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Temperature compensated current measurement

Discussion in 'Electronic Design' started by Christian Walter, Jan 15, 2008.

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  1. Hello,

    I am currently looking for a way to measure and control the current
    through a resistive load. I found same standard circuits often used for
    voltage/current conversion but all of them are temperature sensitive
    because they simply measure the current through an additional sense
    resistor. This does not provide the desired accuracy over the entire
    temperature range. For example if I use a 50ppm resistor and a resistive
    value of 100R I have about 0.25R over a temperature range of 100K. This
    is 0,25% which allows maybe 8-9Bits of resolution.
    Does anybody know a good solution for this problem or can provide some
    hints on that?

    Kind regards,
    Christian Walter
     
  2. Phil Allison

    Phil Allison Guest

    "Christian Walter"

    ** Easy - don't let the resistor get hot.

    Use a wire wound type ( typical tempco +/- 30ppm), mount it on a heatsink.

    If its temp don't change, neither will its resistance.

    Surely you are not subjecting the whole circuit to temps of 120 C ???



    ......... Phil
     
  3. linnix

    linnix Guest

    What's wrong with temperature compensating the data? Anything is
    possible with digital data.
     
  4. Guest

    The answer is fairly simple - spend more money on your resistor.

    Farnell stocks a number of Vishay bulk foil resistors offering
    temperature coefficients of +/-0.2ppm per degree Celcius. Look at the
    100R 0805 part (order code 110-924) the 1206 part (110-9037) or the
    through hole part 120-3343 - the last only offers a temperature
    coefficient that is less than 0.6/ppm per degree Celcius form 0 to 60
    Celcius. They cost around 15 euro (say $20) each.

    Farnell also stocks several 15ppm metal film resistor - the MMA0204
    range has a 100R part (order code 308-5703) which is very cheap, while
    their E96 ranges of axial lead resistors include a couple of 100R
    parts for about one euro each.

    If you are doing seriously precise work you could go directly to
    Vishay or other specialist suppliers to get four-leaded (Kelvin
    connection) resistors.
     
  5. D from BC

    D from BC Guest

    One time I made a current sensing resistor so small*, I got worried
    about noise from amplification.

    *To reduce self-heating, to reduce R drift.


    D from BC
    British Columbia
    Canada.
     
  6. I care more about the environment. The power dissipated by the resistor
    is very small. Certainly less than 125mW. Of course I don't want to
    subject the circuit to 120C. And you are right. If I calibrate the
    device at 20C the whole spawn is only 0.125R if I go up to 70 and down
    to -30 using R = R0*(1+alpha*delta_T).

    Regards,
    Christian
     
  7. Phil Allison

    Phil Allison Guest

    "Christian Walter"

    ** I think you have misread the spec for resistor tempco.

    The figures are all " +/- xx ppm " = not just plus or minus.

    You would do well to get some sample components and see just how the value
    varies over your required temp range.

    The actual variation may well be no where near as bad as you assume.




    ........ Phil
     
  8. neon

    neon

    1,325
    0
    Oct 21, 2006
    An lm117 regulator can do that connect it as a current source. must remenber that temperatures do not rise or fall quickly.
     
  9. Joerg

    Joerg Guest

    If it is really, really critical you could pick the lowest tempco
    resistor money can buy and then peltier-cool that. But be prepared to
    shell out close to $100 if you want a TE controller module that is ready
    to use (meaning no design work there) out of the box.
     
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