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Jan 20, 2007, 10:33 PM
Impact Testing Specialist
drcrash's Avatar
Here are some resistances of plain (not stainless) guitar strings. (Namely Ernie Ball Custom Gauge, which is the most common thing in music stores.)

8 (.008") 2.0 ohms/ft
9 (.009") 1.40 ohms/ft
10 (.010") 1.17 ohms/ft
12 (.012") 0.83 ohms/ft

Here's some resistances of straight Nichrome-60 wire, from infraredheaters.com:

24 ga. (.0200") 1.671 ohms/ft
22 ga. (.0253") 1.055 ohms/ft
21 ga. (.0285") .8310 ohms/ft
20 ga. (.0320") .6592 ohms/ft
19 ga. (.0359") .5208 ohms/ft
18 ga. (.0403") .4219 ohms/ft
17 ga. (.0453") .3333 ohms/ft
16 ga. (.0508") .2595 ohms/ft

Doubling the diameter decreases the resistance by about 4x.

I think stainless steel has about three fifths of the resistance of nichrome, so it should have about 80 percent of the diameter, to give the same resistance. I'm not sure about that, though, because its resistance may go up more when it heats up. (Nichrome's only goes up 10 or 20 percent across 1000 degrees of temperature.)

EDIT Jan 26 07:

I measured some 304 Stainless Steel wire from Small Parts, Inc.:

.011" dia.: 3.7 ohms/ft
.016" dia.: 2 ohms/ft
.020" dia.: 1.33 ohms/ft

Eyeballing the numbers, I guess that means that stainless is about three or four times as resistive as guitar-string steel, not five. But not bad, and about four fifths as resistive as nichrome.

(These numbers are very approximate. I wouldn't trust my meter on low ohms to less than about 10 percent.)
Last edited by drcrash; Jan 26, 2007 at 10:56 AM. Reason: new numbers
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Jan 21, 2007, 11:02 AM
Registered User
Quote:
Originally Posted by drcrash
Bob, I couldn't find the meter you're talking about on the Harbor Freight site. (It may be there, but listed oddly; I think I may have found it before in an odd way.) The $10 yellow one I found doesn't go up to 10 amps. I edited my list to refer to the $10 red one, which does. (The $7 black one doesn't have a description anymore, because it's discontinued or something.) Do you have a model number I can search for?

The yellow one here: http://www.harborfreight.com/cpi/cta...emnumber=90899
Will do 10 amps even though the writeup doen't indicate this. It is listed at $9.99, but, I have purchased at least 10 of these, and never paid more than $3.99 in the store and the last 3 were on sale for only $1.99.
BobK
Jan 21, 2007, 12:06 PM
Dismembered Member...
arx_n_sparx's Avatar
Doc/Paul: That's a great! breakdown on hot wire info! I've added a link to it from here: https://www.rcgroups.com/forums/show...7&postcount=32


Brad
Jan 21, 2007, 12:11 PM
Dismembered Member...
arx_n_sparx's Avatar
Variacs are very expensive compared to dimmers. You also need to know what you are doing to isolate the line from your output, or the results can be quite shocking The dimmer/transformer circuit is easy do do, cheap, and should pose no danger. It's the danger thing that most concerns me.

Brad ('lectrician who has been belted by 120, 240, 380, and 480 I guess 600 is just a matter of time.....)
Jan 21, 2007, 01:44 PM
Impact Testing Specialist
drcrash's Avatar
Thanks, Brad.

Would you look it over and let me know about anything you think is wrong, or even iffy? I'm especially wondering about the AC vs. DC thing---is it that you get more effective amps when you convert to DC, or more volts? (On reflection, I'd think volts, but I don't understand the issues.)
Jan 21, 2007, 02:19 PM
Dismembered Member...
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Paul: "iffy" items (going by your point numbers)

1) I dunno about a watt per inch - I think that's probably a good number so that you don't stress things too much. Hwere did you get that figure?

2) The 40% is only if a cap is added - as you suspected. You need a big cap for this (electrolytic - and get the polarity right, or you won't appreciate the noise/smell/mess )

4d) - it's close enough to 4x - the resistance won't jump very nuch at all.

8) I haven't seen what I'd call an ampy enough wall wart. They may be out there, but I suspect they'd be heavy enough to just fall out of the wall if plugged in. This would be driving it at its max, and I'd keep a very careful eye on it, because that sucker is gonna get hot
Jan 21, 2007, 04:58 PM
Impact Testing Specialist
drcrash's Avatar
Quote:
Originally Posted by arx_n_sparx
8) I haven't seen what I'd call an ampy enough wall wart. They may be out there, but I suspect they'd be heavy enough to just fall out of the wall if plugged in. This would be driving it at its max, and I'd keep a very careful eye on it, because that sucker is gonna get hot
Altex (www.altex.com) has some wall warts that can do 2 amps or so, including
a 12 V 40 VA transformer (3 1/3 amps) for $9
a 16 V 40 VA transformer (2 1/2 amps) for $9, and
a 24 V 50 VA transformer (just over 2 amps) for $10.

(Those are slightly better prices than Radio Shack's for plain transformers. I have a local Altex, but I don't know how widespread they are.)

If I'd known the tricks when I started, I'd probably have gotten the HF router speed controller, a 3-prong extension cord with three outlets on the end, and two of the 24 V 50 VA transformers, and wired them in parallel like I did the plain RS Tx's. (You could probably just cable-tie the wall warts to the extension cord to keep them attached and securely plugged in.)

BTW, the pictures on the Altex site don't show it, but at least some of these are center-tapped. (With those screw-down connector thingies). You could do the parallel wiring on the primary by just plugging them into the same extension cord, and on the secondary side with crimp-on connectors.

EDIT... I didn't mean to make it sound like you need center-tapping to do the parallel wiring; you don't. Center-tapping just lets you use half the range if you want. And I didn't meant to suggest putting crimp-on connectors on the screw-down posts, just on the wiring.
Paul
Last edited by drcrash; Jan 21, 2007 at 05:19 PM.
Jan 21, 2007, 05:12 PM
Dismembered Member...
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Class 2 transformers are current limited - just like those train and doorbell transformers

TANSTAAFL

Brad
Jan 21, 2007, 05:32 PM
Impact Testing Specialist
drcrash's Avatar
Quote:
Originally Posted by arx_n_sparx
Class 2 transformers are current limited - just like those train and doorbell transformers

TANSTAAFL

Brad
I'm not sure what you're saying.

Just checking...

By the specs, the Altex wall wart transformers are as ampy as the RS non-wart transformers, so you can use them pretty much the same way. (E.g., if you want 3 amps or so at 12V, no frills, you're fine just plugging one into a speed control.)

They're not as ampy as the ampier non-warts from All Electronics, so if you want a higher-amp system, you have to parallel them. (E.g., if you want 3 or 4 amps at 24 volts, you've got a little wiring to do on the secondary side.)

Seems like a wash to me. Some people may prefer to avoid doing any wiring on the high-voltage side, and other people may prefer to put it all in a project box. (If I didn't have that Girls Bath Buddies box handy, I'd probably be in the former category, just strapping it together and letting the secondary-side wiring hang out. )
Jan 21, 2007, 06:00 PM
Dismembered Member...
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TANSTAAFL = there ain't no such thing as a free lunch. Class 2 transformers (and doorbell/train tx's fall into this category) have a lot of winds on the secondary. This basically means that they are "safe" in a short circuit condition - the current is limited. Class 2 txfmrs are the only Tx's that don't need a fuse on the primary or secondary side (electrical code) because of this. They will deliver the current - as long as you don't pull them down too hard - they aren't much good for hot wire cutting though.
Jan 22, 2007, 12:01 PM
Impact Testing Specialist
drcrash's Avatar
Quote:
Originally Posted by arx_n_sparx
1) I dunno about a watt per inch - I think that's probably a good number so that you don't stress things too much. Hwere did you get that figure?
Partly from ballpark figures from BobK's setup, partly from a couple of articles on the web, including one about a bandsaw-type setup with a fine stainless wire 13 inches long dissipating 13 watts, and another where somebody using very fine wire says they only need about 0.7 watts.

I'm thinking that if the wire is mostly vaporizing the foam without touching it, this should be mostly independent of wire diameter---the kerf is more dependent on the watts dissipated than how the wire generates them, and essentially all of the heat is going into the plastic. (That probably won't be true with very big wires cutting on contact in a "hot knife"-typesetup.)

When I get my 3 diameters of stainless and rectify and meter-ize my setup, I'll do a few controlled experiments.

Quote:
2) The 40% is only if a cap is added - as you suspected. You need a big cap for this (electrolytic - and get the polarity right, or you won't appreciate the noise/smell/mess )
Does this reduce the max amps at the higher voltages? If not, where do the watts come from? (Maybe making the transformer do more work between peaks, without overloading it?)

BTW, I reorganized the list a bit and renumbered it, and put some more stuff in.

Paul
Last edited by drcrash; Jan 22, 2007 at 12:06 PM. Reason: tag typo
Jan 22, 2007, 12:46 PM
S55
S55
Registered User
S55's Avatar
I have many different wire cutters for various jobs: straight wings, tapered wings, thinning sheets, scooping, etc. Some use nichrome, some steel wire and the gauge is in the 10-30 mils range.

Nichrome wire is not the only one that can be used. Any wire will get hot if the current is high enough. In fact what matters is the total power. So nichrome may get hot when 5V are applied and a 4A current is established. The same result can be achieved with some steel wire when 1V is applied and 20A current is established. The numbers are for example purpose only, but you get the idea. The main thing to look for is to have a thin strong wire, so copper is not a good choice because the tension will stretch it.

S55
Jan 22, 2007, 01:55 PM
AMA# 851123
brucea's Avatar

Pretty Much a Standard Plan


Hey Guys,

I have attached the plan for a very standard. I have all the materials to make this hot wire cutter. I have looked at a lot of posts. Although the materials vary, this configuration seems to be pretty standard.

Thanks, Bruce
Jan 22, 2007, 01:59 PM
AMA# 851123
brucea's Avatar

Nice Design


Quote:
Originally Posted by Skinner46
This is my version of a hotwire JigSaw.
Dear Skinner,

I got to looking at your hotwire jig saw. I really like the adjustable angle cutting feature.

Thanks, Bruce
Jan 22, 2007, 02:51 PM
Impact Testing Specialist
drcrash's Avatar
Quote:
Originally Posted by S55
I have many different wire cutters for various jobs: straight wings, tapered wings, thinning sheets, scooping, etc. Some use nichrome, some steel wire and the gauge is in the 10-30 mils range.

Nichrome wire is not the only one that can be used. Any wire will get hot if the current is high enough. In fact what matters is the total power. So nichrome may get hot when 5V are applied and a 4A current is established. The same result can be achieved with some steel wire when 1V is applied and 20A current is established. The numbers are for example purpose only, but you get the idea. The main thing to look for is to have a thin strong wire, so copper is not a good choice because the tension will stretch it.
S55
The problem is that plain steel (or nickel steel guitar strings) is just not very resistive---about a quarter as resistive as stainless and something like a seventh as resistive as nichrome. So you need several times the amps if you use plain steel wire.

A 10- or 20- amp variable power supply usually costs several times what a 2- or 4-amp variable power supply costs, so plain steel is usually not the most economical choice unless you have a high-amp variable power supply lying around. (Or you have a high-amp fixed power supply and are willing to jigger the wire length & diameter to match.)


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