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Safe Hot Wire

 
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Old 04-29-2006, 02:54 PM   #1
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Safe Hot Wire


Hi, I am new here. I am a specialty painter who mostly works on films. Currently I am building a whole lot of styrofoam mountains. My hot wire machine is in repair and I need a couple back ups. I know they are easy to build But I need some advice to keep it safe.

I am making a 2-8 foot hot wire. In the past I have used ac 110v/15amp outlet with lighting dimmer dial to control wire temp. Attaching the two wires to each end of the nichrome wire. Basically making the hotwire a fuse that heats up. This method I think is a rather dangerous electrocution risk.

I have also used the same system with a variac to control the wire temp. But I assume this still carries the same risks.

I have used a welder as a power source. This converts the power to dc and drops the voltage and raises the amps. I assume I am safer now since I am isolated from the the ground. But it seems a little overkill.

I am now trying to hook a dimmer switch to the household power for wire temp control. Then go through a car battery charger to give me isolation and drop the voltage. With a 10 amp charger I can only heat an 18 inch wire. I am not sure if its the amps or volts that I should increase.

Someone recommended I pick up an isolating transformer and put in a dimming switch after it. This will give me all the power I need, but will keep me isolated from electrocution if something goes wrong.

I would appreciate any ideas on this. My goal is of course making the tool and operation as safe as possible.

Thank you
Tom
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Old 04-29-2006, 03:48 PM   #2
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Re: Safe Hot Wire


I bought my first one just to see how they were done and built every one since. I stick with your #1 design. Where are you worried about electrocution? The risks should be minimal. What are you using to build the frames?

I used to build hydrofoils and aicraft wing sections for composite designs.

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Old 04-29-2006, 04:35 PM   #3
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Re: Safe Hot Wire


Thanks for your response.

Currently I am using a small wooden bow type frame for one person cutting with 2 feet wire. I also use two plastic or wood handles (and two people) to stretch a 8 foot wire. I clip the live wire ends with aligator clips or tie to the screws holding the cutting wire. Since it is the people moving, not the foam, it is more likely that someone may step on their live wire and disconnect it from their handle and end up touching a live wire. The hot wire can break easily as well, therefore leaving a live bare wire flopping through the air. There is nothing to ground since the only metal is the screws holding the cutting wire to the wooden handles. I am assuming (perhaps incorrectly) that by transforming to dc or isolated ac the only risk is burn (not electrocution)if someone touches both ends. And that since ground is out of the equation it would be also then become safe to touch any of the ends alone, even if standing in a puddle of water.

?
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Old 04-29-2006, 07:14 PM   #4
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Re: Safe Hot Wire


Here are some general electrical facts that might help you.

lower voltage is safer. 50 volts AC and DC seems to be the threshold when it comes to safety.

DC is safer than AC.

To make your heart stop takes about .1 amp at 120 volts AC (12 watts)

An arc welder is about 30 volts AC or DC from ground to the welding rod.

For your application I would look at a 120 VAC to 24 VAC transformer. You would use the 24 VAC to heat the wire to cut the foam. Converting the 24 Volts AC to 24 Volts DC and then using 24 volts DC to heat the wire would have added advantages.

I will stop here and wait for your reply just to make sure I understand what you need.
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Old 04-29-2006, 07:46 PM   #5
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Re: Safe Hot Wire


I really appreciate these responses since I need to be up and running on Monday.

I suspect the battery chargers are not working because of the polarity check they do. This might explain why I cannot hook two battery chargers together (red to black).
I have read that guys are getting 4 feet of wire to heat with 24vac/2amps. I would like to get 2 - 8 feet. Perhaps 24v/5amp might do it.

What would the advantages be to use 24v dc over 24v ac?
..and how would I make this conversion?

Does a 120v to 24vac also act as an isolation transformer?

Currently I am thinking plug a 24v/5amp transformer into the wall. One wire goes from the transformer through a dimmer switch, then to the hot wire. The other wire goes from the transformer directly to the other end of the hot wire.

If this is the best solution, is there anything I can do to make it safer.

Thank you
Tom
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Old 04-29-2006, 08:17 PM   #6
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Re: Safe Hot Wire


Here are some answers to your questions

You asked "Does a 120v to 24vac also act as an isolation transformer?"
Yes it does.

You asked “What would the advantages be to use 24v dc over 24v ac?
..And how would I make this conversion?"
The wire would probably last longer, the heat produced by the wire would be more controllable and the tool would probably perform better. There are a lot of unknowns about the tool that need to be specified but it still will work with 24 Volts AC.


Hang on I am going off line to work on this.
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Old 04-30-2006, 04:45 AM   #7
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Re: Safe Hot Wire


Back again:

After thinking about this awhile I need some more information in order to help you out. The real important information concerns the cutting wire properties. I need to know what is the wire size, the resistance in ohms per inch/foot/mm, what material it is made of and the source/ manufacturer where you got it. (I hope you do know or can get this information) How hot the wire will get and how much power (amps, volts) required is directly related to this information.


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Old 04-30-2006, 10:37 AM   #8
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Re: Safe Hot Wire


Tom
Something like this might work for you.
http://www.jandrweldingsupply.com/st...ster85TIG.html

Can you find a welder somewhat close to this one to try it out? What you need to look for is something that will vary the amperage and not necessarily the voltage. An arc welder will do this.
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Old 04-30-2006, 11:07 AM   #9
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Re: Safe Hot Wire


27-Gauge Inconel Wire (0.0126) is the wire I bought at www .demandhotwire. com. It is for the MAG8 machine they also sell. The input on the machine is 120vac/4amps, but it gives no output voltage.

If I remember correctly, an enclosed variac has a 3 prong output socket. I would guess this means a variac is not an isolating transformer. I have used a 3amp variac before and it sat about at the 50% markings for a 5 foot wire. I know it works, I just question its saftey. I have also used a welder before and it works. Again, is it safe?
The details about the welder I used I cannot get until monday.

Tom
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Old 04-30-2006, 11:18 AM   #10
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Re: Safe Hot Wire


Tom

The MAG8 machine looks ok and should be safe to use for your purpose.

Note: I learned something new today because of your post. Thanks

Last edited by CE1; 04-30-2006 at 02:08 PM.
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Old 05-03-2006, 12:49 AM   #11
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Re: Safe Hot Wire


My dad has the standard "buzz box" welder. It has only one problem. It will shock the snot out of you if you are change the rod bare handed when standing on wet ground. I would suggest a GFCI in the line going to this set up. Also a fuse or circuit breaker just a bit bigger than the load. More safety is better than less.
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Old 05-04-2006, 09:59 PM   #12
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Re: Safe Hot Wire


I tried hooking up a 25v/2amp transformer and I was able to heat 3 feet of inconnel wire. Then I tried a 24v/4amp transformer and only gain a few inches. I could try a 50v/5amp transformer but I think a cheap stick welder with a dial (opposed to a high/low switch) would probable give me enough juice. If anything I might not be able to get it low enough for shorter wires, but that is where my other rigs come in.

I bought another mag8 machine and will open it to see how it is rigged up as soon as the warrenty runs out. It runs 2-8 feet.
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Old 05-06-2006, 11:41 PM   #13
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Re: Safe Hot Wire


Quote:
Originally Posted by CE1

You asked "Does a 120v to 24vac also act as an isolation transformer?"
Yes it does.
I disagree with this. I would ONLY say isolation, if the transformer actually said it. The neutral on the high & low sides could be tie together. This is so on some furnaces. Just grab the hot wire for the T-stat when standing on a wet concrete floor. Not bad, but I have been shocked.
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Old 05-12-2006, 10:18 PM   #14
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Re: Safe Hot Wire


I don't know if it would be large enough capacity, but we build radio controlled model airplane wings out of foam and use an automotive battery charger as the power supply. I have used a bow with cutting wire up to 3 feet in length and it cut just fine.

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