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Fuel gauge adapter

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by jelliott, Nov 26, 2011.

  1. jelliott

    jelliott Member

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    IC pin voltages w/ input = 2.0 V, 12V bus = 12.5 V (I = 27.5 mA):

    1.1 6.19 V
    1.2 0.01
    1.3 12.50
    1.4 0.01
    1.5 1.05
    1.6 1.27
    1.7 1.27
    1.8 0.01
    1.9 12.50
    1.10 12.50
    1.11 12.50
    1.12 4.74
    1.13 4.79
    1.14 4.88
    1.15 5.21
    1.16 5.85
    1.17 6.17
    1.18 6.36

    2.1 12.50
    2.2 0.01
    2.3 12.50
    2.4 1.27
    2.5 1.05
    2.6 2.54
    2.7 2.54
    2.8 1.27
    2.9 12.50
    2.10 12.49
    2.11 12.49
    2.12 12.50
    2.13 12.50
    2.14 12.50
    2.15 12.50
    2.16 12.50
    2.17 12.50
    2.18 12.50
     
  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    The voltages on pins 15-18 of IC1 are a good 10% higher and the current of 27mA is 20% lower than the sim predicts, but I don't think that's the root of the problem.
    The sim shows that a discontinuity in the +12V supply line, somewhere between R15 and R21, would result in a step function like the one you're seeing. So worth getting the magnifier out and checking for a hairline crack in the pcb trace in that area, also metering the whole length of the +12V line.
     
  3. jelliott

    jelliott Member

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    I couldn't find any problems with that +12V supply to IC2; to verify I applied power by clipping the power supply lead onto the +12V side of a resistor instead of where the screw terminal normally provides +12V to IC2; I could move the power supply lead amongst those resistors and didn't see any change in circuit behavior.

    I did discover that my R28 trimmer seemed to have a bad spot where it was set when the mishap occurred, so I thought maybe screwy trimmer behavior was to blame; replacing both of them didn't improve anything.

    I guess I'm at the point where I need to just take the plunge and re-fabricate the whole thing from scratch, but I had one final thought: Does your sim account for the fact that I had to reduce the value of R29 to 1.0 kΩ to make it work correctly when it was installed in the car?
     
  4. dave

    Dave New Member

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  5. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Current changes smoothly in the sim from empty to full whether R29 = 1k or 1k5. No big step as per the problem. I'm out of ideas :(.
     
  6. jelliott

    jelliott Member

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    I took the plunge and made an all-new circuit, and it displayed the same behavior on the bench! I was beginning to wonder if there was a flaw in the design that went unnoticed back in January because the car was driven so little before I broke it. This weekend I'm visiting my brother and have a chance to work on the car in person, and even just turning the trimmer to adjust the 100% indication, the big step was apparent.

    I had a hunch that this might be related to a transistor turning on and off instead smoothly varying the current, so I replaced Q2 this morning and it seemed to be cured! (Note that I had already replaced all of the transistors at least once on the original circuit, and it still didn't work.) But it doesn't quite work correctly. The fuel gauge reading falls faster than than it should (but seemingly without any big jumps); it read nearly empty when it should have shown ~50%, and I think it finally showed zero somewhat in advance of the input going to zero. My initial reaction is that this must represent a flaw in our model of the car's fuel gauge, but I don't recall ever seeing any such lack of response to 0.5 - 2.0 V (for example) inputs on the bench, so I'm curious if maybe another transistor or something might be defective. Does that ring any bells for you?

    Thanks again,
    Joe Elliott
     
  7. jelliott

    jelliott Member

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    Update: It works! I was able to make it reasonably accurate by playing with the R3 and R4 values. It seems to read a little high in the top half of the scale and a little low in the bottom half, but 100%, 0%, and the all-important 50% (point of no return) are dead on. Now we'll see how long it lasts...

    Is there anything more robust that I can use in place of Q2 if it gets jumpy again? Would two such transistors in parallel be less likely to develop problems (I recall that something was said about excessive base current when I had to reduce the value of R29 to get it to work properly the fist time)?

    Thanks again!
     

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