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I'm looking for guidance on building an arduino-controlled heating element. I like to have a rotary encoder switch control the over heat delivered to the device.

Because heating is such a power drain, I'd like to connect the device directly to the power source, but run it through a relay that is controlled by the Arduino. I've attached the design below.

Design Diagram

In general, I have two categories of questions:
1. Power Management
2. Heating Element Design

Power Management Questions

  • How can I use AC outlet and manage via Relay? Please give me the brief overview of mindset there.

  • What precautions should I take?

Heating Element Design

My plan is to take a nichrome rod, (or any other high resistance rod), and wrap it in nichrome coil. I'd then like to encase this rod/coil in a cover. When plugged in, The coil will heat up, thus heating the whole unit.

  • Is there a simpler approach? ie, connecting the rod directly to power? I could just build an encasing around that.

  • Is nichrome the right choice?

  • Are there any pre-made materials that look like this? I need it to get red hot.
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  • \$\begingroup\$ Are you attempting to build an adjustable temperature soldering iron? \$\endgroup\$ – Samuel Nov 11 '16 at 20:55
  • \$\begingroup\$ @Samuel Sure looks like it. An open loop control type anyway. (No feedback.) \$\endgroup\$ – jonk Nov 11 '16 at 20:58
  • \$\begingroup\$ @jonk I think the loop is closed by the operation when it's "red hot". Fancy optical-biological feedback. \$\endgroup\$ – Samuel Nov 11 '16 at 21:01
  • \$\begingroup\$ @Samuel I specialize in pyrometry. So I understand optical methods. But regarding "red hot" the color also depends on the emissivity and not only temperature. This is also changed by the shape of the object, as well as the materials, and also the surrounding environment and its temperature. It's very complex, actually. So I'm not even sure what "red hot" means here. \$\endgroup\$ – jonk Nov 11 '16 at 21:03
  • \$\begingroup\$ @jonk Indeed. Several unknowns here. \$\endgroup\$ – Samuel Nov 11 '16 at 21:07
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First, I'd recommend looking into Cotronics' various epoxy and/or ceramic supplies for coating your Nichrome wiring and sealing it. I don't know what temperatures you expect, but it's something to look into. They support quite a range. Of course, I've no idea what you are really doing here. So I don't know if this suggestion will be at all useful. But there it is.

Second, you may need a way to monitor the temperature. It's unusual to operate Nichrome heating elements without some kind of temperature feedback so that you can achieve some desired level of temperature control. You could observe this temperature and get an idea if it is "red hot" enough, then. How you do that will depend on the exact range and/or why you are seeking such a temperature.

Also, keep in mind that the shape of your object and the materials used and a host of other factors will affect the color appearance of it. You can take a block of aluminum, drill a small, deep hole into one side of it, and heat it up to a certain temperature. The hole will appear "red hot" while the rest of the block doesn't even show color, at all, to your eye. And if I used copper, it would be all different besides. This idea of "red hot" is vague and poorly expressed in your question.

Finally, I think the idea of a relay is fine enough. Resistive loads like this are pretty tolerant of different methods of control. You could consider solid state relays (SSRs), too, though those will have a voltage drop across them which makes them dissipate power and possibly require heat sinking.

I can't suggest any better guidance without knowing a lot more about what you are doing, and why you are doing it.

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