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Need to design precision 5mA constant current source (wide temp range).. need help

Discussion in 'Electronic Basics' started by stevo, Nov 22, 2004.

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  1. stevo

    stevo Guest


    I need to design a 5mA constant current source for a Wheatstone bridge
    application. The problem is the source needs to be +- 3% accurate and
    hold that accuracy over a -40 C to +85 C temperature range.

    I have thought about using a simple FET CCS, but don't have much
    experience and am not sure if I can make it accurate enough.

    Can any of you more experienced guys point me in the right direction?

    Thank You

    Ohter info : V avalible +15V; bridge resistance ~ 850 ohms
  2. John Fields

    John Fields Guest

  3. Are you sure that a constant voltage across the bridge would not
    improve its accuracy?
  4. stevo

    stevo Guest

    I currently have the end of the bridge connected to ground, but I
    suppose it wouldn't have to be.
  5. stevo

    stevo Guest

    The spec sheet for the bridge has the following:

    Sensitivity tempco:
    Vbridge= 8V (constant voltage) 0.3 %/C
    IBridge = 5mA 0.06 %/C

    So, it appears that the bridge is more stable w/ the constant current.
  6. But those values are very different excitation levels. 8 volts across
    850 ohms gives an excitation current of 9.4 ma, and a bridge
    dissipation of 75 mw, while 5 ma of bridge current gives a bridge
    dissipation of 21 mw. A 4.25 volt excitation would be more equivalent
    to the 5 ma excitation, and may improve on its stability.
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