Voltage monitor for car battery

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  1. - 3rd August 2007, 15:41 #1
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    Impedance of a DC voltage divider.

    If I remember right

    R or (Z) = (R1*R2) / (R1+R2)

    Using the 3 to 1 example with 10K resistors - Z = 6.6K
    Using the 4 to 1 example with 10K resistors - Z = 7.5K

    No problem here, Besides that, I use the 3 to 1 example, It does work.

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  2. - 3rd August 2007, 21:41 #2
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    Quote Originally Posted by mackrackit View Post
    Impedance of a DC voltage divider.

    If I remember right

    R or (Z) = (R1*R2) / (R1+R2)

    Using the 3 to 1 example with 10K resistors - Z = 6.6K
    Using the 4 to 1 example with 10K resistors - Z = 7.5K

    No problem here, Besides that, I use the 3 to 1 example, It does work.

    Asbestos <> RoHS Compliance
    The OP hasn't specified a device. If using the popular 16F88, the maximum recommended analog source impedance is only 2.5k. It is always best to stay within the specifications of the device for reliable results.

    Regards,

    Andy
  3. - 3rd August 2007, 22:33 #3
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    Quote Originally Posted by Andy Wood View Post
    The OP hasn't specified a device. If using the popular 16F88, the maximum recommended analog source impedance is only 2.5k. It is always best to stay within the specifications of the device for reliable results.

    Regards,

    Andy
    Are you sure you have the correct data sheet. Section 12.1, paragraph one in the data sheet titled PIC16F87/88 clearly states "The maximum recommended impedance for analog sources is 10K."

    I do agree one needs to stay within the specifications if you do not want to see the magic smoke
    Dave
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  4. - 3rd August 2007, 23:44 #4
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    Quote Originally Posted by mackrackit View Post
    Are you sure you have the correct data sheet. Section 12.1, paragraph one in the data sheet titled PIC16F87/88 clearly states "The maximum recommended impedance for analog sources is 10K."

    I do agree one needs to stay within the specifications if you do not want to see the magic smoke
    Now there's a trap - the current datasheet from Microchip states 10k:

    http://ww1.microchip.com/downloads/e...Doc/30487c.pdf


    However, the (older) datasheet on the melabs site states 2.5k:

    http://melabs.picbasic.com/devicedata/30487a.pdf


    A good lesson - always ensure you have the latest datasheet!!!

    Regards,

    Andy
  5. - 4th August 2007, 00:11 #5
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    And to make sure you are not using old parts. MicroChip seems to make improvements every so often. With all of their products it is hard to keep up.

    But I have to wonder, impedance being an AC term, does this apply more to an ADC source that is switching? Like an old load cell driver?

    But as far as the original question on this thread, I think it has been answered and designed. Thanks to Adam.
    Dave
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  6. - 4th August 2007, 00:58 #6
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    Smile Circuit Design Committee

    Thanks goes to the Circuit Design Committee for the input and to passion1 for asking the all important question.
    Great job nailing down the 2.5k 10k discrepancy, I replaced my old PIC16F88 sheet with the newer one.
    -Adam-
    Ohm it's not just a good idea... it's the LAW !
  7. - 4th August 2007, 01:17 #7
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    Circuit Design Committee

    I like that.
    Dave
    Always wear safety glasses while programming.
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