Yes you can interface K-type thermocouple with PSoC3/PSoC5. You can interface it with the thermocouple that you have chosen.
A K-type thermocouple gives about 41 uVolt/deg Celsius. PSoC3/PSoC5 can measure voltages as low as 2 uV when set to operate with 20 bit resolution. In addition to the thermocouple itself, you will need an external ic for cold junction compensation. Also to compensate for the negative voltags from thermocouple you will need a couple of resistors. Typically the measurement procedure is as described below:
- Measure the cold junction ic voltage and determine the equivalent temperature . For example if you use LM35 for cold junction compensation, at 25 deg Celsiuis, you will get 250 mV(10 mVolt/deg Celsius).
- Find out the corresponding thermocouple output voltage from the table available at the following link , http://srdata.nist.gov/its90/download/type_k.tab. This has to be done in PSoC firmware
- Now measure the K-type thermocuple differential votage.
- Add this voltage to the previously determined Cold juctiontion ic voltage. Using this value you will be able to find the actual temperature from the table.
I hope this helps. Let me know if you need further help.
Thanks Udayan, for the great reply.
You might want to have a look at the following post as well, if you have not looked at yet.
The attached file is the project for Precision analog temperature sensor EBK mentioned in the previous post. The project contains the interface and code for thermocouple, thermister, RTD and diode and the user moves through the each sensor on the touch of a button.
I would also recommend AN2226 which talks about thermocouple interface to PSoC1.
Long time back, I had written an app note on measuring with J and K type thermocouples, AN2148. But that app note is not active at this time. However, I found the below link where there is a copy of the app note in psocdeveloper. Maybe this can be of some help too.
Im trying to build an exhaust gas temperature data logger using the psoc 3. Im using 8 thermocouples. Is this possible with the psoc 3 and how difficult will this be?
Just a few thoughts,
While the above solutions are all really technically exquisite(atleast I think so),a better way would be to use either the MAX31855 ( www.maxim-ic.com/datasheet/index.mvp/id/7273/t/al ) or the AD595.
These are Accurate Thermocouple-to-Digital Converter ICs,which have inbuilt CJC and sampling via an ADC(12bit in thsoe 2,I think... which is good enough for a wide range of applications),so the MCU(here SoC) just reads the digital value via SPI.
Although I havent really looked into the power consumption comparison,(assuming a battery operated sensor module),the psoc running all the thermocouple processing continuously(..or that depends..) sounds like short battery life.
Though the PSoC is a truly awesome piece of hardware,using a whole PSoC1/3/5 sounds like a design sin(if all you need is a sensor interface to a bigger system,like a PC or so),since its an overkill.If I recall correctly,there used to be these 8pin SOIC PSoC1 chips....(cant find them on the part selector now)..Anyhow,Better use a smaller MCU,or if your design allows,no MCU at all.
Instead,use an FT232 or 2232 to bitbang the SPI and pass it to the PC(assuming a typical datalogging application) over its own USB interface.There are many examples on their website to do precisely this. ( http://www.ftdichip.com/Support/Documents/AppNotes/AN_114_FTDI_Hi_Speed_USB_To_SPI_Example.pdf ).This can also be done via Labview itself (the site details that too .. www.ftdichip.com/Support/SoftwareExamples/CodeExamples/LabVIEW.htm ) etc for analysis.
Oh,and another thing that occured to me,since the PSoC uses the 20bit ADC for this,and we're talking about microvolts,its recipe for bad signal data,if youre PCB layout and design arent up to the mark.
Another reason to use an external chip to do the "heavy"(kidding..nothing is really that heavy for the PSoC :P ) lifting.
One correction,the AD595(for K type) and 594(J type) are thermocouple amplifiers,you'll need to sample data via the parent controller,so analog is still involved.But.the other maxim chip does this for you.
- It is quite possible to implement a design using PSoC3. Once a measurement unit is developed using PSoC3, in order to add more thermocouple units, all you need to do is use the same set of constants that was used for the first thermocouple. The same set of code that was used for the first thermocouple can be used. For each thermocouple unit you will be making two measurements(Cold junction and the thermocouple itself). Hence you will need a maximum of 16 analog pins and a single ADC. Two scan through the cahnnels you can use the Analog Mux channel available in PSoC.
- The only external components that might be necessary are a reference sensor for CJC.
- The MAX 31855 is sold at $3.11 when bought in bulk. For 8 measurements you will need ($3.11*8 ~ $25). Compare this against PSoC?
- Consider the board space used, PSoC as one single IC would accomplish the task !
- And in addition to this, you are left with lot more resources and more than just a microcontroller when you go for the PSoC style design !
Well put U2.I shouldve thought of the expansion and multiple sensor angle. To add your point,(about using one ADC and a Mux) we can also get "simultaneous" temp readings from them(in case we need) based on what The Filter Wizard recently shared http://www.analog-europe.com/en/sample-multiple-channels-simultaneously-with-a-single-adc.html?cmp_id=71&news_id=222902424