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Voltage Divider

Discussion in 'Electronic Design' started by Marco T., Jul 26, 2006.

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  1. Marco T.

    Marco T. Guest

    Hallo,
    I have thought to use a voltage divider to have an input voltage range
    of -10v to +10v and and output of -2,5v to +2,5v.

    Following the schema: Input -> R1 ->output
    |
    R2
    |
    Ground

    I can choose between:
    R1 R2
    7500 1500
    10000 2000
    12000 2400

    What is the difference in choosing R1 R2 couples?

    Many Thanks
    Marco
     
  2. Marco T.

    Marco T. Guest


    What is the difference between using Ground or a reference voltage?

    Thanks Again
    Marco
     
  3. John  Larkin

    John Larkin Guest

    No difference; they're all wrong.

    John
     
  4. Marco T.

    Marco T. Guest


    Why?
     
  5. Greg Neill

    Greg Neill Guest

    Show your math for determining the resistance ratios.
     
  6. Marco T.

    Marco T. Guest


    I have used a script into TI site.

    http://www-k.ext.ti.com/srvs/cgi-bin/webcgi.exe?Company={5761bcd8-11f5-4e08-84e0-8167176a4ed9},kb=analog,case=obj(32620),new

    I have re-used the script and it has showed different values.
    R1 R2
    12000 3000
    30000 7500
    Maybe I typed a wrong value.

    Into the script there is the possibility to choose resistor scale 10, 100,
    1000, ecc.

    What is the difference, in this case between the 2 possibilities?

    Many Thanks
    Marco
     
  7. Marco T.

    Marco T. Guest



    Input stage of voltage divider should be a strain gauge with signal
    conditioning to obtain 0 10v in single ended, or -10 +10 volt in
    differential.

    Instead the output should be a voltage follower.

    Which could be a good resistor configuration?

    Many Thanks
    Marco
     
  8. They're still wrong. There's not much data input validation on that
    script. You will need to conform to American practice and use "."
    rather than "," as the radix point. One exact possibility would be 39K
    & 13K
    Loading on the 10V input and output impedance (about 10.3K in the
    example I gave above).


    Best regards,
    Spehro Pefhany
     
  9. Greg Neill

    Greg Neill Guest

    Do you have any additional specifications on the
    strain gauge such as its output impedance or
    allowable current range?

    You want to choose resistor values such that
    they won't represent too low an impedance
    compared to that of the gauge in order to
    avoid loading it down.
     
  10. mc

    mc Guest

    I think you want the output to be 1/4 of the input, not 1/6. So: 7500:2500,
    12000:4000, etc. (none of which are standard resistor values).
    The amount of current that will flow into the input and the amount of
    additional voltage drop caused by current flowing out the output.

    What is connected to the input and to the output?
     
  11. John  Larkin

    John Larkin Guest

    You want the output to be 1/4 of the input.

    So imagine a string of four equal resistors, +10 volts on one end and
    ground on the other. Call the voltages at the connections A B and C


    A B C
    +10------r-------r-------r-------r------gnd


    By symmetry, B os halfway from +10 to gnd, so Vb must be +5 volts.

    Again, by symmetry, Vc must be 2.5 volts, which is what you wanted.

    You can combine the left three resistors into 3R.

    So R1 must be 3x R2.

    John
     
  12. John Perry

    John Perry Guest

    The strain gauge specs don't matter; he has a signal conditioner. What
    he needs is the output current limit of the signal conditioner, which
    will be more than adequate for any of the total resistances that anyone
    has mentioned, including Spehro's correct one.

    Since he intends to put a voltage follower on it, the same applies to
    the output. Any total resistance from ~2K to ~500K will be fine for
    just about any combination of signal conditioner and voltage follower
    I've ever heard of. Choose something convenient in the middle. Just
    keep in mind John's demonstration of 3R -- R, and make sure 4R is great
    enough to keep from loading your signal conditioner, which you should
    have specs for.

    John Perry
     
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