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  • Monitoring Wideband O2...Safe?

    I have a wideband O2 sensor connected to my car's ECM. The output of the wideband is 0-5V (perfect!) Is it safe to tap into that signal line and feed one of the FB's Analog inputs so that I can also display a gauge of the A/F ratio, or can the connection to the FB affect the signal that goes to the ECM?

    Thanks, Steve
    Steve



    VIN 02650 "Project Delorean"

    http://www.projectdelorean.com



    or on CarDomain at:

    http://www.cardomain.com/ride/2448292

  • #2
    It may skew the readings to the ecu. Almost every wideband I have seen, or used, has a programmable output or a second output for an external gauge.....this can be used to feed the FB.

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    • #3
      Originally posted by tom04841 View Post
      It may skew the readings to the ecu. Almost every wideband I have seen, or used, has a programmable output or a second output for an external gauge.....this can be used to feed the FB.
      Hmmm. That's what I was afraid of. I'm using the PLX wideband unit. It seems to have a single NB and and single WB output. I just shot PLX a note directly to see what they think.

      Thanks!
      Steve



      VIN 02650 "Project Delorean"

      http://www.projectdelorean.com



      or on CarDomain at:

      http://www.cardomain.com/ride/2448292

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      • #4
        If the impedance on the FB input is high enough in relation to the impedance the ECU is reading the sensor at, it should not vary the signal enough to many any significant difference. The question is... how do they compare? If you know the impedance of both inputs, you can actually figure out how much of a voltage drop it will cause, as that will essentially turn the circuit into a voltage divider.
        "stop with the REINSTALLS, what do you think we got some lame-o installer!!!" - mitchjs
        RevFE
        My Shop

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        • #5
          Originally posted by malcom2073 View Post
          If the impedance on the FB input is high enough in relation to the impedance the ECU is reading the sensor at, it should not vary the signal enough to many any significant difference. The question is... how do they compare? If you know the impedance of both inputs, you can actually figure out how much of a voltage drop it will cause, as that will essentially turn the circuit into a voltage divider.
          Yeah, it'd be good to use a voltage follower here.

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          • #6
            Holy crap! I'd never heard of a "voltage follower", but I just looked them up and that's exactly what I need. Any tips on components to put one together, keeping in mind my 0-5V signal range?

            Thanks for your help. I really appreciate it.
            Steve



            VIN 02650 "Project Delorean"

            http://www.projectdelorean.com



            or on CarDomain at:

            http://www.cardomain.com/ride/2448292

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            • #7
              Originally posted by protodelorean View Post
              Holy crap! I'd never heard of a "voltage follower", but I just looked them up and that's exactly what I need. Any tips on components to put one together, keeping in mind my 0-5V signal range?

              Thanks for your help. I really appreciate it.


              its just an op amp.

              http://electroniccircuitsforbeginner...r-circuit.html

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              • #8
                Apparently those PLXs are good for 20mA so I doubt any buffering is needed.
                What's the FB impedance - a few meg?

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                • #9
                  its been a while, but I think PIC18F series input impedance is real low, like 3kohm

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                  • #10
                    Originally posted by greenman100 View Post
                    its been a while, but I think PIC18F series input impedance is real low, like 3kohm
                    With my math that's 5V/3,000ohms = 1.7mA (if I did that right)

                    If the PLX can handle 20mA, should I still be looking to buffer? (be gentle. I'm outside my specialty here.) : )
                    Steve



                    VIN 02650 "Project Delorean"

                    http://www.projectdelorean.com



                    or on CarDomain at:

                    http://www.cardomain.com/ride/2448292

                    Comment


                    • #11
                      From the PIC18F2455 data sheet (AFAIK the FB uses the PIC18F2455), they recommend a max series resistance of 2.5kR (R=Ohms), and....
                      "A maximum source impedance of 10 kΩ is recommended for the analog sources"

                      The only other parameter I can find is an internal ADC input leakage of 100nA (and 500nA for the comparators). 100nA is equivalent to 50M-Ohm leakage.

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