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How to build op-amp amplifier for thermocouple that can switch 12v relay using lm358 ?

don fila

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I have 2200 watt heating element with thermocouple embedded inside the heating element. I used triac and other components to control the low and highness of the temperature. But my problem is how to use thermocouple to detect the temperature of the heating element and turn off automatically when it get overheated and restart when it cool down
 

KlausST

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Hi,

There are many documents telling how a thermocouple works.
There are many documents and discussions about amplifying thermocouple signals.

They show you the difficulties and how to solve them.

What exact information is missing?

Klaus
 

don fila

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Hi,

There are many documents telling how a thermocouple works.
There are many documents and discussions about amplifying thermocouple signals.

They show you the difficulties and how to solve them.

What exact information is missing?

Klaus
A lot because if you see anyone here asking about something he or she doesn't understand then you should no the reasons why discussion forums where created.
 

KlausST

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Hi,

.. and we should guess what you need to know?

Why don't you tell us what you did so far, what documents you've read, what you understand so far ... and what exactly you want to know?

Klaus
 

don fila

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I had an op-amp amplifier that can detect the low and highness in temperature of thermocouple but I need a way which that millivolt can be converted to switch 12v relay
 

don fila

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The internet is always my friend but some lessons are not well organized , complete and informative. That is why I choose to meet professional if my problems can be solve!!!
 

betwixt

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You need a dedicated thermocouple amplifier and a voltage comparator. There are devices with both in one package. It is possible to switch 2.2KW loads with a relay using the output of the comparator but if you have a triac already it would make more sense to use it, either as an on/off switch or to provide phase control to get precise temperatures.

Read up on how thermocouple amplifiers work though, they are not the same as normal op-amps and in particular be careful to isolate the tiny thermocouple voltage from sources of interference or other current paths that might shift the voltage.

Brian.
 

don fila

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I'm already using triac ,potentiometer and other components to control the heater. I used thermocouple op-amp amplifier to detect the temperature and it gives me a millivolt reading . For now my problem is how to convert the millivolt to a voltage that can switch 12v relay ?
 

don fila

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Do
You need a dedicated thermocouple amplifier and a voltage comparator. There are devices with both in one package. It is possible to switch 2.2KW loads with a relay using the output of the comparator but if you have a triac already it would make more sense to use it, either as an on/off switch or to provide phase control to get precise temperatures.

Read up on how thermocouple amplifiers work though, they are not the same as normal op-amps and in particular be careful to isolate the tiny thermocouple voltage from sources of interference or other current paths that might shift the voltage.

Brian.
You have any reference circuit diagram on this?
 

KlausST

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Hi,

do an internet search for "application note thermocouple amplifier"
--> As far as I can see all of the first 10 hits are very informative and rather complete.

*****
before designing an electronic circuit you need to know/decide the requirement.
At this point there is no need to know any circuit solution, nor OPAMPs

* There are different thermocouples. Without knowing which one you have it´s hard to give good recommendations
(No one can answer this for you)
* If you have read a couple of documents, then you already know that a thermocouple measurement is a relative measurement with respect to cold junction temperature. It makes a big difference whether it´s O.K. for you to have a relative temperature or you need an absolute temperature measurement. (No one can answer this for you)
* no one knows at which temperature (range) you want to switch ON and at which to switch OFF.
* no one knows how accurate and how precise you need the switching points

After knowing these "key parameters" one can do some calculations, start design, look for circuits, look for OPAMPs...

Klaus
 
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don fila

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Hi,

do an internet search for "application note thermocouple amplifier"
--> As far as I can see all of the first 10 hits are very informative and rather complete.

*****
before designing an electronic circuit you need to know/decide the requirement.
At this point there is no need to know any circuit solution, nor OPAMPs

* There are different thermocouples. Without knowing which one you have it´s hard to give good recommendations
(No one can answer this for you)
* If you have read a couple of documents, then you already know that a thermocouple measurement is a relative measurement with respect to cold junction temperature. It makes a big difference whether it´s O.K. for you to have a relative temperature or you need an absolute temperature measurement. (No one can answer this for you)
* no one knows at which temperature (range) you want to switch ON and at which to switch OFF.
* no one knows how accurate and how precise you need the switching points

After knowing these "key parameters" one can do some calculations, start design, look for circuits, look for OPAMPs...

Klaus
Wow good answer! Thank you for your reply
 

don fila

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Those who weren't able to help me I wish I could be long term member here to show them how successful this wanna be if no hanna help. Thank you all for helping me.
 

BradtheRad

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To maximize relay life it's a good idea to build hysteresis into your amplifier.

It can be done via window comparator (generally using two devices, one setting low temp, one setting high temp)...
or with a single device by adding positive feedback.

Without hysteresis the relay is liable to cycle every few minutes, as the heating element warms and cools in a narrow range. The result is to wear out the relay.

By adding hysteresis we create wider temperature variations. Thus the relay cycles On-and-Off once an hour or two.
 

KlausST

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Hi,

because of "first things first"
--> Post#11

Different requirements result in different solutions.

You may build a heater for an incubator, or a hot air gun or a, precision temperature controller for a chemical process.
Maybe a one piece project, maybe a 1 million pieces.

Hope you don´t mind, when I´m leaving now. I really tried to help, but I see no progress at post#16...
and I don´t want to waste your time by asking the same questions again and again.


Klaus
 

KlausST

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Hi
You called that help? Sorry ! You have read all what I have said and you know what exactly I'm looking for
No, I don´t call that "help". I said "I tried to". But I was unable because you made it unable for me to give good assitance.
You did not answer my questions. So "no", I don´t know exactly what you need.

Yes, blame it on me. I can stand it. Good luck

Klaus
 

FvM

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Possible circuit of a simple thermocouple limit switch with hysteresis, no cold junction reference provided.

1638792235577.png


LM358 has however max. 5 mV input offset voltage which translates to about 100 K temperature error with type K thermocouple. Initial adjustment can reduce the error to e.g. 10 to 20K.
 

don fila

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Hi

No, I don´t call that "help". I said "I tried to". But I was unable because you made it unable for me to give good assitance.
You did not answer my questions. So "no", I don´t know exactly what you need.

Yes, blame it on me. I can stand it. Good luck

Klaus
I'm sorry my friend because I don't mean it that way
--- Updated ---

I'm sorry my friend because I don't mean it that way
You should take it so serious because that is not what I mean!!!
 

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