Reading a thermocouple ?

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11 May 2017 13:03 #93021 by andypugh
Replied by andypugh on topic Reading a thermocouple ?
Can I repeat my suggestion of using the MAX thermocouple chips and the dedicated HAL component?
You can get them pre-mounted if that is the problem.
www.adafruit.com/product/269
(Adafruit are out of stock, eBay has them, search for max31855)
The following user(s) said Thank You: akb1212

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11 May 2017 19:49 #93042 by calvin.d
Replied by calvin.d on topic Reading a thermocouple ?
Andy, that is a very solid point. I went back and re looked at the hal component you linked to.
so if I wanted to get 4 separate temp readings, could I use the set of pins on a mesa card to read the input ?

this actually seems like less work because I do not need to modify the firmware to get A channel only encoders

any idea what the refresh rate would be ?

Thanks Guys

C

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11 May 2017 20:13 #93044 by calvin.d
Replied by calvin.d on topic Reading a thermocouple ?
also digikey has the adafruit breakouts so, i grabbed on to give it a try

Thanks again for the suggestion

C

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12 May 2017 00:41 #93056 by andypugh
Replied by andypugh on topic Reading a thermocouple ?

calvin.d wrote: so if I wanted to get 4 separate temp readings, could I use the set of pins on a mesa card to read the input ?

Yes, one output pin for clock, one input pin for data and then one output pin per thermocouple channel for select.

Though, you could also use the Mesa version of SPI for a much faster update rate. The HAL component works at 1 kHz, whereas the Mesa firmware SPI is (up to) MHz. But 1000 bits of data is still 5 thermocouple channels at 1Hz.

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08 Sep 2017 03:55 #98652 by calvin.d
Replied by calvin.d on topic Reading a thermocouple ?
So I have hardware in hand to go the MAX ic spi route.

I now have a hardware question that breakout board ( www.adafruit.com/product/269 ) is expecting 5v in and has 3.3v io lines for the data

I have my 7I76 setup to use 24v for the general IO

anyone have an opinion on the right way to get the GPIO voltage down to 3.3v or 5v

or should i just grab a separate mesa board to extend off the 7i76 to deal with this IO.

Thank You
C

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08 Sep 2017 12:15 - 08 Sep 2017 12:17 #98673 by andypugh
Replied by andypugh on topic Reading a thermocouple ?
You could use a parallel port or serial port, if you have one.
(There are enough IO pins on the serial port to use it for a few GPIO lines )

There is also the option of using the second port on the 5i25, or adding something like a 7i73 to the 7i76.
Last edit: 08 Sep 2017 12:17 by andypugh.

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08 Sep 2017 14:12 #98690 by rodw
Replied by rodw on topic Reading a thermocouple ?
Might be a bit late to the party here but another simple option where you are using a Mesa card with analog ADC inputs would be to use something like this
www.sparkfun.com/products/10988

And scale the voltage to suit the Mesa 36.3 V ADC range with one of these:
www.ebay.com.au/itm/LM358-Weak-Signal-Co...p2057872.m2749.l2649

I've ordered one of the amplifiers to read an analog output current sensor this way on my 7i76e. The analog pin shows the ADC voltage so then it would be a simple matter to scale the output in hal with mult2.

I have played around with temperature sensing on an Arduino and I will say that Andy is on the right track. The SPI or one wire interfaced sensors are superior but the analog devices are easy to implement with the Mesa devices.

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13 Sep 2017 03:19 #98909 by calvin.d
Replied by calvin.d on topic Reading a thermocouple ?
the sparkfun option will not work for what I am looking for I need actual thermocouples for what I want to do.

I think you are correct though, doing this via analog or PWM into the GPIO on a Mesa board is starting to make more sense to me. I think I want a board that has a micro controller and a max3155. The code to go from spi to a khz PWM output is easy and if I just use a mosfet to switch a 24v line then I am right in voltage range for the GPIO on the board I am using.

anyone have any thoughts on reading a 10kHz PWM on a 7i76 GPIO ?

Thank You
C

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13 Sep 2017 06:02 #98912 by andypugh
Replied by andypugh on topic Reading a thermocouple ?
Mesa BSPI to a MAX3155 ought to work, too, if the bit-banged component offends you.

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11 Jun 2019 21:30 #136661 by superman
Replied by superman on topic Reading a thermocouple ?
Hello together,

it is a very late reply to this topic, but actually we have a problem exactly on this topic.

We are using a MESA 7i76 with one of the analog inputs to measure a NTC100k@25deg thermocouple. We have wired as following:

+12V --- NTC --- Analog0-Input
Parallel to the Analog0-Input, we have wired a 100k resistor to GND.

We expect to measure around 6V at the Analog input @25deg.
As long as the potential between NTC and 100k-resistor is NOT connected to the Analog0-Input, this is the case.
But as soon as we connect the potential to the Analog0-Input, the voltage drops to around 2V.

I cannot explain myself this behavior since i assume that the input impedance should be (theoretically) infinite.

Can somebody please explain, why this behavior appears or what i am doing wrong?

Thank you very much in advance!

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