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MAX 6675 Thermocouple to serial interface chip. — Parallax Forums

MAX 6675 Thermocouple to serial interface chip.

MorteAzureMorteAzure Posts: 8
edited 2010-01-29 21:51 in Accessories
HI, I am again having the same problem with this chip as I was with my old circuit that I built. The purpose of this circuit is to convert thermocouple signals to something that my basic stam can understand. I have the circuit built right onto the thermocouple head to eliminate any possible outside inerference, but something is going on.·It is in a loop. and this is suppose to be reading the temparature as I am heating up the thermocouple. I am Suspecting the power supply or feedback from the stamp, but is still am not sure. Tonight I will be putting the scope on it so see, but I am not sure if I am going to find anything. Any Idea's? My screen in run mode looks something like this.
600
600
612
540
500
0.0
0
14.25
590
600
·Thanks, MorteAzure.

Comments

  • James LongJames Long Posts: 1,181
    edited 2009-12-31 00:24
    Morte,

    I have used this chip a lot. It is very reliable. I have tons of them.

    The chip is very specific on the Thermocouple connection. Also, the thermocouple will read the ambient temp.

    The readings are usually very accurate in Celsius. They are designed for K type thermocouples, and will have some error with different types.

    I'm sure this doesn't help much, but should get you started.

    James L

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    James L
    Partner/Designer
    Lil Brother SMT Assembly Services

    Are you addicted to technology or Micro-controllers..... then checkout the forums at Savage Circuits. Learn to build your own Gizmos!
  • MorteAzureMorteAzure Posts: 8
    edited 2009-12-31 15:19
    WHat Kind Of thermocouple connections am I to Use. There is nothing in the datasheets about this. I Am using 1/2 " copper wire to connect this chip to the thermocouple. It is an industrial well and head assembly, which allows me to build the circuit right onto the head at close proximity to the thermocouple connections. I also found that there is some kind of feedback going on in the power supply from the Stamp. Do you know what the maximum permissible ripple is allowed at the power input?
  • Tracy AllenTracy Allen Posts: 6,656
    edited 2009-12-31 16:35
    There is an article about the MAX6675 interface to the stamp here:
    www.emesystems.com/OL2therm.htm#MAX667x_description
    It is a good idea to include a bypass capacitor on the power supply right at the chip. If there is ripple on the MAX6675 power supply, you should be able to see that on the oscilloscope.
    It looks like there may be something wrong with the PBASIC code.

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    Tracy Allen
    www.emesystems.com
  • MorteAzureMorteAzure Posts: 8
    edited 2009-12-31 18:03
    The link you gave me is where I got the demo pbasic code from. I also have a bypass capacitor by across the power supply leads at the chip. My scope shows a 20 mv ripple p-p everytime the basic stamp initiates the chip. I am going to try and get a computer power supply and Isolate the Maxim chip power supply.

    Post Edited (MorteAzure) : 12/31/2009 11:18:32 PM GMT
  • MorteAzureMorteAzure Posts: 8
    edited 2010-01-29 21:51
    Finally resolved the problem. I Changed·four things. First, I have an Isolated computer power supply attached to the chip, Second, I moved the ground for the stamp's serial connection to the screw on the basic stamp, third, I used shielded cable on all of the data lines. The ripple was caused by the ground and power supply interweaving with all of the other i/o wires on the board. And since my basic stamp runs at 10mhz, I thought that it should have a 10mhz signal line.· Now my ripple is gone and I can accurately scale my data.··Amazing where noise comes from. Thanks
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