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So first of all, I'm a mechanical engineer and a terrible electrical engineer.

I'm trying to build a failsafe for a heating element. This heater has an integrated k-type thermocouple and is limited to 350 degrees C. At this temperature, the thermocouple should produce a voltage of about 14mVdc. The heater is powered through a solid state relay which conveniently asks for a 12Vdc control voltage in order to relay the mains voltage.

I wish to use the thermocouple output voltage to control a switch. Once a voltage of 14mVdc is produced, the 12Vdc control voltage line should be broken which powers off the heating element.

Unfortunately, I have no clue how to go about this problem. I can imagine that the thermocouple voltage needs to be amplified but that's about it.

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The easiest approach is to buy a commercial limit controller or temperature controller and attach it to the thermocouple. Configure it for the thermocouple type (if necessary) and for on/off action and set your desired temperature. Use a relay type and directly break the mains current. Or buy a mechanical thermostat or thermal fuse and use that to directly cut the mains current. That may or may not be good enough for your particular situation.

'Fail-safe' is not trivial to achieve or even to approximate. For example, if your SSR fails 'on' (as they are wont to do) cutting the 12V control signal will have no effect. A commercial controller will pass a small current through the thermocouple to detect a break in the wires or the thermocouple alloy extension wires. If you wanted to make something like this, it could be as simple as an op-amp and a comparator driving a relay (with a power supply and a voltage reference- you could probably eschew cold-junction compensation since your accuracy requirements are likely modest), but the details could cause problems and it's apparently got some safety consequences.

Any kind of safety-related device should be as simple, reliable, independent (of the primary control) and robust as possible. You should consider the consequences of any failure and take appropriate measures to prevent any danger to humans and to reduce any financial losses to an acceptable level.

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    \$\begingroup\$ In case anyone was curious, the Mythbusters have shown what can happen when a hot water heater control fails; terrifying! \$\endgroup\$
    – rdtsc
    Jun 2 at 13:28
  • \$\begingroup\$ @rdtsc I assume they replaced the pressure relief valve with a plug to get that to impressive pressure-vessel failure to happen... no way would a single point of failure be allowed to create a risk like that in a mass market home environment. Fun anyway. \$\endgroup\$ Jun 2 at 13:55

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