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I would like to build a simple control circuit for a 12v silicone heating pad. I think what I'm looking to make is a thermally controlled relay: I want current to flow when the temperature is below freezing and turn off when above freezing (or thereabouts - precise temperature control not necessary).

What could I use to sense and control temperature? I'm looking for small, simple, cheap and low power.

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  • \$\begingroup\$ Welcome to SE EE. We're a bit "nasty" here and prefer you to do some research before you ask basic questions like "how do I...". Many have build temperature switches (thermostats) before, you're not the first. So browse the internet and see how others do this. Example: electronics-project-design.com/Thermostat.html There is also the option to not build something yourself but to buy a ready made module, BigClive discusses one here: youtube.com/watch?v=dkxR4A5l58U \$\endgroup\$ – Bimpelrekkie Jun 13 at 12:46
  • \$\begingroup\$ Believe it or not, I have been researching this for over a year! Much of what I find is thermostats - much to big and complex. I am not an 'electronics' person and there are a bewildering number of electronics components out there. In the past I came across a PTC which seems like it might be exactly the kind of thing I need but I can only find them for high temperature. I'm hoping someone with knowledge of electronics components and design can help me find the type of circuit I need amongst all the unsuitable things that are way more common and dominate my search results. \$\endgroup\$ – RadOD Jun 13 at 13:04
  • \$\begingroup\$ I am not an 'electronics' person Then please, just buy a ready made module. The one from Clive's video will do fine. Buy it on Ebay, it's not expensive either. There is no shame in that. Only build your own if you need something special and for educational purposes. \$\endgroup\$ – Bimpelrekkie Jun 13 at 13:11
  • \$\begingroup\$ A 'thermostat' is exactly what you're describing. Simple Thermostat Circuit gives you an abundance of schematics to choose from. \$\endgroup\$ – brhans Jun 13 at 13:13
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Look into "PEPI" brand thermal switches. In your search you will also find a number of other types as well. However, PEPI are the most compact. They are all reasonably priced. The PEPI's are typically smaller than the other brands.

These are as simple as it gets for thermostatic control. It is simply a mechanical switch with two wire leads, which switches on and off at a fixed temperature. So, you have to choose the model of the switch according to the control temperature you want. They come in a wide range of temperatures.

You can often use these switches with no external circuitry. It depends on how much load current you need to control. Do not exceed the PEPI switch rating. If you need to control a larger load, use the PEPI switch to drive a relay and use the relay contacts to drive the load. Choose the relay accordingly.

You do need to understand a little bit about "trip points" and "hysteresis" to use these types of thermostatic switches effectively and correctly. You can find lots of info on this topic on-line. However, simply put, the thermostatic switch does not turn on and off at exactly the same temperature. Typically, they turn ON at a slightly lower temperature than their "rated" temperature, and OFF at a slightly higher temperature. These two, slightly different temperatures are typically called the "Trip On" and "Trip Off" temperatures or "points". The difference between them (in degrees C or F) is called the "differential" or "dead-band". Also, known by the fancy word "hysteresis".

The dead band plays a big role in how accurately your temperature controller will operate in the real world. The wider the dead band the less accurate will be the temperature control - in the sense that the temperature will cycle up and down as the switch cycles between the slightly different trip points. You'll get the idea once you hook it up and monitor the temperature of your controlled object with a digital thermometer or similar.

Good Luck!

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  • \$\begingroup\$ Thanks - maybe I don't know how to ask the question properly, but this is exactly the sort of thing I'm looking for. \$\endgroup\$ – RadOD Jun 13 at 15:11
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    \$\begingroup\$ That's a common problem when you are new to a field - you don't know the vocabulary, so you don't know what to ask for or about. Especially so in electronics where we have many conflicting terms and synonymous names for components. E.g. choke, inductor, coil. Diode, rectifier. Wire, conductor, net, connection, node. Etc. And lots of sloppy grammar to boot! \$\endgroup\$ – FiddyOhm Jun 17 at 21:42

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