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Constant Current Source Design

Discussion in 'Electronic Design' started by confused soul, Jan 24, 2005.

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  1. I need to design a constant current source of 0.1 mA. The load varies
    between 1Mohm and 1 Kohm. Using a constant voltage source, is it
    possible to design a constant current source?
    Any help in this is greatly appreciated.

    --Confused Soul
     
  2. Jim Thompson

    Jim Thompson Guest

    0.1mA thru 1Meg is 100V. What supplies do you have available?

    ...Jim Thompson
     
  3. Good enough approximations may be possible.
    Is the load resistance connected to a positive supply, a negative
    supply, ground, or none of the above?

    What supply voltages are available?

    What kind of frequency response must this current source have?

    0.1 mA through 1 Mohm implies 100 volts drop across that resistance,
    so a voltage supply greater than 100 volts will be necessary to feed
    the current regulator.
     
  4. I read in sci.electronics.design that confused soul
    z.googlegroups.com>) about 'Constant Current Source Design', on Mon, 24
    Jan 2005:
    Yes. It sounds like homework to me. We don't do homework but we may give
    hints to polite enquirers.

    Have you given all the data? How much variation in the 0.1 mA is
    allowed? Do you have to use just a voltage source and resistors, or are
    transistors allowed?

    0.1 mA through 1 Mohm means that there is 100 V across the 1 Mohm. If
    you had a 1 kV supply with a 9 Mohm resistor in series, how would that
    work out? Do you think that's a practicable solution?
     
  5. John Woodgate wrote...
    Hints: check out common bipolar-transistor current-source circuits,
    with opamp feedback. Think about using two MPSA42 (npn) or MPSA92
    (pnp) transistors, wired in the Darlington configuration.
     
  6. Looking at the previous replies, I kinda feel dumb that I didnt even
    think about the possible voltage required. 100 V seems impractical in
    my case. It is possible for me to change the constant current value to
    0.01 mA or even 1 uA instead of 0.1 mA. In that case, what kind of
    circuits can be used for building a constant current source.
     
  7. I read in sci.electronics.design that confused soul
    z.googlegroups.com>) about 'Constant Current Source Design', on Mon, 24
    Jan 2005:
    A few weeks ago we had a thread here in which a FET-based current source
    could give output source impedances of gigohms. That should be OK for
    you! If you can't search the group archive, maybe someone will give you
    a message ID to look up or a copy of one of the relevant posts.
     
  8. Nicolai

    Nicolai Guest

    Have a look at the Fixed Current Source schematic on page 10 of
    http://cache.national.com/ds/LM/LM124.pdf
    Exchange the PNP transistor with a P-channel MOSFET with low Vgs threshold
    and recalculate the resistor values for your own needs and it should do the
    trick. If you need better accuracy just use a better OP-AMP and resistors.

    The High Compliance Current Sink on page 12 will do the same trick if you
    need to sink instead of source the current. Again just substitute the NPN
    transistor with an N-channel MOSFET and recalculate for your needs. The
    current sink circuit has the advantage that it can be made to work with any
    voltage as long as you use the right MOSFET.

    You can keep the transistors if you don't like MOSFETs but then you will
    have a small error contribution from the base current. Old datasheets are
    abundant with similar circuits.

    HTH
     
  9. A FET-based solution is well worth considering!
     
  10. With a bipolar op-amp such as the LM324, there's an error component
    from the input bias current. At 1uA, it could be as much as 50% error
    (typically 4%). It also varies with supply voltage and somewhat with
    temperature. There are better op-amps.


    Best regards,
    Spehro Pefhany
     
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