Random thoughts on hot wires

So, the approved method of cutting styrofoam for model airplane wings is to get a nichrome wire (or stainless -- you pay extra if it's called "heater wire") hot, and run it through the styrofoam.

Many folks do this using a lamp dimmer connected straight (no isolation transformer!) to the wire that they're playing with just inches from their hands. For some _really odd_ reason I don't want to do this. (It can't have anything to do with not trusting that neutral is really neutral, of course).

As near as I can tell, the required current in the wire is on the order of a couple of amps. I know from extensive reading (but practically zero real experience) that having an adjustable supply to heat the wire is a good thing (hence the lamp dimmers -- egad!).

I'm thinking that, in this degenerate age, the best way to achieve this end would be to dig through the bucket-o-wall-warts and see if I have a laptop computer power supply, or some other supply capable of delivering a few amps at 12V, and chop the current with a sufficiently herky transistor, driven by a 555 for duty cycle control. _Real_ super-zoot systems would monitor the overall resistance of the wire and adjust the current to maintain constant resistance (hence constant temperature). I ain't gonna do that -- I'm too lazy, and I suspect that it's very much gilding the lily.

An alternative, depending on what I have in my junk box, would be to use a transformer from a quartz-halogen light. These have the right current rating, but they supply AC which is harder to switch. You really want to make the PWM reasonably fast -- the wire's thermal time constant is on the order of a half second or less. But fast PWM, without rectification and filtering, means that there's a very real danger of inducing some pretty significant DC in the transformer secondary. The notion of synchronizing the switch to the AC so that it starts and stops at the cycle boundaries is a tempting one, but then the controller complexity goes through the roof.

The easiest in principal would be a variac and a transformer -- but I don't have a variac.

So -- thoughts?

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Reply to
Tim Wescott
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You're over thinking this. Music wire (.008" - .015") works fine for the hot wire. My setup uses a MOT w/ the secondary stripped off and a new one with a dozen or so turns of 12ga stranded added. I use a lamp dimmer to feed the primary and it works just fine. An old, non-electronic, battery charger will work for the xfrmr too. Art

Reply to
Artemus

[snip]

I've used Nichrome so-heated to make straight-line bends in Plexiglas. (In a slot in Bakelite to keep from direct touching.)

Beware, You need to spring-load it to keep it straight, it stretches when heated. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Styrene monomer is very nasty, so you'll want to do this standing upwind from the hot wire.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
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Reply to
Phil Hobbs

It's possible to safe the hot wire somewhat, by using a ground fault interrupter in addition to the dimmer. You will want to spring-load the wire (it stretches quite a bit on heating, it will NOT stay taut).

Reply to
whit3rd

Feed the halogen transformer ( assuming a *real* ironcore transformer ) from a dimmer.

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Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
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Reply to
Adrian Jansen

Get some nichrome:

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and play!

(remember: E = I * R)

(I did it a couple years ago, about 7 inches of #36 nichrome and a 12V wall wart - it worked "real good," but the cuts weren't straight because I did it with my bare hands and the piece of nichrome between a pair of insulated alligator clips.)

Have Fun! Rich

Reply to
Rich Grise

Here's my untested, untried idea I made up.

Automotive battery, little wall wart charger and light bulbs.

If you're lucky, your car battery is due to be changed (~every 5 years), so use that.

When you go to get your new battery(for your car), buy some light bulbs. Put as many in parallel to get the current you need. The same store may have a little charger.

One stop shopping and a 1 hour build.

Sometimes less is more.

Reply to
D from BC

The $ 11 one works pretty good.

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It uses a D cell battery. I used it to decorate a Mardi Gras float some years ago. I bought it at a hobby shop. If you are interested I can look for the one I have and send it to you. Harbor freight has one that looks more substantial.
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Never tried it though. The mail address is rike2

Reply to
Rick

(It

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ttdesign.com

Have you seen Charles Wenzel's site?

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George H.

Reply to
George Herold

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Or smash up some old Wire Wound resistors from the junk box... cannibalize the old toaster that's been in the attic for ages...

Reply to
Oppie

Ha! That I can do! I even have a dimmer switch in a box with a plugin, for my wife's stained glass shop. She hasn't been doing that since we started home-schooling the kids, so it's mine mine mine!!!

Temperature regulation would still be a fun challenge, though.

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Reply to
Tim Wescott

That's way too small for wings -- you're hot cutting along the length of a 30 inch hunk of foam.

But it'll be cool for details!!

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Reply to
Tim Wescott

Although it is not very difficult, there are still a number of things that can go wrong.

- If you get the wire too hot, use too thick a wire or cut too slowly, the wire will melt a huge hole around it, resulting in a wing that is too thin.

- If you don't get the wire hot enough, get too little tension or cut too fast, the wire will lag behind in the middle, causing your wing to come out thinner in the middle.

- Get the communication wrong (you will need an assistant), and your wing will not look like a wing at all.

I simply use my old lab power supply. It is good for 0-60V at 5A, so I can experiment with different wire thickness, length and also adjust temperature as needed.

Don't give up if the first one fails miserably. Remember: If at first you fail, you belong to the majority.

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RoRo
Reply to
Robert Roland

I keep a couple of supplies like this around-- they come in handy sometimes, like when running big motors, hot wires, anodizing or plating, giant Helmholtz coils etc.

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No experience with this particular seller, I got them from a guy in the Chicago area, but the Chinese brand is the same.

With a name like "Manson" it has to be good, right?

Reply to
Spehro Pefhany

This is what they sell receptacle testers for. If you don't have one, you could substitute a meter. The hot will still be hot of course, so there is still something to be said for isolation and/or reduced voltage, such as a dimmer powered transformer or car battery charger.

That seems a bit like overkill for the application. I just use my old

350 Amp Miller Dialarc HF welding machine for heating nichrome wire :-). It has other uses also, like thawing frozen pipes, heating the workshop, and with the high frequency turned on it makes a pretty good EMI source too!
Reply to
Glen Walpert

Reply to
m II

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Wirewound resistors: Too messy; You might be able to find suitable wire, but I wouldn't want to go through it.

Toaster: WAY too fat of wire! It takes 10 freakin' amps!

But, do your own thing. :-)

Cheers! Rich

Reply to
Rich Grise

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I guess one could simply get some constantan wire ?

Besides, the wire used in the heater elements of toasters tend to increase in R as they get hot. Well, most that I've seen any way.

I have used steel wire for making air flow sensors :)

Jamie

Reply to
Jamie

That's a _man's_ big blue power supply.

Best regards, Spehro Pefhany

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Spehro Pefhany

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