temp controlled switch

I wanting to put together a temp controlled switch using a k type thermocouple to simply turn a relay on and off with the set temp is reached. Any help or suggestions would be great

As it is our nature to try and understand the greater context, I wonder:

- why use a K type thermocouple rather than a thermistor or semiconductor temperature sensor? The thermocouple requires a bit more interface circuitry and so is not the most convenient choice. A K type is not the most accurate choice either so the alternatives, which are not particularly accurate either may be considerably more convenient.
- while a relay is easy to use, it is also slow, noisy, less power-efficient, larger and perhaps even costlier than a semiconductor solution so, why use a relay?

However, to answer your question as originally asked, start with an accurate thermocouple interface:
**broken link removed**
https://www.sparkfun.com/commerce/product_info.php?products_id=306

then wire the output of this to a comparator circuit
**broken link removed**
and use a potentiometer in the second input of the comparator to set your desired switching point. Then use the output voltage of the comparator to drive a relay:
**broken link removed**

RadioRon said:

As it is our nature to try and understand the greater context, I wonder:
However, to answer your question as originally asked, start with an accurate thermocouple interface:
**broken link removed**
https://www.sparkfun.com/commerce/product_info.php?products_id=306

then wire the output of this to a comparator circuit
**broken link removed**
and use a potentiometer in the second input of the comparator to set your desired switching point. Then use the output voltage of the comparator to drive a relay:
**broken link removed**

Click to expand...

hi Ron,
Thanks for a well explained link [Comparators.html], I will point OP's asking about Comparators to that site.
Saves constantly repeating the same answers.

Eric

RadioRon said:

As it is our nature to try and understand the greater context, I wonder:
- why use a K type thermocouple rather than a thermistor or semiconductor temperature sensor? The thermocouple requires a bit more interface circuitry and so is not the most convenient choice. A K type is not the most accurate choice either so the alternatives, which are not particularly accurate either may be considerably more convenient.
- while a relay is easy to use, it is also slow, noisy, less power-efficient, larger and perhaps even costlier than a semiconductor solution so, why use a relay?
{snip}

Click to expand...

I wonder why some of you cant give the OP the credit for having a little bit of common sense. He obviously wants to control an oven at some high temperature, the K type is good to over 2000°F and its accuracy is more than adequate, in most instances. Substituting a thermistor or semiconductor is NOT an option at those temps.
And why should one even consider controlling an oven with anything other that a contactor (relay)?
This is all rhetorical .......... of cause.

Some thermistors will work at those sorts of temperatures.

I would use a small light bulb as a thermistor as it's pretty cheap and reliable, I'd go for a 5W 240V bulb as it'll have a higher resistance that's easier to measure than a 12V bulb.

Also, it's the origional poster's fault for not providing enough information.