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 Post subject: Hot Wire Post Mortem
PostPosted: Sat Feb 28, 2009 9:06 pm 
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Any dub-E's wanna help me analyze the hot wire foam cutter failure, I'm getting various advice from multiple channels, but am having trouble formulating a solution.

So as mentioned before it was pretty simple, single pole light dimmer controlling current to a 25V/2A output transformer.

It worked great for a while but eventually overheated and burned out the transmitter (or so it would seem).

One guy suggested cheap radio shack transformers would always be sacrificial no matter what I did to try to control heat or whatever and that I should really be using a variac.

Here is a quote from somebody else "you probably overloaded it refer to ohm's law to figure your amps. I bet it exceeds the transformers output." Not really sure where to take that.

Any input would be appreciated.

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Sat Feb 28, 2009 10:46 pm 
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Any more info on the circuit? Simple schematic?

Didn't I read that you followed an instructable somewhere?

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Sat Feb 28, 2009 11:22 pm 
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I only saw it in action and working, and it worked SWEET.

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Sat Feb 28, 2009 11:24 pm 
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Do you know what the resistance of your wire was?

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Sun Mar 01, 2009 6:17 am 
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I used this Instructable. http://www.instructables.com/id/Hot-wire-foam-cutter/

Not sure of the wire resistance, can work on that.

It seems like maybe it wasn't really an overheating issue, but an overload issue, the heat I was was feeling after the fact may have been because it was already fried.

I may try it again with a slow burn 2A fuse on the hot-wire side.

There are probably simpler and more reliable (maybe not as cheap) solutions to this, but what fun would that be, I wouldn't learn anything.

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Sun Mar 01, 2009 11:28 am 
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Yea, figure out the resistance of your cutting wire.

If you used the two yellow wires, the cutting wire should be at 25V

If 25v/(resistance of cutting wire) > 2A that could have fried the transformer

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Sun Mar 01, 2009 12:02 pm 
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there are some decent slow blow breakers that might be worth the investment. much vchaeper in the long run than a fuse. personally if i see a tool thats fused the fuse should *only* go in the event od complete failure. breaker is protection that youre ok with tripping


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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Sun Mar 01, 2009 12:07 pm 
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metis wrote:
breaker is protection that youre ok with tripping
Absolutely yes.

Also, I can probably provide a 12V DC computer-style fan or two, if you (Noise) still need some heat management after you've got the electrical formulae worked out.

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Sun Mar 01, 2009 2:51 pm 
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Thanks Pyro, that's what I wasn't able to put together. My multimeter is less than accurate at these resistances, but it seems like the it could be from 2 to 5 ohms which means that it is pointless to even run the transformer at 25 watts since any resistance less than 6 ohms can take me over 2 amps.

So I'll try running it at 12 volts, add a fuse and see where it gets me. I already picked up a fuse holder and bag of fuses. If I burn through them too fast I'll think about getting a breaker.

Not sure what to do about a fan since I would need a separate power supply to run most of them, I suppose I have some room in the enclosure so I fit a wall wart in there. I would also like to add a bulb so we know when the thing is on, a small incandescent bulb inside a cage would have a nice retro look to it (because such things are important!). Of course if I'm going that way I should build a nice wood enclosure... :)

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Sun Mar 01, 2009 7:00 pm 
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What am I looking for if I want to go with a breaker?

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