the problem is amplifying the 1mv voltage with lm324

[faraday]

Hi

I wanna send the output of a thermocouple to a micro but the
thermocouple voltage is maximum 3mv. I used lm324 Op-Amp for amplification. This
circuit works in simulation but practically there’s no output (Vout=0)

What’s wrong?

What kinda changes are needed to amplify this little voltage
(about 1mv) and rise it to 4v?

 

[alec_t]

The LM324 is not suited to measuring such a small voltage, because its input offset voltage can't be nulled and swamps the voltage you are trying to measure, driving the output hard against either power rail (0V in your case). You need a precision op-amp (google for that) with offset-nulling capability.

 

[Reloadron]

There are several things you need to consider to amplify the output of a TC. There are precision instrumentation amplifiers designed just for that purpose. The AD594 and AD595 come to mind for a Type K TC.

Among a few problems you would need to address is first a thermocouple while having a somewhat linear region is a non linear device so linearization curves need to be applied to its output, check a thermocouple output versus temperature chart to see what I mean. Also, there is a matter of CJC (Cold Junction Compensation) that needs addressed. CJC alone if not done right will cause large errors.

What exactly are you trying to do and what is the thermocouple type you plan to use? I have been planning a Thermocouple Temperature Blog but have yet to get around to publishing it. I have many images ready, and might be able to help you out.

<EDIT> I do have a full collection of thermocouple tables available in a zip folder which can be downloaded from here. Clicking the link will give the option to download all the files as a zip folder. Just extract the contents. The most common thermocouples are in there with degrees F and C listings. The files were compiled from Omega Engineering. They were going to be used in the blog I have yet to publish. </EDIT>

Ron

 

[faraday]

Your posts are very useful, thanks.
I have a thermocouple; but I do not know its type.
how can I determine its type?
I wanna measure water temperature by connecting thermocouple's output to microcontroller.
Maximum water temperature is about 80-90 degrees Celsius.
--------------------------------------------------------------
I apologize for grammar probable mistakes

 

[Reloadron]

If your TC has leads exiting it that are insulated, note the colors. For example Type K generally uses Red and Yellow with the red being the negative. Type J generally is Red and White with Red being negative. That isn't always true but may help. Next on a Type K TC the negative is magnetic while on Type J the positive is magnetic. Types J and K just being a few of the more common.

However, in your case considering the grief involved using a TC I would think about using a thermistor for the temperature range you mention. Thermistors being much easier to work with. Likely the best candidate for your range would be one of these units. Check out the linked data sheet. They can easily be embedded in an epoxy bead with leads exiting. I would really consider an LM35 circuit or similar.

Ron

 

[KeepItSimpleStupid]

A PT100 RTD would be even better if you need accuracy. A T thermocouple would be used in that range.

A thermister is the easiest. Depends on what you need.

 

[bountyhunter]

I wanna measure water temperature by connecting thermocouple's output to microcontroller.
Maximum water temperature is about 80-90 degrees Celsius.
--------------------------------------------------------------
I apologize for grammar probable mistakes

make your life simple and use an LM34 or LM35 temp sensor. No calibration required, very easy to use.

 

[faraday]

thanks all for replies
I bought max6675 and wanna use it.

 

[Reloadron]

That will work for you. Slight overkill for range but there are countless tutorials out there to interface it to an Arduino so that is good.

Ron

 

[faraday]

make your life simple and use an LM34 or LM35 temp sensor. No calibration required, very easy to use.

thanks, the thermocouple can be screwed in water tank but I can't screw the lm35 in it.

 

[Reloadron]

thanks, the thermocouple can be screwed in water tank but I can't screw the lm35 in it.

Attached are a few images of a Type K TC. This one is a 6 inch inconel sheath (pretty common) with a compression fitting. You can see the compression fitting on the TC sheath plus the compression fitting open. This is how thermocouples are generally inserted into a tank with as much TC protruding into the tank as you wish. The TC is 1/8" OD and it is a 3/8" NPT pipe thread for the tank.

Actually with some doing the LM35 can be waterproofed but given a choice the TC is a good way to go. Read the data sheet carefully for the chip as to connecting things. Some of the data is important as to good accurate measurements.

Ron

 

[bountyhunter]

thanks, the thermocouple can be screwed in water tank but I can't screw the lm35 in it.

An LM35 can be epoxied to anything you want to use for mechanical support.

 

[Reloadron]

An LM35 can be epoxied to anything you want to use for mechanical support.

I mentioned that earlier but the OP seems bent on using a TC? Another advantage is the span versus volts, especially considering this will feed a uC. For a furnace yes a TC but for a hot water tank? Something like the LM35. However, not my project so TC it is.

Ron

 

[Dhandehelu]

Hi
Can any one pls help me!
im gonna calliberate a j type thermocouple using AVR atmga16 microcontroller..
i want know which IC i hav to use, to amplify this small millivolt of TC.

thanks in advance..tc

 

[Reloadron]

You may want to start with a T/C amplifier along these lines as it takes care of your CJC (Cold Junction Compensation). There are dozens of solutions out there to scale and make linear a thermocouple output. Just becomes a matter of how accurate you want thinks to be and what your budget is? Just note, using a chip like the link requires paying attention to detail when making the board it sits on. There are also many off the shelf solutions like these.

Ron

 

[Dhandehelu]

Thanks 4 the idea..i will try with that.