voltage not sensitive to temperature

Discussion in 'General Electronics Chat' started by z_mx, Apr 8, 2013.

  1. z_mx

    Thread Starter New Member

    Apr 7, 2013
    9
    0
    Hi!
    I am young engineer after college. i am trying to solve next problem:
    [​IMG]
    i need to stabilize voltage between points A and B (constant 0.7V) in temperature range from -45 to +85. Haw can I do this (minimal change in the circuit)?:confused::confused:
    thanks very much
    https://www.dropbox.com/s/4g59uc864z0jl1s/4A.jpg
     
    Last edited: Apr 8, 2013
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Load?
    Precision +/- ?
    Source range?

    Ken
     
  3. timescope

    Member

    Dec 14, 2011
    298
    44
    To obtain a voltage of 0.7v across 0.33 ohms implies a current of 0.7 / 0.33 = 2.12 Amps.

    No load is shown on the schematic and you will not get 0.7v without one.

    This circuit is a constant current source

    Timescope
     
  4. z_mx

    Thread Starter New Member

    Apr 7, 2013
    9
    0
    I need to get 4A (+-10%) on the source ( pin 3)
    typical Load 1-7 ohm
    thank you

    Max
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,392
    1,605
    You are depending on The B-E drop of Q4 to regulate that voltage, and that drop will vary with temperature... 2mv per degree C as I remember.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    You can build a (relatively) temperature stable constant-current source using a low offset op amp and a temperature stable voltage reference. The circuit would be similar to that shown here, but substitute you P-MOSFET for the PNP transistor and generate the reference voltage with the reference connected between the +28V and the Vref point (remove R1 and R2).

    Note that Vref must be referenced to the V+ (+28V) rail, not ground. Low voltage references usually go no lower that 1.2V, so you can use a voltage divider between the voltage reference and +28V to reduce the voltage to your desired 0.7V below +28V.
     
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