# Electric furnace - Controller box (plan + photos)



## Noxx (Jan 3, 2008)

Hello guys !
I finally got my thermocouple (K type) so I finished my controller box.
This box will control the heating element and show temperature. There are also a SSR (solid state relay) and a switch to turn heating on and off.

Here is the schema (click to enlarge):

http://www.goldrefiningforum.com/images/electricfurnaceshema.JPG

And a few pictures of the box:

http://www.goldrefiningforum.com/images/controller003.jpg


http://www.goldrefiningforum.com/images/controller004.jpg

Picture of the inside
http://www.goldrefiningforum.com/images/controller005.jpg

I know the box is ugly but it's functional :lol: 

Please note that the switch is only controlling the heating element but not the PID. This way, I can turn off the element without loosing the temperature monitor.

Any comments or recommendations ?

Thanks


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## lazersteve (Jan 3, 2008)

Noxx,

I like your ingenuity. Clever design using the plastic tote. 8) 

Here's a few concerns I have about the design and schematic.

1) The plastic box may melt if too close to the furnace during operation.

2) The switch in the schematic should remove power from the relay control coil, this will de-energize the furnace coil for added safety. 

With your current switch configuration the coil could remain energized by the PID even with the neutral line removed by the switch. If you happen to touch this floating neutral leg when the relay is still energized you will complete the circuit and suffer a nasty shock or worse. :!: Additionally, by placing the switch on the relay control leg, you can use a smaller switch to control the power to the unit. In your current schematic your switch has to handle all the current the coil gets. You could also add a failsafe switch to the box lid to kill the power when you open the box.

A isolated ground is another typical safety feature with AC circuits. You need to isolate the house circuit from the unit circuit in case of a short in the unit. You can achieve this with a simple isolation transformer. At the very least you should put an in line fuse in the AC feed.

Steve


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## lmills148 (Jan 4, 2008)

Very creative, but steve is right you dont want to open that nuetral for many reasons, including having to replace that relay more often than you have to. 2 other things I noticed the romex your using doesen't hold up to heat very well, I would suggest using MC and and increasing your wire size to 10AWG to help dissipate the heat faster and finally using a steel box would allow you to ground the box and any metal parts on the furnace to eliminate any possable difference in potentials. 

note: TTL, or solid state semiconductors are very sensitive to heat I think there function derates around 90 degrees F

steve 
using a quality surge strip fused accordingly may eliminate the need to isolate the ground if you eliminate the difference in potential between non current carrying conductive parts. Assuming it operates from a single power source.

sorry if i overcomplicated a simple Idea
Lloyd


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## Noxx (Jan 5, 2008)

lmills148 said:


> I noticed the romex your using doesen't hold up to heat very well, I would suggest using MC and and increasing your wire size to 10AWG to help dissipate the heat faster...



Sorry, I'm not sure to understand what you're talking about...

But thanks


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## lmills148 (Jan 5, 2008)

The wire you are using is called romex ( or some other brand there of). If you read the jacket it will give you a temperature rating, I think around 90 degrees. When the ambient temperature around the wire reaches that temperature it changes the impeadance of the conductor. Convection from the furnace is even more of a factor. I don't know what your load is but considering your proximity to a major heat source like a furnace I would Increase your wire size to #10 AWG I also suggest using a type of wire called MC it has a steel jacket rather than pvc and will also carry a ground better. Use the right connectors for the MC, a metal box and make sure all the metal parts are bonded together with a ground wire. Its not that big of an issue but the right wire size and type will make your components last longer. Grounding and bonding will make you last longer.


this may help

http://www.usawire-cable.com/product.asp


look at the diagram for armored cable
its sold at lowes, depot its easy to get
#10 awg / 2 conductor with ground type MC or MC lite


Lloyd


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## necromancer (Jul 13, 2014)

any way to view the above images ??

thanks !!


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## Noxx (Jul 14, 2014)

Fixed.


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## necromancer (Jul 14, 2014)

you are awesome Noxx, thank you

you have been quite lately, hope you are going to sing Harold_V Happy Birthday (in french)  

did you figure out the wire configuration comment above ?


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## Noxx (Jul 14, 2014)

Yes, the wire should be a bit more thick, however the wires inside the box aren't very long and it didn't cause much problem.

I didn't pursue this electric furnace idea since I melted two Kanthal elements. You will need some sort of voltage controller like a triac to limit current to the wire.


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