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Stable voltage divider circuit

Discussion in 'Electronic Design' started by Alan Adrian, Apr 23, 2006.

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  1. Alan Adrian

    Alan Adrian Guest

    I'm using a Basic stamp to parse out a GPS string and send a beep into my
    motorcycle intercom in time to warn me of the info screen on said GPS
    showing me the details of an upcoming turn

    I intend to use the microcontroler to read the temperature and also to
    monitor Bike voltage via and ADC.

    My ADC0831 8 bit A-to-D converter reads 0 to 5v... I need to adjust my bike
    voltage down to fit... A resistor voltage divider seems to be the thing..
    but is it going to be pretty stable (I'd like to keep a reasonable accuracy
    of say.... .05V I know... overkill, but I want to do it if I can...) in the
    temperature range of less than freezing to 40 deg C?... .is there a better
    way?

    Al...
     
  2. T. Atkin

    T. Atkin Guest

    Sure there is a better way.
    I'm not sure if you need a reference voltage or a regulator but you can use
    a TL431 to get 5.00V. The TL431 is cheap yet very precise and stable, I use
    to read 4.99V with power supplies using it as a reference.

    Tom
     
  3. saeed

    saeed Guest

     
  4. Guest

    There are better ways, but you don't need them. Regular metal film
    resistors have temperautre coeffiicients of 50ppm/C. Two combined give
    a worst case error of +/-100ppm/C which is 4,000ppm or 0.4% over 40C,
    almost exactly the resolution of your 8-bit A/D converter (one part in
    256).

    It you wnat to do better, you can buy +/-15ppm/C metal film precision
    resistors off the shelf from broad-line electronic distributors like
    Farnell, who also stock thin film precision dividers which offer ratio
    temperature coeffiicients down to about +/-5ppm/C.

    For the sort of divide ratio you'd need, the dividers cost about $5
    apiece - about twice what you'd pay for a pair of precision resistors.
     
  5. Fred Bloggs

    Fred Bloggs Guest

    It depends on the bike voltage and the scaling factor of the attenuator
    to make it fit the full scale A/D range. The A/D range is fixed at 5V
    and measured with approximately 0.02V maximum error. If your input
    scaling is S then your scaled reading error will be S*20mV, making
    maximum allowable S=2.5 for maximum battery voltage range of
    2.5*5=12.5V. If this is not good enough, then you will have to scale and
    offset so that final reading is of form S*Vm+OFFSET where S<=2.5 and
    OFFSET= BATT,max-S*5.00. In any case, be sure to low pass and clamp the
    battery voltage making its way to the A/D input.
     
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