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Dazza

Making an electric furnace.

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Hi Ante,

It's pretty much as you described ;), the picture below shows three of the four ingredients for the refractory, top left is the perlite, below that is the silica sand and the third is Portland cement, the fourth ingredient to be added after these three are mixed and moistened, is fireclay or bentonite.

The pic on the right shows two spiralling formers, which will create the cavity for the heating elements, with insulating tape joining the two to help create a taper, that will allow the heat from the elements to escape into the main cavity more freely. The spiral formers are glued to the main former body, the tape at the ends of the formers are to hold the ends down until the glue has taken.

Top left shows three drinking straws passing through the outer shell penetrating into the spiral formers, at the point where the heating elements will be connected with machine screws passing through to the outside. There are steel rods inside the straws to prevent them from moving and deforming under the wait of the refractory.

It hasn't all gone without any glitches :(, I'm pretty sure I got a few air pockets in the refractory, which is bad news for when I fire it up, it's likely that it will shatter :o. I fully expected to run into these sorts of problems, it's a learning experience with some trial and error ;), and the expense of making a mistake is very little 8).
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Hi Dazza,

Thanks, that explains a couple of things. So, you worry about air pockets in there? Is the mixture very dry when you work with it? With a wet mixture you could vibrate it to avoid cavities. Just a thought. ;)

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Ok, so far so good, every think is coming along nicely better than I expected :), I have applied heat to the furnace for several hours to drive out moisture, a few more hours and she should be right to crank up the heat, once I make the new heat controller unit, and install the second element 8).

Ante, It is said to use this refractory mixture fairly dry, like cookie dough. It is also said that this concoction is sticky as I don't know what and it certainly is, what a nightmare to work with :o. Yes Ante, I was thinking the same thing ;), but then I realised if I made it to wet there would be too much shrinkage, I did wet it more than I knew I should, and still this gunk did not want to play nice >:(. Now that I have first-hand experience and know how nasty this stuff is, I'm pretty sure I have worked out a way to tame the beast, and make it do what I want :),

Oh yes did I mention this stuff is very nasty to work with ;D ;D ;D ;D.

I don't understand what's going on when I measure the voltage out from the heat controller unit, it measures around 330VAC ???

Here is a couple more pics :)
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Hi Dazza,

I am sure you know what you are doing, I find this project very interesting. I suppose the 330Volts must be the peak value of the mains voltage that you see on the meter. Please keep reporting as you proceed! ;D

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Well it looks like it works 8), and I am ahead of schedule :). In the first pic you can see my you beaut trolley ;D ;D, with the furnace mounted on it, I was planning on salvaging the wheels from this old lawnmower to use for making a trolley, then I thought why do that when all I have to do is sit the furnace on it and she's all done.

The other pic shows her in action, the aluminium was not properly melted down, it would of taken about another 15 minutes to melt down to the point where it would have been right to go, unfortunately I had to cut this test run short :(.

Sorry for the bad pic quality, there wasn't enough light :-\.

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Hi Ante,

Nothing went wrong, it was just that I had to go out and I didn't want to leave the furnace running unattended, always safety first ;).

I have fired her up again this morning, the furnace temperature control works great, with a large heat sink it doesn't get hot enough to need to use a cooling fan. Here's a couple of better quality pics 8).


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Hi Dazza,

This looks really great, like a professional device! If you run it down the street people would think you are part of the bomb squad on a mission! ;D Just kidding, I think this is a very smart way to handle a devise like this. If something goes wrong it

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Guest Alun

Dazza,

Just a few questions:

What are you planning to use as a mold?

How long does it take to melt the aluminium?

Do you think it would be more efficent to use a higher powered heater?

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Hi Ante, Alun

Melting point for aluminium is around 660C. I don't know what the temperature should be for pouring into the mould ???, when I was working in the foundry we knew it was at the right temperature, by its texture and colour and it was very rare that we ever got it wrong, hopefully I still have that ability 8).

This is the most common method for a backyarder to use as a mould, green sand (http://www.backyardmetalcasting.com/book_sc.html) and this is the method that I'll be using to begin with, there are many other methods as well for example, the loss foam method and dry sand.


The time it takes to melt the aluminium, is about 2 to 3 hours, each melt after that takes about 30 to 45 minutes roughly,

High-powered heaters, will help to a certain point, that is to reduce the time it takes to raise the furnace temperature up to its maximum temperature, which is governed by the maximum temperature the elements can handle which is 1200C, for the elements that I'm using, Nichrome 80.


So now it looks like I'm going to have to maintain a constant temperature in the furnace, if I want to avoid the temperature rising above that of which the elements can handle and to help them last as long as possible. I was really hoping that I wouldn't have to do this, man nothing is simple is it.

Ok, I can get a K type Thermocouple that is rated to over 1200C, for 13.00$ that is meant to be used with a DMM, that I think should do the job. Next I will need a Thermocouple Preamp, I have been searching the net for a circuit, that uses commonly available components, I haven't had any luck yet. I already have a LCD digital panel meter, that I could use to monitor the temperature, once I get that sorted out I don't know what to do, that is how to connect it to my existing temperature control circuit? Maybe I could connect a relay between the DIAC and the gate of the TRIAC, to switch the power on and off to the elements, this might be the simplest way for me to do it, but I would much prefer to be able to have the output from the temperature sensor, to very the power to the elements to maintain a preset temperature, I will also have to be able to set a maximum, as I do now or the elements will burnout straightaway.

Any help please, I don't really know what I'm doing, this project is beginning to become more complicated than I expected
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Hi Dazza,

Which range on the DMM do you use for the thermocouple? Is it perhaps possible to drive a LED instead through an inverting op-amp, and then you could swap R4 for an LDR and put both of them in a tube or similar. I think you get the idea it just might work. Please speak up anyone if I am way of here! :-\

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Guest Alun

Ante you're on the right track but you can buy special Triac opto isolators for this purpose.

Dazza you've used the wrong symbols for the Triac and Diac. The symbols you've used are N-channel JUGFET for the traic and a rectifier diode for the Diac, here are the correct symbols:

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Hi Ante,

Yes Ante I do get the idea, and it is a good one, and I think it will work.
Your idea means that I won't have to worry about mains power, while trying to get it to work.

I have no idea what range is used on the DMM, the thermocouple that I'll be using, is meant to be used with a DMM that is designed for measuring temperature.

I guess the most important thing to know, would be how much current a K type thermocouple can handle.
I'll also be wanting to use a LCD panel Meter to display the temperature, as well as drive the LED.

The accuracy of the LCD temperature display and the temperature regulation, doesn't have to be all that accurate, near enough will be plenty good enough, for this application :).

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Hi Alun,

I don't know much but Yes I did know they were the wrong symbols ;), ExpressSCH unfortunately didn't have them available, so I went with near enough, I could have made a custom component but I guess I was being lazy ;D.

It is very likely I will make mistakes along these lines in the future, I do hope you will keep an eye on my future projects :), thanks for your reply Alun.

Alun, what do you think of Ante's idea :D, do you think using a optoisolator would be better, I do have available to me a(4N25) optoisolator, I would like to try to keep things as simple as possible.

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Alun,

Yes I know about optoisolators, but the once I have used are digital not analogue as was needed here. The idea was to replace the pot with the LDR and use the rest of the circuit as is. If I was to use digital (on / off) isolators I must convert the measuring voltage to PWM or a schmidt trigger circuit with a max and min temp on / off. This would mean a completely different triac circuit, that

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Ante, I'm going with the LDR and LED ;), I just need to find a thermocouple Preamp circuit, that uses commonly available components to work from, I'm just not to good at starting from scratch :-\.

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Guest Alun

Ante, look at the attached data sheet and you'll se what I mean.

The circuit Dazza is useing is a Phase control power controler put simply R2, R3 & R4 form a timeing circuit with C2 when the voltage is high enough the Triac is triggered, if it's triggered at the start of the AC cycle the power will be high and if it's and the end of the cyle the power will be low.

See this link for a more detailed explanation: http://www.opamp-
electronics.com/tutorials/semi_theory_ch_007.htm


Phase controllers are very good for light dimmers but they do produce a lot of RF interference. This opto isolator I've shown here has a built in zero crossing unit this means the triac is only triggered at the 0V point of the cycle, this makes it power efficient (you could get away with a smaller heat sink). The only thing with this is the power is either on or off but this doesn

moc3041.pdf

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Guest Alun

The power control doesn't need to be continuously variable, a simple on/off switch will do the job very well. Most temperature regulated heaters don't use analogue control, they use a bi-metalic switch, electromechanical relay or solid state relay.

If you set an electric iron to a specific temperature it will turn off the element when it's hot enough and back on again when it cools down. These devices often have some hysteresis to prevent the switching frequency from being too high. A springy bi-metal strip has some hysteresis, and a device using an electromechanical relay uses an op-amp to give this hysteresis.

Now as Dazza's circuit is using a solid state relay the switching speed is unimportant and as long as it's less than 100Hz there wouldn't be a problem. The furnace has a relativity large mass it would the have a long enough thermal time constant to prevent the frequency from being this high anyway.

So I would recommend connecting a comparator with some hysteresis (just in case) to the input of this solid state relay. You wouldn't even have to reprint the circuit board, just make the following modifications:
Remove R3
Replace R2 with the triac side of the opto-isolator
Remove C1 & C2
Replace the diac and R1 with tinned copper wire.
And there you have it, a solid state relay.
The comparator circuit could be on a separate board and will depend on the nature of the thermocouple.

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Alum, thank you so much for your help and advice :), what you are suggesting, would basically mean a complete do over , I think that the big problem that I will find with the switching on and off, is that it may be very stressful on the type of elements I am using Nichrome 80 wire. This is one of the reasons why I wanted to avoid using a on off type of control. With Ante's idea, all I have to do to the original circuit is replace R4 100K with an LDR which will be varied by a LED, the LED has to be varied accordingly to the temperature I want to maintain, I also want the temperature displayed on an 3.5 LCD panel Meter,

I could really use some help on getting started on this circuit, any suggestions of a suitable opamp that is commonly available, and some resistor values to get me going please :-\.
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Guest Alun

Dazza,

What you need is an inverting amplifier that turns and LED fully off when the furnace is up to temperature and fully on when it's cold. You also said it might cause a problem if the elements are suddenly turned on and off (though I personally disagree with this) you will need a soft start because when the furnace is first turned on it will be cold so you need to turn the power up slowly until it reaches it's temperature.

With incandescent filament in light bulbs switching can shorten the life but they run at 3000C, not 660C. Sudden switching of this furnace is not likley to be a problem an electric oven doesn't use phase control nor does any other heating appliance (at least to my knowledge) So I would still recommend the switching idea.

The circuit's lot simpler too and there's less experimentation, you use a comparator (- pin) to look at the voltage on the thermocouple and set the (+pin) to the voltage given by the thermocouple when the furnace get too hot. You may need some hysteresis and depending on the thurmocoupler output you may need a pre-amp but this is very simple and I can easily fix that for you - I will need to look it up though.

Ante, PWM for AC is called bust control, a zero crossing unit is used and the load is turn on for a certain number of cycles and off for a number of cycle, and the switching frequency is - very low typically <10Hz

Anyway Dazza forgive me if you've already answered this question before but what voltage does the thermocouple give when the furnace is hot?

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