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I'm building a device which is designed to be built by people of varying skill and education levels. Part of it needs to climb to and maintain a temperature of 220C, but I don't need very tight regulation. Swings from 215-225C are acceptable, but not desired.

My normal goto for this is a microcontroller, thermistor, MOSFET, and heater cartridge. I know for a fact this is above my target audience's skill level. My next goto would be digital PID controllers, and those are going to be out of my budget by a significant portion.

My project has 5v & 12v available (5v would be preferred actually, saves me a 12v power supply). I've seen temperature fuses, but those seem to be one shot so as not to set fire to something. I've seen PTC thermistors, but ... I don't know how well strapping one of those to the hot part of this assembly is going to work.

Any suggestions? My ideal solution could be had from either a local B&M store, or Mouser, for $5 or less qty 1.

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    \$\begingroup\$ Nichrome wire? Good old bimetallic strip thermostat? \$\endgroup\$
    – pjc50
    May 13 '14 at 19:55
  • \$\begingroup\$ No need for MCU, ADC, triac etc.. A fixed value thermal switch (thermostat) like these ones electronics.stackexchange.com/questions/68481/… ? \$\endgroup\$
    – Cornelius
    May 13 '14 at 19:58
  • \$\begingroup\$ For the heating element I was thinking a power resistor shorted to ground. Are there cheap sources of bimetallic strip thermostats? My mechanism is kind of small, perhaps the size of a postage stamp. The bimetallic thermostat would have to fit there. \$\endgroup\$ May 13 '14 at 19:58
  • \$\begingroup\$ @Cornelius: that may be it. Want to expand it into an answer? \$\endgroup\$ May 13 '14 at 19:59
  • \$\begingroup\$ I'm terrible with analog design, but I really feel like what you want could be done with a quad channel op-amp chip and any reasonably temperature-linear resistance. Configure a voltage controlled current source through the resistance and then a threshold detector to respond to the voltage drop across the resistance. Not sure how to handle giving it a dead band - does it need one? Do you have split 5 volt rails available? \$\endgroup\$
    – Sean Boddy
    May 13 '14 at 21:04
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It is quite simple to do it without any digital electronics (MCU with ADC) or analog comparators (thermistor) with triac.

You can use only a component: a fixed temperature thermostat. The circuit becomes very simple and reliable. Just connect it in series with the heating resistor. Also there is no need for auxiliary power supplies. Just make sure the switch is rated at a higher than the required amperage.

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  • \$\begingroup\$ I see where you're going with this, but I'm finding it difficult to find anything in the 220C range for under $40. Right now these are still out of budget. \$\endgroup\$ May 13 '14 at 20:12
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There is a partial answer, cheap and readily available.

Use the thermostat from a toaster oven. These normally operate up to 450 F (232 C), and are certainly cheap (I've seen toaster ovens advertised online for $20, so you could buy the oven and rip out the thermostat, if necessary).

What I cannot guarantee is that the temperature swing of the thermostat is within your limits. I'd suggest some experimentation.

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  • \$\begingroup\$ That's actually really clever. +1 \$\endgroup\$ May 14 '14 at 14:24
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You could use a halogen bulb and a reflective enclosure to heat your project, like a childs toy oven. If the enclosure is well insulated, you could then use optical feedback to set the temperature with a photo resistor, rather than sensing the temperature directly.

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