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I am wanting to make what we will just call a furnace that will always operate near 1800°F. I'm looking for circuit ideas or a direction in order to maintain the temperature of the furnace.

The current thought process is using a thermocouple and relay to turn the heater on when it drops to ~1700°F and turning it off when it hits ~1900°F. The goal is to do this without any complex digital controller. How could the mV generated from the thermocouple be taken advantage of to control a relay in such a manner?

Thanks in advance

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    \$\begingroup\$ that should say turn the heater on when it is below ~1700°F ... you have to be able to turn the furnace on \$\endgroup\$
    – jsotola
    Commented Jan 27, 2022 at 0:18
  • \$\begingroup\$ a microcontroller is probably the most flexible option ... something like an Arduino, or a Raspberry Pi .... both could have an option for monitoring and controlling the furnace remotely \$\endgroup\$
    – jsotola
    Commented Jan 27, 2022 at 0:22
  • \$\begingroup\$ find a controllable soldering iron project for ideas about the complexity of the project \$\endgroup\$
    – jsotola
    Commented Jan 27, 2022 at 0:25
  • \$\begingroup\$ Just as one tidbit: silver will change its volume by about 6-7% about where you'd want to turn on the furnace in its on-off control. An appropriate quartz bubble partially filled with silver, with contacts entering from the top, would make contact when the silver is liquid and would open when the silver crystallizes. Simple, safe, and it won't wear out over time. \$\endgroup\$
    – jonk
    Commented Jan 27, 2022 at 3:39
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    \$\begingroup\$ @jonk Thank you for this insightful comment - this is exactly the type of idea I was looking for. I'm now doing more research over this approach. \$\endgroup\$
    – 4DF
    Commented Jan 27, 2022 at 5:10

2 Answers 2

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Head on over to adafruit.com and get a thermocouple and a little board with a MAX31855 thermocouple amplifier. That will get you a pretty accurate reading in degrees.

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The simplest is to get a ready-made heater controller, which directly takes power supply (e.g. mains) and thermocouple wires and does the rest for you.

If you insist on building it yourself, you can look at simple mechanical thermoswitches, such as bimetallic switches.

Electronically, you can condition the TC voltage with an opamp and then control a relay with a hysteresis comparator. Attention to noise is needed because TC leads can readily pick up EMI, e.g. from the heater.

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