How to build op-amp amplifier for thermocouple using lm358

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

"Normally" one thinks of using a micro and a relay or SSR to switch the heater power.

If you want to stay with discretes, and not concerned with high accuracy a Vref, and a comparator
to generate the appropriate trip point in order. You can use the LM358 as a comparator. You
would need a discrete to drive relay, but most SSR have low power inputs for effecting switching.

Also you would want to use hysteresis on the comparator to reject noise related switching.

Some thermocouple design considerations -


A single chip approach would look like this family that has all components onchip (except R's and
T sensor) -



This is more for a measurement type application. eg. display on LCD and such. But could easily function as
simple switch.




Regards, Dana.

Google the AD8495 thermocouple amplifier with integrated cold junction compensation.

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danadak said:

"Normally" one thinks of using a micro and a relay or SSR to switch the heater power.

Click to expand...

Proud to be seriously ab-normal.

ak

Taking the signal from a thermocouple and turning it into a temperature reading is not terribly difficult, but getting decent accuracy is complex. Specialty chips make a huge difference. But . . .

If all you want to do is determine if things are above or below a setpoint, then you can do this with a comparator, plus maybe a preamp before it. The LM358 is not anything near a precision opamp, but it should be good enough to produce a go / no-go signal. (See what I did there - I read the title.)

Is this school work, a job assignment, or - ?

ak

I'll give you the same answer here as I did in another forum.

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

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OK, some of this simply does not make any sense. First a 2200 (2.2 KW) heating element at 220 volts would be drawing 10 Amps and the wires extending from that do not look like 10 Amp wires. Thermocouple wire color codes, when used would be Red (Neg) and Yellow (Pos) for a Type K thermocouple. Type J would be Red (Neg) and White (Pos). A 2.2 KW heating element would be larger than what I see in the images. While a common light dimmer circuit works fine for low power such as 300 / 600 watt applications I doubt you will find one for a 2.2 KW heater element, not a common off the shelf home improvement store version anyway. On a Type K thermocouple including thermocouple extension wire the Negative lead is magnetic while on a Type J the positive lead is magnetic. You may want to see if any of your wires are attracted to a magnet.




What temperature range are you looking to control? What exactly is that heating element heating? What is the element voltage?

Ron

I left you a reply in the other forum. You may want to share the details here. Someone else may have a suggestion. This is what I posted:

OK, I get that. With the rest of the parts there is more to this. It's a hand held small heat gun. With the open exploded view I see what looks to be the small blower with a red and black going to the blower. Heat Gun Hot Air Desoldering Gun Handle FOR 858 8858D 878A 878 Rework Soldering Station BGA Repair Tools. The handle is used on several heat gun tools. They all have adjustable heat and fan speed settings. They also come with assorted end tips. If all you have is the handle assembly as pictured I would use an ohmmeter and start measuring the resistances between wires. That may give some idea as to which two are the element and which two are the sensor. The assembly should have overtemp protection.





You would have to know exactly what sensor was used and if a thermocouple which type?

Ron