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Thread: System monitor w/RoadRunner

  1. #11
    Fusion Brain Creator
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    Quote Originally Posted by nserpa9248 View Post
    Im self taught with alot of this stuff.
    Me too.

    If you have specific questions, I'm happy to answer them... it's just impossible to teach 4 years of electrical engineering to everyone.

  2. #12
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    Update on my "research"


    I have been going over the wiki and alot of other stuff ive searched from google on these things (op-amps)

    I seem to have two options
    either i need a differential amplifier, so that i can take the two positive outputs from the shunt (one being the "un-altered" voltage and the other being 0-50(or 100)mV less) and get an output voltage from the op-amp which is a function of the difference of the two different voltages...which...hopefully i can get to be 0-5v (doesnt look like this is possible) if i can not do this, i would need to add another op-amp after the differential amp in series into the FB input
    Shown below


    Or

    Use two separate op-amps one having the input voltage being the "un-altered voltage" from the shunt, the other op-amp having the input voltage as the "altered voltage" and having the fusion brain see the two voltages from the op-amps outputs and doing the math via software.
    I am not quite sure if i would need to use inverting or non-inverting OP-AMPs (trying to do the math with the resistance values to see which will suit my needs)
    so i drew a diagram for both

    Dual Non-Inverting Op-Amps


    Dual Inverting Op-Amps


    Using the second option would obviously take up four of the analogue inputs into the FB, which is fine, i only need two more for the temp sensors, and one for the voltage sensor

    so 7 ports will be used, three left for future sensors

  3. #13
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    your first option is exactly what you want, just make sure the FB and the op amp share the same ground.

    nice work btw.

  4. #14
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    thanks.

    I did the math on the differential op-amp, and found a resistor combo that should work great for me.

    Here is the calculation


    I am figuring the V1 input being the "un-altered" voltage (distro side of shunt) and V2 being the "dropped voltage".

    I figured these resistor values would work:
    R1-9,500 ohms
    R2-10,000 ohms
    Rf-3,800 ohms
    Rg-10,000 ohms

    With those calculations, if i did my calcuations correctly this table is correct.
    The concern that i am having is the very slight voltage changes when the amperage changes.

    For example:
    When the voltage is 8V and the current draw is say 50% (50mV drop) the output voltage is 7.995V. When the voltage is 8v and the draw is 0% (0mV drop) the output is 8v. This is only a .005V drop between these two situations.

    I am concerned about if the fusion brain will be able to read that little of a difference.

    Are my calculations correct?
    Will the fusion brain be able to read these small differences?

  5. #15
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    i did some searching on fusion brain accuracy

    I came up with a thread on the temperature sensors.
    Greenman, this post you made
    Thermometer - Range/Accuracy/Direct Contact?


    Quote Originally Posted by greenman100 View Post
    Displayed are the raw voltages. Converted, they are as follows:
    Code:
    Raw Volts	deg F
    
    0.879765	76.286
    0.869990        75.38369231
    0.879765	76.286
    0.889541	77.1884
    0.889541	77.1884
    0.874878	75.83489231
    0.874878	75.83489231
    0.899316	78.09070769
    0.889541	77.1884
    0.889541	77.1884


    Shows that one (of many)of the raw voltage values you were able to read was 0.879765 volts
    which is an idication the fb can read up to the millionth place, it seems that i would need accuracy up to that point.

    Can somone verify this as well as the previous question on my calculations.

  6. #16
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    The FB can read 1023 different voltage ranges, between 0 and 5v.
    IE, x.xx5, x.xx0.. so its accuracy is +/- 0.005v

    so, it can tell the difference between 2.780 and 2.785 volts, but that is where it draws the line. Due to math, some of the actual voltage numbers come out very long, but they only actually "step" every 0.005v.


    Well, actually they step every 0.0048875855327468230694037145650049 (5.0/1023) volts... but that is just too many numbers to remember =D


    peace
    Mike

  7. #17
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    you're not setting up the op-amp right. re-evaluate your calculations

  8. #18
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    what is incorrect? The calculation i am using? The variables i am using? Or how i am doing my math?

  9. #19
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    i looked up some op-amp calculators online and found a few.
    This page http://www.bcae1.com/opamp.htm
    has an differential amplifier calculator, and when i entered the values i have for resistances and voltages, the results match my math for the output voltage.

    You said i was setting up the op-amp wrong. Howso?

  10. #20
    Fusion Brain Creator
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    try 10k for feedback and 20k for the rest

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