Making an electric furnace.

[Alun]

Just thinking you could use an op-amp PWM circuit like the dome lamp dimmer and either connect it to a lamp and use an LDR like Ante says or reduce the frequency of the PWM circuit to about 5Hz and connect it to a solid state relay circuit as I would strongly reccomend.

Either way you're going to need a ramp generator that generates a ramp as long as a few hours - I don't think you can do this easily with simple RC circuit as you would need a very big low leakage capacitor. Maybe a DAC with a  binary counter would do the job, even so I don't think this would be too complicated.

[Ante]

Alun, Dazza;

I would prefer to wrap the furnace with the kind of bandage you use for turbo exhaust manifolds (the kind you can touch at full working temperature) and on top of that you just might put some foil (shiny side inwards). This will save some energy and time for you.
BTW, I can see you have got the LF PWM to stick now.  But what

[Alun]

Hi Ante,
I would agree with everything you said but I would put the foil matt side down. If you dont't believe me the you could do my Thermos flask experiment.  Take 3 plastic bottles wrap on in foil with the shiny side up, the other with the shiny side down and leave the last one unwrapped as a control. Fill them up with hot water and leave them to cool.

I don't really know whether shiny side up is better than shiny side down I don't think it will make that much difference in this case. I just know that the more shiny a surface is the less will radiate the heat. The side of the foil touching the hot surface can't reflect the radiation back as it's in physical contact with the surface so the heat will be conducted straight to it. I don't really want to discuss this at great length just do the experiment and see.

About the ramp generator
Regardless of whether Dazza wishes to use your idea or mine a ramp generator is required. Because Dazza says that the nichrome wire will be damaged if the furnace is turned on too quickly it needs to be turned up very slowly. When the power is first applied the ramp generator could be connected to the PWM unit that's  connected to a bulb coupled to the phase controller via and LDR or as I recommend a lower frequency PWM unit could be connected directly to a solid state relay, as there are no bulbs or spiky RF causing waveforms to worry about.

To make a ramp generator you use a constant source to charge a capacitor. A 1F capacitor will take 1 second to charge to 1V  with a constant current of 1A, So for 10s to 1

[Ante]

Alun,

Why to you think a picnic thermos flask is shiny inside? The shiny side of a foil will reflect light and heat; this is why I want the shiny side inwards to keep the heat in the furnace. Light and heat are radiation just with different wavelengths. This is used in space to protect satellites and spacecrafts from heat and radiation (often using gold foil). And here on the ground or almost on the ground we use it in our shoes on insulating soles to reflect back the heat to the underside of our feet. And it

[Alun]

Alun,

Why to you think a picnic thermos flask is shiny inside?

I bet it's more shiny on the vacuum side (the part you don't see)

The shiny side of a foil will reflect light and heat; this is why I want the shiny side inwards to keep the heat in the furnace.

This is a common misconception as I said the foil's in contact with the furnace, thus it can't reflect any heat, so the heat is immediately conducted to it. Try this, hold an open biscuit tin up to your face and will feel warm because the shiny reflective surface on the tin is reflecting your body heat back at you, but if you touch the tin it will feel cold.

Light and heat are radiation just with different wavelengths. This is used in space to protect satellites and spacecrafts from heat and radiation (often using gold foil).

Yes, it works both ways.

And here on the ground or almost on the ground we use it in our shoes on insulating soles to reflect back the heat to the underside of our feet. And it

[Alun]

Try this, it's not a very good linear ramp generator it will just output a voltage wave form of 16 steps - more than adequate for this application.

[Dazza]

Hi Ante, Alum,

Ante, sure I like the idea, but it's the putting it all together I'm not too sure about, by the time I'm finished that is if I can work out where to start or how to start, I should be able to pick up satellite TV with it, the way things are going .

Alum, Sorry my electronic knowledge is very limited , I don't understand the circuit you have provided or how to use it, I understand it is for creating a slow rise and fall response to the elements .

[Ante]

Alun,
That

[Dazza]

Yes Ante , amplify the voltage to the correct level, to display the temperature on to my 3.5 LCD panel Meter, then I can work on the inverting amplifier to drive the LED, then look at what else can be done ones that is working correctly .

I can borrow a thermometer from my neighbour that he uses for his homebrew, its maximum temperature read out isn't very high, but it should be enough to get a fairly accurate idea of what my thermocouple voltage output represents.

I will post the results tonight .

[Alun]

Hi Dazza,

By the way it's Alun not Alum

The circuit I've posted is a 4 bit binary counter with a poor man's analogue to ditital converter on the output.

the circuit counts from binary 0000 to 1111 thus connecting resistors on the A B C D outputs in various combinations these in turn form a potential divider with the 1K connected to ground.

The counter is counts a stream of pulses on the clock pin I've not shown it here but when the counter reaches 1111 it needs to stop or it will keep generating the ramps every 16 clock cycles.

I would connect this to the burst control unit. This will then turn the power up slowly. Then you use switch control to turn the furnace off when it gets too hot and back on again when it's too cold. Suppose you set the turn on temperature to 750

[Dazza]

Hi Ante, (Alun )

Ante, here are the results of comparing the thermocouple voltage output to the thermometer.  There is quite a difference between the temperature of the Molten aluminium and from where I measured the temperature of the furnace.  I think I should be able to use the 200mV range of my 3.5 digital panel Meter, to represent the temperature. Ok 18.8mV = 560 so 188mV as it would be seen with out the decimal point on my digital panel Meter, times it by 3 A gained of 3 = 564mV, so what value resistors should I use for R2 and R1.  Also I should be able to use the second amplifier of the TL 061 as an inverted amplifier to drive the LED?  Amateur I M does it show .

18.8mV = 560C air temperature inside furnace.
25.2mV = 780C temperature of Molten aluminium.

200C = 07.3mV          
190C = 07.0mV          
180C = 06.6mV
170C = 06.1mV
160C = 05.6mV
150C = 05.1mV
140C = 04.7mV
130C = 04.2mV
120C = 03.7mV
110C = 03.3mV
100C = 02.8mV
 90C = 02.4mV
 80C = 01.9mV
 70C = 01.6mV

Thanks Alun, I understand much better now how it works. You still want me to go with your idea, of the on-off switching, very persistent aren't you .  I helped my neighbour lay a few slabs of concrete, I told him that he needs to soak the ground before pouring the concrete, as the ground is very dry and it will draw the water from the concrete very quickly and it will crack, he just wouldn't have it, and sure enough it cracked , man I was very persistent as I can be , and he was starting to get very annoyed with me , he has moved house now not because of me, we are still good mates, last week I went for a visit and he had laid a slab of concrete leading from the front of his house, yet again he still didn't soak the ground first and it was full of cracks, he believed that it would be sufficient to occasionally hose down the concrete, sometimes there is just no changing someone's mind .

I'm sticking with the way I am going with the temperature control, but you have convinced me that your way will work fine , once I have got everything set up for my foundry, I want to build a small furnace that is capable of casting a large heat sink or a good size project box, and offer it as a project in the project section , that is if mixos wants it there. So when the time comes I would greatly appreciate your help in setting up the temperature control, your way .

[Alun]

It's like when my boss at work told me to cover a table in a very thick coat of varnish. I told them it would better to use 3 thin coatings and one thick coat would take weeks to dry and look awful. But he insisted I used one thick layer and it would take too long to do 3 coatings. So I did as I was told and applied the thickest layer possible and it took more than a week to dry and looked terrible he was not amused.

Anyway Dazza, your results seem to be very linear and go up by approximately 0.35mV per 10

[Ante]

Hi Dazza,

If you set up the op-amp to a gain of 3 your display will show 56.4mV at 560C. Of curse you can

[Dazza]

Hi Ante, Alun

Thanks for your help, its greatly appreciated .  I shouldn't have too many problems getting this project to work now .

Ante, I was thinking there was two opamps in the one package, but of course there isn't , so I will have to use another opamp to drive the LED to influence the LDR.

I have been busy setting up the rest of the equipment I will need for the foundry, it's coming along nicely with the exception of obtaining a product called (Bentone) which is an ingredient used in high-quality moulding sand. I have done a test casting using (green sand) which is a very basic moulding sand, and I'm not happy with the results , so I need to use a better quality moulding sand which is called K-Bond or O-Bond, which contains the Bentone.

I have come up with a few product ideas, that I can apply electronics and aluminium casting to, to produce a product I can market locally .

I now have to get my foundry set up, and produce a marketable product as quick as possible , to produce some extra income, as we have recently found out that there will soon be a new member to the family .

Here are some smiley faces to show how excited and happy I am.