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thermo couple

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Well thermocouples have other uses as well.
They're used to measure high temperatures which cannot be measured with a thermistor. The common types are K-type and J-type, but there are many other more exotic types. All generate only a millivolt signal. They are all actually differential devices, the voltage is proportional to the difference between the hot junction and the cold junction (where the thermocouple's wires made of special alloys are connected to the copper wires leading up to a meter).
 
so the thermocouple generate milivolt, could you tell me where i can buy it, or the distributor, i would use the thermo couple to measure the temperatur with microcontroller interface
 
There are any number of places to buy one. First you have to decide whether you need a type J or K, they operate in different temp ranges.
A microcontroller is generally very inaccurate measuring these small signals, and an ordinary op amp often has a significant offset error and noise. A chopper op amp, "thermocouple amplifier", or a quality A/D chip can do it. You may need a thermistor to measure the cold junction temp to compensate the temp rather than just assuming a particular temp for the cold junction.

A thermocouple is actually just 2 types of wire touching at the "hot" side. You can even solder them, although the solder will melt at high enough temps. They often have teflon or other high-temp insulation.

Here's a measuring trick. The hot junction you're measuring is sometimes electrically connected to a grounded object because it's tough to make high temp insulation and it can really slow down the response time and accuracy. But that electrical connection can be a problem since one wire may then have a voltage slightly below ground and a normal op amp can't measure that without a negative supply. However, a beyond-the-rails op amp or ADC like the LTC2014 can do it!
 
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