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> resistance wire temperature controller?, setup and components and values
noobelectron
Posted: June 20, 2004 02:38 pm
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Hi! I'm new here, and to electronics.

I've got a problem maybe someone here would find easy. This is what I want to do.

I want to use mains electricity and a controller or some description, a rheostat or variable resistor, to crudely control the temperature of a section of nichrome wire approximately 10ft long.

Household electricity in my country, australia, is 240V@15Amps.

Here is a table, stating wire widths, then relating the amount of current required to heat each particular wire to the specific temperature.

http://www.pelicanwire.com/tempNichrom80.htm

The .036 diameter is the wire I'd like to work with, this one reaches 1093C when the amperage across it reads 14.91Amps.
I could settle for a smaller diameter if you say so...

Also available on that site is the resistance per foot for a each type of wire! For the .036 diameter wire, it's approx. 0.5 Ohms/ft.


What type of components would a good, simple and safe control box for something like this include? blink.gif
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Sch3mat1c
Posted: June 20, 2004 06:24 pm
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Depending on how much you want to heat (oh, and you forgot what kind of environment it's in - horizontal, vertical, no wind, insulated with firebrick, etc.) you'll need a lower voltage... A light dimmer will work for control since it's just a cooler light bulb. wink.gif You can make an electric dimmer, add in a thermocouple and some other stuff and you've got temp control. smile.gif

Tim


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Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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noobelectron
  Posted: June 21, 2004 06:59 am
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Tim, thanks for the reply.

your right about the voltage, because I looked at another page on the site where the other URL's came from, that states the recommended wattages at the operating temperatures for the individual wire diametres.

http://www.pelicanwire.com/sizeapproxcoldresistances.htm


The environment will be a tubular vessel made of ceramic fibre, and the nichrome will be coiled vertically around the perimeter.

As far as the desired temperature, I suppose a maximum of 1000-1100C would be really nice!


So first step is to select a wire, second is to step-down the voltage (I should be fine selecting a transformer) so that I'm in the recommended wattage range.

Then I'd buy a dimmer appropriate for the now stepped down voltage @ 15A at the tranformers output terminal.

The dimmer should allow a maximum of 15A through at it's maximum setting, if I required 15A to attain my maximum temperature. (But, I think Ill settle for a lower guage by the way.)

1. The dimmer would control the current only, right?

2. What purpose would a thermocouple have?

3. Would it be safe to use a metal box for the controller, If I grounded both the box, and the tranformer, where the transformer is screwed down to the metal box?

(Ill pay special care here, and won't test it until I post a circuit diagram explaining exactly how it is wired, and what's grounded) dry.gif

4. Where in the circuit do you think would be the best place to add a switch?

5. How would varying the length of wire at ohm/ft rating effect things? Which component choices would I have to re-evaluate if I decide I want a much longer heating element?



That site does sell in small quantities by the way. 20Ft, 50Ft coils on quite a few selected items, that section is a bit hidden on the site though.
http://ec-securehost.com/PelicanWireCo.Inc./

This post has been edited by noobelectron on June 21, 2004 07:21 am
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Sch3mat1c
Posted: June 21, 2004 07:13 am
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Hm, you makin an electric furnace? Kiln? Look at my website, you are in good company. wink.gif

If you want just manual control, that's fine. With a thermocouple wired in, you can have it maintain a certain temperature, for heat treating for instance. You doing pottery, metalwork, ?

There are many controllers already out there that may suit your needs.. unless you want to design one yourself, I can identify with that. cool.gif

Tim


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noobelectron
Posted: June 21, 2004 10:52 am
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Hey tim, your quite an admirable one! Inspiring aswell!

Due to the fact that, when your interest first budded in 'casting', you could have thrown in the towel before you got started when you realized you couldn't afford, or otherwise have access to proper industrial-grade equipment, but you've persevered and made-do.
You lugged a 55 gallon steel drum home a mile, and the list of similar feets goes on and on by the looks of it.


Well I have grown up with kilns, my mum has done ceramics over the years, we have two kilns, both electric, reaching 1300C.
Now, because of this current project, I realise why electric kilns aren't suitable for porceilon, which requires 1450C, because nichrome compositions melt at 1400C.

A kiln is really convenient for producing charcoal though btw.

Anyway, back on to the real topic..

What I'm building is a miniture kiln, or retort to prepare relatively small amounts of metal oxides from carbonates. Metal carbonates + heat ==> corresponding metal oxide.

Just fun little experiments like that. wink.gif

As for the wire gauge, I think Ill just buy a few types and go from there.

The transformers Ill rip out of old stuff, I've got a few laying around anyway, in addition. Hopefully the ratings are written clearly on them.


unless you want to design one yourself, I can identify with that.

haha I can tell. Yeah, Ill definetly design it myself, things seem quite a bit clearer and straightforward now. thumbsup.gif

http://www.dansworkshop.com/00-index/14-Am...0foamcutter.gif
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damien
Posted: June 21, 2004 11:37 am
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Where abouts in Australia are you?

I live in Adelaide........

Damien


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Wow you look quite normal for a geek.

The statistics on sanity are that one out of every four persons is suffering from some sort of mental illness. Think of your three best friends -- if they're okay, then it's you.
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Sch3mat1c
Posted: June 21, 2004 06:31 pm
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Thanks thumbsup.gif So you just want to calcine stuff? I've done that in the furnace before, put some (limestone, etc.) in a tin can, fire it for about an hour (around yellow heat), remove, viola, lime! Mind you there's nothing left of the tin can...(but hey, you want metal oxides, now you have some iron oxide too. wink.gif )

If you need to know when some things break down, I found these pages:
A-K and L-Z.

QUOTE
when your interest first budded in 'casting', you could have thrown in the towel before you got started when you realized you couldn't afford, or otherwise have access to proper industrial-grade equipment, but you've persevered and made-do.


Actually, I realized early on (from reading web pages) that you can use nothing but JUNK and make some nice stuff! That's the beauty of metalwork. smile.gif

Tim


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Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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kor
Posted: June 21, 2004 08:25 pm
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Just a little note, you're acting like you'll only get 15A out of the mains. That's the maximum, how much you'll get out is dependent on the voltage supplied and the resistance.
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noobelectron
Posted: June 22, 2004 01:30 pm
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Tim, I haven't read about why you melt large amounts of metal yet, or what you mould it in to, but Ill get to that.

Exactly right, calcination, decomposition, oxidation and reduction. wink.gif


Damien, g'day, I live in Melbourne.

QUOTE
Just a little note, you're acting like you'll only get 15A out of the mains. That's the maximum, how much you'll get out is dependent on the voltage supplied and the resistance.


Kor, g'day.
So how should I factor that in properly?

Say I choose to step the voltage down to 115V, and I use a piece of wire which has 10 Ohms resistance in total.

115V / 10Ohms = 11.5 Amps

So would I have to step up the amperage, and aswell, step down the voltage (to keep the wattage in the recommended range), if I required 15A to reach it's maximum temperature?

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kor
Posted: June 22, 2004 06:54 pm
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Start with ohm's law:
voltage=current/resistance

You know the current you need for your desired temperature, and the resistance of the wire you're using. From that, you can get the voltage you desire. You can then either get an appropriate transformer that can handle the desired current, or you can get a variable transformer (someone help me out here, are those variacs?) that can handle more than the required current, and vary the voltage until you get what you want. This is most useful if you'll be changing voltages a lot, but just getting a transformer that's close off the shelf will probably be cheaper.
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Sch3mat1c
Posted: June 22, 2004 07:44 pm
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To a close approximation, power in = power out of a transformer. P = V*I so say you have a 1kW heater at 110V, 9A.. at 240V that would be 9 * (110/240) amps at the primary side. Current and voltage ratios are inversely equal, one drops in return for another rising so that the product (power) is the same.

Tim


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Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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