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???

i want to construct a temperature sensor using a thermocouple.
the emf which produced from a thermocouple is very small amount to enter to the ADC 0808 ( about 0 to 69.55 mV ). therefore i want to amplify this emf with an amplifier.

can someone help me?

???

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mrtrhs,

Take a look at these:

http://www.embeddedtronics.com/public/Electronics/thermocouple/schematics/thermocouple_schematic.pdf

http://www.embeddedtronics.com/public/Electronics/thermocouple/schematics/thermocouple_layout.pdf

http://www.embeddedtronics.com/public/Electronics/thermocouple/schematics/thermocouple_bom.pdf

Ante ::)

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  • 2 weeks later...

You probably don't need an opamp to amplify the 65mv to the ADC. How do you bias the thermocoupler? I feel you must need a voltage across the wire which has a variable resistance. Is this how it works?

Just a reminder. Opamps used for common mode rejection reject the common mode noise if it is there. That is, when you expect there to be noise that you cannot get away from. Don't be afraid of noise. A nicely grounded case is normally sufficient if you are in the RF range. Also, the signal is plenty large enough to not worry.

Also, what part of the design do you need help with?

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Let's see if we can't come up with a good combination for his thermocoupler. To start, we don't need the inversion. I think he should just ground the input to the opamp and ground one of the wires. You need some phase compensation to keep the opamp on a 20db per decade at higher frequencies. We need to keep the supply voltage at +-18. What else? Oh, the output current is a hot debate, but I think we should keep it close to 20mA. Also, you should have a voltage differential between the inputs when the signal is present.

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Hi Kevin,
You are correct to choose a non-inverting opamp circuit, since the ADC0808 only has a positive input.
Most opamps have built-in compensation, so we don't have to add any.

Why did you choose such a high supply voltage? Your +,- 18V is the absolute maximum voltage for most opamps and the manufacturers recommend using a supply that is only as high as +,- 15V. They say that their spec's don't apply when using supply voltages more than +,- 15V. The ADC0808 has a maximum input voltage of only 5V, so even a +,- 8V supply for the opamp will work well here.

Hot debate? Just read the spec's. Why are you loading down the output with 20mA? Opamps work very well without any load current, and 20mA is beyond the maximum limit for most opamps. The maximum specified and guaranteed output current for most opamps is only 5mA (10V into 2K ohms). Special high-output-current opamps are designed to drive 600 ohms loads. They have a guaranteed output current of a whopping 16.7mA (10V into 600 ohms).

The voltage differential between inputs of an opamp is determined by the output voltage divided by the open-loop gain of the opamp. You have no control over it so don't even think about it. With a 5V output and a typical gain of 200,000, the input differential is a whopping 0.000025V.

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J-type thermocouple

Definition:

A thermocouple best suited for measurements in the 0 to 480 degree © range. The positive conductor is made of iron, and the negative conductor is made of constantan.

May i put my two cents in here ?
mrtrhs, how about a radioshack thermistor? i realize that you may not have a "shack" around the corner ,but the thermistor ,in my opinion , is the way to go.
You could set up a voltage divider using 5V as a supply voltage.
Just a suggestion.. ;D

Oh ... What temperature range are u thinking of measuring?

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Hi Harsh,
As for your two cents, the reference table that Ante posted shows that a J thermocouple that is generating 69.55mV is at 1200 degrees C!
That's hot enough to burn down your "shack", their thermistors and everything else that's there.
Isn't that the thermocouple that is directly in the flame of my gas furnace? Ouch. ;D

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