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Size of Resistors

Discussion in 'Electronic Basics' started by GraemeC, Aug 25, 2006.

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  1. GraemeC

    GraemeC Guest

    I'm a digital disginer who is having to some some pesky analog stuff.


    I've been desiging some circuits using pen and paper and a little with
    spice. As far as I can tell the designs theoretically work.

    I've designed a a constant current source using npn and pnp
    transistors. My only concern is that im using resistors in the 1-10ohm
    range.

    I need to do this because I need a high output (around 250mA) and only
    have a small supply (+/- 5v)

    If I build this am i likely to encounter problems. Will the tolerance
    of the resistors make it difficult to control. If so does anyone have
    any solutions.

    Thanks in advance

    G

    The spice netlist is below if anybody is interested

    This circuit provided a constant current through the load LED (D1) when
    Vi1 is greater than approx 3v
    The value of the current is dependent on R2 and R5

    V1 V+ 0 SINE(0 5 100K)
    R1 V+ N005 1k
    Q2 N003 N001 V+ 0 PNP
    Q3 N001 N001 V+ 0 PNP
    Q4 i N001 V+ 0 PNP
    Q1 V+ N005 N003 0 NPN
    Q5 N005 N005 0 0 NPN
    Q6 N003 N007 0 0 NPN
    Q7 N001 N003 N007 0 NPN
    R2 N007 0 10
    R4 V+ N006 1k
    Q8 N004 N002 V+ 0 PNP
    Q9 N002 N002 V+ 0 PNP
    Q10 i N002 V+ 0 PNP
    Q11 V+ N006 N004 0 NPN
    Q12 N006 N006 0 0 NPN
    Q13 N004 N008 0 0 NPN
    Q14 N002 N004 N008 0 NPN
    R5 N008 0 10
    D1 i 0 QTLP690C
     
  2. Chris

    Chris Guest

    Hi, Graeme. Try small wirewound resistors -- they're commercially
    available down to 0.1%. They cost a little more, but they're
    remarkably durable, and also have a lower temperature coefficient of
    resistance.

    Good luck
    Chris
     
  3. I've only just begun to look at the arrangement. You have 14 BJTs in
    order to provide a constant current into an LED!?? Can you explain a
    little more about why?

    Jon
     
  4. PeteS

    PeteS Guest

    I ditto this reponse.

    If you are trying to provide 250mA into an LED as a constant current
    (which seems awfully high), it can be done with only a single
    transistor and a few other parts.

    Apart from that, the part you specify (QTLP690C) is listed as obsolete
    http://www.fairchildsemi.com/sitese...temap+id&ia=1&text=QTLP690C+&as=1&render=1&w=

    Please tell us the application requirements :)

    Cheers

    PeteS
     
  5. GraemeC

    GraemeC Guest

    I can confirm that the 250ma is correct

    The QTLP690C is not the device that will be used. I just added it to
    the design for illustration. I need to drive the LED at a range of of
    constant currents to check the beam profile.

    My design is two bootstrapped current sources, hence having to use
    14BJT.

    I accept that I might be over complicating the issue but I can't seem
    to find anything in the literature.

    thanks for the help so far.
     
  6. Not that much help, so far. I already could see the two current
    sources, in parallel. What I didn't understand are some things you
    still haven't discussed. If you want 250mA into an LED, how important
    is the accuracy, precision, and drift over temperature and time? What
    about information about the power supply? You are using a test
    voltage that ranges and it's not possible to tell what you intend,
    here. How critical is the voltage threshhold? Over what voltage may
    the transition from 0mA to 250mA take place? How 0mA must it be below
    this point?

    It's hard to know if you are over-complicating. But it would help to
    know why you put the pieces in that you did and what your design
    thinking is. My own take, from the fact that you include a SINE
    voltage source, is that you are looking at power supply variations
    against diode current. On that score, I don't consider it all that
    good. The current source impedance is only some 700 ohms with typical
    3904's and 3906's substituted and working into a simple LED model and
    requires some three times as much current to operate it as the LED
    requires. I can only imagine the heating going on and I think better
    can be done with a lot less. Transition spans about 400mV. That may
    be just fine. Or not.

    You are asking for others to suggest some thoughts about a particular
    design you have in mind. I'm not familiar with the details, as I
    haven't tried to work through them in each half-section and don't
    really want to try with so little information. You haven't disclosed
    your work and thinking on it, either. If someone else has already
    worked through this particular design, you are lucky indeed and may
    get some excellent comments. But with no clear information from you
    about what you are trying to achieve, and otherwise in unfamiliar
    territory with your design and no information from you about why you
    did what you did to help me think about it, it's hard for me to either
    recommend something else or find the time to try and fathom your own
    thinking when you won't just write it out on your own.

    You also haven't said anything about cost or using parts that are
    designed to be current sources for driving LEDs or LED displays. Some
    may be a good fit without design time, but I'm just a hobbyist so I
    probably cannot recommend them. I'm sure others can.

    I wish you'd written more so I could have written less. But there it
    is.

    Jon
     
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