doesent use a resistor at all. then again it is a wooden frame. the part that confuses me ( and that is an easy task some times) is why it doesent work like a battery, when i hold a piece of wire between + - it doesent heat up :(. is it because the transformer shuts itself off? as in read a short? so if the wire was long enough between +- it would heat up right?

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LasVegas (author)motley2007-03-21

in flywoodkb's tutorial, he's not using a current-limited power supply. Your supply is limited to a maximum of half an amp (5W). He's controlling the current with the Dimmer switch. You don't have that luxury. He also noted that he improperly is not using a fuse in the circuit. This could prove very dangerous.

A battery's current is limited by the internal resistance of the battery itself. While it would heat a short wire, you'll also note that the battery itself will get very hot.

Your power supply is not a battery. It has a stated limit of 5 Watts. 5W/10V = 0.5A. This is why I suggested that you add a 20ohm/5Watt resistor (10V/0.5A = 20ohm) This would allow your power supply to put out it's maximum current. I can't say it would be enough to heat the wire though.

It would be best to duplicate flywood's design. The transformer isn't too expensive.

BTW: I don't know what format your document is in.

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motley (author)LasVegas2007-03-22

aa things started to get a bit more clear now! thanks!! i tried ading rezistors to the contraption but i forgot about the 5w thing when i was buying them, so i got 5 10ohm 1/4w rezistors... and now my room smells like burnt plastic yay! i will try ading a dimmer and proper rezistors to the equasion tomorow. i also have a nother smaller transformer that is 4.5 v, 400ma. do you think that would be a better option then the biggre transformer? here is my setup

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LasVegas (author)motley2007-03-22

You cannot have a metal frame! The wire must be totally insulated from the frame.

400mA = 0.4A @ 4.5V would give you 1.8W. Quite a bit less. No. It won't work.

Get a 25V, 2A transformer (That's up to 50Watts!) from RadioShack. I know it's more expensive, but it's necessary. So is the Dimmer!

Do not do anything with your contraption until you get rid of the metal frame!

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motley (author)LasVegas2007-03-22

ok, will do now i am all excited about this. i hope it works this time. 25v 2a and a dimmer it is. do you think that my 10v would do the trick as well with just a dimmer? my wire currently isnt longer then 1', and i dont see it getting any longer.

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LasVegas (author)motley2007-03-22

A 1" wire might get hot enough with the 10v supply, but the 20ohm/5w resistor is absolutely necessary. No substitutions! If you used 2 10ohm resistors they would still have to be 5 watt resistors. These will look like a ceramic brick about 1.5" long and 1/4" square or brown ceramic tube about the same size. You cannot use the dimmer with the 10v supply since that supply is already using an internal regulator. If you use the dimmer/transformer setup with that tiny wire, start with the dimmer full down and turn it up just until the wire appears to smoke a tiny bit. It won't take much. Again... Don't use that metal frame!

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motley (author)LasVegas2007-03-22

awesome! thanks a lot! i truly appreciate it. im using the metal frame but im replacing the attachment parts with dowels, so there wont be current going through it. i like the metal frame cos i can tension the wire really easily. oh and the wire is 1 foot long :) here is my final shopping list then: dowels for insulating, 25v 2a trans, dimmer. and i will attach them just like in flywoodkb's tutorial, wall outlet ->dimmer ->transformer -> wire. no resistors. thanks again for being patient.

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trebuchet03 (author)motley2007-03-22

I'm using the metal frame but I'm replacing the attachment parts with dowels, so there wont be current going through it. I like the metal frame because I can tension the wire really easily.

I don't think LasVegas stressed it enough..... for safety, you need a frame that will not conduct. That is, should something go wrong... and eventually something will go wrong (guaranteed) - you need to be isolated completely.

Wood frames can be just as easy to tension ;) It's up to the designer (you) on how easy it is to build and adjust ;)

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LasVegas (author)trebuchet032007-03-22

If you keep the metal frame with the dowels, pick up a roll of black electrical tape (This is real cheap!) and cover the frame with the tape. You don't want any metal exposed around the filiment.

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motley (author)LasVegas2007-03-22

ok got me some tape the transformer and a dimmer! tonight i shall construct the deamon. hm im going to try to isolate it as much as i can but what would happen in case of the 24 v toucing the frame? i imagine if the 110 touched it it would be a bit different. my faulty highschool physics tells me that not much would happen because the metal frame has a higher rezistance and the current will prefer the thiner cutting end... tell me it is so and i wont blow a fuse or my house.

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LasVegas (author)motley2007-03-22

Actually the metal frame would have lower resistance than the stanless steel wire. If you short out the transformer you run a high risk or burning out the transformer. The dimmer, if kept below about half way should prevent dammaging the transformer.

I would recommend adding a fuse to the circuit at the earliest convenience. Being that the transformer is 50Watts (25V * 2A) then you should have a 1/2 Amp (50W / 120V = 0.4A) fuse inline with the transformer's primary and dimmer. 1/2A is close enough to protect the transformer.

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motley (author)LasVegas2007-03-26
  • insert uber frowny face*

well hmm i think i got the wrong transformer. or dimmer. nothing happens really, the transformer heats up a bit and when the dimmer is set all the way there is a scary electrical humming sound. I tried it without attaching it to the table, so there is only a piece of wire in between

the transformer is 24 v i think but i will tell the more specifically when i get back home today, and probably some pictures. i have made it a personal vendetta now to complete this project :D its not for my model any more.

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motley (author)motley2007-03-26

ok it is a 8vac-10va, 16vac-10 va, 24 vac-20va low voltage transformer from ace harwear store. i am using the 24v option. i still have the receit and am probably going to take it back tomorow.

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LasVegas (author)motley2007-03-27

No. No! No!!! That is not a transformer. It's a power supply! A transformer is nothing more than two coils of wire wrapped around a core (usually iron). A power supply will have switches and a plug and extra circuitry to protect the supply (and you). You cannot substitute one for the other in this situation because this application really does need the current. You also cannot substitute wood (a natural insulator) for metal (a conductor).

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motley (author)LasVegas2007-03-27

yeah, it is a transformer though. i dont know if it makes a difference but it has options in wiring. here are some pictures. and i am definatly getting rid of the metal frame! im not even attaching it to the frame at the moment. i have the components in a case usually but i took them out so i could show the wiring. i dont think there is anything wrong with the wiring but one of the main wall outlet goes to the dimmer and the other one goes directly to the transformer. thank you for your patience and help though. i do appreciate it a lot.

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LasVegas (author)motley2007-03-27

Yes... That looks like a transformer with a center-tap. Now. What are you using for wire? Is it a stainless steel guitar wire? Is it one of the thinner ones? The finer the wire the better it will work. Do not turn the dimmer up full! You don't have any fuse attached to protect against over current.

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motley (author)LasVegas2007-03-28

Hm, it seems not to want to work. yeah I turned it down as soon as I heard the sounds. maybe the 24VAC- 20 VA is too low. But I would think with even that the wire would get a tiny bit warm. The wire I use is a really thin copper from some left over wiring and some stainlesteel wire that is fairly thin gauge as well. they both work well with my battery operated foam cutter. or maybe the wrong time of dimmer? or im going to test the wires as well. maybe it is too thin and the wire heatsup as opposed to the stainless steel part. but some thing tells me i would have noticed that. When I was using the 4.5 v adaptor I got a spark when I made a circuit, I don’t get it when I have the dimmer attached although there is a current going through it. Hm curious indeed.

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LasVegas (author)motley2007-03-28

I wouldn't recommend any copper wire. The reason is that copper is a very good conductor with not much resistance. As a result, it wouldn't loose much in the form of heat. Go to a music store and pick up a cheap (Cheaper, is better for this!) steel guitar strings. The steel isn't near as good conductor and would heat much better at lower currents.

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LasVegas (author)2007-03-21

The wire cannot be connected to a conductor at all! Also, you need enough wire to provide enough resistance so the power supply doesn't read a short. For that power supply you need at least 20 ohms of resistance.

If you're working with a short piece of wire, try adding a 20 ohm, 5W resistor in series with the wire. This would give you about .5A running through the wire and drive the supply at or near it's limit. Remember, the wire must be insulated from the metal frame.

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