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Battery-low detect using Voltage Detector and RC

Discussion in 'Embedded' started by Robert Scott, Mar 7, 2005.

  1. Robert Scott

    Robert Scott Guest

    I thought I had a really clever way of implementing 3-volt battery-low
    detection with only 3 parts: a 2.7-volt reset detector, a resistor,
    and a capacitor. The idea was to periodically raise and output which
    will would be filtered by the RC filter, and then feed that to the
    reset detector. By timing when the reset detector declares the
    filtered voltage to be above 2.7 volts, I could infer battery voltage.
    The trouble is those reset detectors (Microchip MCP 130 series) have a
    built-in additional power-up delay that swamps out the effect of my RC
    filter.

    Does anyone know of any tiny 3-terminal device that acts like a
    self-contained voltage detector but without any delay? If so, then
    this method could still be salvaged. I plan on performing an
    in-circuit calibration on each piece, and then custom-flashing the PIC
    to compensate for variations in threshold voltage and RC values.


    -Robert Scott
    Ypsilanti, Michigan
     
    Robert Scott, Mar 7, 2005
    #1
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  2. Robert Scott

    Guest Guest

    What would be wrong with periodically lowering the output and timing when
    the reset detector declares the voltage to be below 2.7 volts?
     
    Guest, Mar 8, 2005
    #2
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  3. Robert Scott

    Robert Scott Guest

    By golly, it works! In a way, it's an even more sensitive measure of
    the battery voltage. The only disadvantage is that the time period is
    much shorter, and therefore harder to measure with accuracy. But on
    my target system, a 3.6-volt battery supply takes about 60 usec. to
    trigger the reset detector at 2.7 volts using the RC filter I've got
    now. My 4 MHz PIC can measure time to within 1 usec, so I guess I'll
    go with it. Thanks for the suggestion.


    -Robert Scott
    Ypsilanti, Michigan
     
    Robert Scott, Mar 8, 2005
    #3
  4. Robert Scott

    Joerg Guest

    Hello Robert,

    Just in case you get hit with inaccuracies due to internal delay or the
    tolerance of the capacitor: Check out the National LMV431 or the TI
    counterpart. That is a very precise chip in SOT-23 and you could build
    something around it.

    Regards, Joerg

    http://www.analogconsultants.com
     
    Joerg, Mar 8, 2005
    #4
  5. Robert Scott

    Robert Scott Guest

    I will definitely look into it. As for inaccuracies, I expect to deal
    with most of them with a system calibration. I will flash a test
    program into the completed module after it is potted. This test
    program will determine the correct time delay for a threshold battery
    voltage, and then reflash the application program based on the results
    of the test. I won't be relying on any precise part tolerances.


    -Robert Scott
    Ypsilanti, Michigan
     
    Robert Scott, Mar 9, 2005
    #5
  6. Robert Scott

    Joerg Guest

    Hello Robert,
    Sounds good, as long as the tempcos of the contributors are within your
    spec.

    Regards, Joerg

    http://www.analogconsultants.com
     
    Joerg, Mar 9, 2005
    #6
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