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constant current source

Discussion in 'Electronic Design' started by kell, May 25, 2005.

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  1. kell

    kell Guest

  2. Yes.
  3. Ban

    Ban Guest

    The trigger current is 0.5uA, so there is enough margin with this value, but
    non-linearities will start popping up with less. If you do not need the
    200mA output current capability, but are happy with 10mA source or 50mA
    sink, you can use the LMC555, a CMOS version, which has only 10pA input
    current on that pin. It will consume much less current and work at
    frequencies up to 3MHz.
  4. Mebart

    Mebart Guest

    Yes, it will work fine. But, the 555 uses quite a bit of current (3 ma
    to 7.5 ma, depending on the supply voltage) just to light the chip.
    And, it requires an extra outboard transistor. I would think you could
    do better depending on the degree of regulation you need.

    Doesn't Maxim make some precision current sources for low power aps???


  5. Robert Baer

    Robert Baer Guest

    One could use a bipolar transistor biased like that for currents down
    to the nanoamp region.
    However, it is a poor "constant" current source (or sink if one uses
    an NPN) due to the Early effect.
    Use a JFET (good to about 30V, the max rating i have seen), or a
    Depletion MOSFET (good to 500V).
  6. Robert Baer wrote...
    Yes. I assume you're talking about figure 13. Just take care, as
    Ban said, to use a CMOS version of the 555 with low timing currents.
    I disagree, for two reasons. First, the Early effect (most usefully
    expressed as a slight change in Vbe vs Vce) is no worse at low
    currents than at ordinary currents, and second, the voltage drop
    across the emitter degeneration (current-setting) resistor overcomes
    the slight Early effect. The transistor current source will work as
    well as before, after scaling the design to low currents.

    Now, if you were using a simple two-transistor current mirror,
    without emitter resistors, that'd be another matter.
    Not necessary, and the large variation of Vgs introduces problems.
  7. Jon

    Jon Guest

    Yes. Make sure that the Re is large enough so that the voltage across
    it is >>Vbe, the Base-Emitter voltage. This ensures that Vbe and
    variations in Vbe with temperature will have minimum effect on the
    collector current.
  8. kell

    kell Guest

    Okay I can use a bigger cap so the chip doesn't load it and affect the
    timing. Looking for some accuracy here. Just need to know the
    tolerance. I have 1uF dip tantalum caps with K printed on them,
    meaning 10%. My 4u7 dip tantalums have an A printed on them. Does
    that mean anything about the tolerance?
  9. kell

    kell Guest

    And another thing: if anybody wants to suggest an alternative constant
    current source, while keeping the headroom down to 3 volts.
  10. Tam/WB2TT

    Tam/WB2TT Guest

    If you have to buy the transistor, I would get the 2N3906. It has twice the
    beta, 60 min at 100 uA. Also, somebody suggested using two transistors in a
    current mirror. You might want to check that out; it will give you more

    For accuracy, at the 1 uF level, I would use some kind of plastic dialectric


  11. You need to worry at least as much about leakage current as tolerance, if
    you're trying to make an accurate long-time-constant ramp based on charging
    a capacitor with a constant current source.

    You might instead consider a microcontroller (or other counter mechanism)
    and a D/A converter.

    ISTR a thread on this group a few years back discussing the various
  12. JeffM

    JeffM Guest

    kell wrote:
    [1 of only 19 Usenet postings (posted from Google Groups)]

    No one has mentioned it yet,
    but you don't include context when you respond.

    Some threads that you may find useful:'t-see-Usenet-the-way-you-do
    Follow the link in that post and follow the link in THAT post.

    The **Show options** stuff will prove useful.

    Grise puts a fine point on it here:
    He may be a bit strong with his rhetoric--but not by much.
  13. kell

    kell Guest

    Okay I went to the links and saw the advice to click show options and
    then click reply; I'm trying that now to see how it works. Does this
    give you the context you need?
    I never used a newsgroup until Google started its groups thing. I
    actually did not know Google leaves newsreaders in the dark until you
    told me.
  14. JeffM

    JeffM Guest

    you don't include context when you respond.
    That's better.[1] Actually, I read on Google as well.
    I just didn't want you to form a bad habit.

    Therein lies the problem.
    A newbie who posts from Google often does it wrong for weeks
    then gets defensive when corrected
    (usually by someone in a bad mood.)

    [1] You don't have to re-post everything from the previous post;
    feel free to snip out as much from the automated blockquote
    as you wish .
    I only leave enough to show what part I'm referencing ("context").
    See above.
  15. kell said
    If you really want to enjoy USENET you might consider a real
    newsreader. Most consider newsgroups-via-Google a pretty poor
    interface. YMMV

    I'd recommend XNEWS.... but there are many options.
  16. JeffM

    JeffM Guest

    I actually did not know Google leaves newsreaders in the dark
    Expanding on what Homer said, if you get a newsreader and use it a bit
    you may like it better than Google Groups.

    I have Gravity, but only use it rarely
    e.g., responding to a poster who cross-posts to more than 5 groups
    (and I don't know which one he reads)
    or when Google doesn't archive a particular group on the Groups line.

    Generally, I prefer reading from my browser.
  17. JeffM

    JeffM Guest

    I actually did not know Google leaves newsreaders in the dark
    :Expanding on what Homer said, if you get a newsreader and use it a bit
    :you may like it better than Google Groups.
    : JeffM

    I forgot to include the major point I was trying to make:
    Even if you go back to Google Groups after trying a newsreader,
    you'll have an appreciation of how others see Usenet.
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