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  • 2 wire sensors

    This is a stupid question that I should know the answer to but bear with me as I have spent as many years trying to forget this stuff since retiring than I spent learning it. In an automotive application you typically have 2 wire and 3 wire sensors, 2 wire sensors like thermistors for measuring temperatures are common. your temp sensor is a 3 wire sensor which makes it easy to hook up to a board that is set up for 3 wire analog inputs, but just the same, a 2 wire temp sensor is an analog input also.
    Now if I wanted to utilize my already present ECT sensor can I just use the 5vdc and sensor return, disregarding the GND, or do I need to build some other circuit to interface it?

  • #2
    http://www.mp3car.com/vbulletin/docu...uspension.html

    the fusion brain senses voltage, 2 wire sensors are variable resistance so they must be wired as part of a voltage divider

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    • #3
      So looking at the schematic you provided in the link then my 2 wire resistive sensor would go between the GND and the Input terminals on the brain and the 5vdc terminal would connect to a resistor of which the other end would tie in with the input terminal right?
      also is the value of R1 a constant or is this going to be something that depends on the resistive range of the thermistor and if so is there a formula to determine the proper value?

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      • #4
        "also is the value of R1 a constant or is this going to be something that depends on the resistive range of the thermistor and if so is there a formula to determine the proper value?"

        the latter, and you can certainly optimize, but a nice rule of thumb is R1 = highest resistance the sensor can be

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        • #5
          So if I have a thermistor that reads roughly 13k at ambient temp and 60k at freezing, I should be able to replace R1 with a 100k pot and adjust that to get the most accurate temp readings across the range I will be using and then replace it with a resistor valued approximately at the resistance I find with the pot?

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          • #6
            yup, that'd work

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            • #7
              just one other problem, what if my thermistor is a negative temperature coefficient device?

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              • #8
                why is that a problem?

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                • #9
                  because as the temperature rises the voltage goes down. with room temp I show about 60 degree F, if I stick the thermistor in ice water It jumps to 125!

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                  • #10
                    Originally posted by jimp View Post
                    I show about 60 degree F, if I stick the thermistor in ice water It jumps to 125!
                    how are you converting to temperature?

                    it's not a problem, you just need a new formula to convert to temperature, utilizing the negative coefficient

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                    • #11
                      #SETTINGS#((((($t$ - .4) * 1000) / 19.53) * 1.8) + 32)|t=VINC[!]

                      thats in the old program, tried the new configurator but haven't yet got it figured out, I get no multiimage showing and if I include the text label display, when I first fire up MDX it shows some reading but just jumps to "0" and stays there when its fully loaded.

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