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Transistor as a current limiter

Discussion in 'Electronic Basics' started by Lauri Alanko, Jul 5, 2013.

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  1. Jon Kirwan

    Jon Kirwan Guest

    He appears to depend upon a slow enough operation with low
    enough duty cycles to allow cooling back towards ambient.
    Saying, "The thermal-runaway effect does not have time to
    develop."
    Yes. As the article states. But as I point out, the effect of
    a larger valued C1 is to cause the leading edge of the pulse
    to have a noticeable higher current edge that gradually drops
    back to the design value. Small values for C1 would have
    little impact on it. Ahyway, he doesn't mention that behavior
    and instead says it "prevents transient oscillations" but I
    don't see how they occur as a result of switching on and off.

    Jon
     
  2. Jon Kirwan

    Jon Kirwan Guest

    anyway
     
  3. Jon Kirwan

    Jon Kirwan Guest

    I should explain more about my experiences, I suppose.

    I've no doubt at all that if one of the BJTs carries a
    different Vce than others in the chain, and if the currents
    are enough, thermal runaway just happens. The BJT with the
    larger Vce is going to have its Vbe decline steadily with
    some bad news.

    But LEDs present fairly consistent Vce's to their driving
    BJTs. (Usually a very low Vce, in fact, if you are keeping
    the margins down to minimize wasted power.) The only
    substantially different Vce is the current mirror BJT that
    sets the current and is under loop control. It's almost
    always the case that the control BJT is the very hottest one,
    more so than any of the LED driver BJTs, since it has the
    highest Vce.

    If the control BJT heats up, then the response by the rest of
    the system is to have lower Ic's. And since its dissipation
    is fixed by design (its Vce is set and so is its Ic), it
    settles at some resulting Vbe and the other BJTs exhibit less
    Ic as they have a lower Vbe drive, less Vce, and are cooler.
    So no runaway.

    Jon
     
  4. Jon Kirwan

    Jon Kirwan Guest

    Yeah. But it's not runaway. And besides, it's fixable with
    ease with a slight design change.

    Jon
     
  5. Jon Kirwan

    Jon Kirwan Guest

  6. Jon Kirwan

    Jon Kirwan Guest

    I'm looking for the mathematical explanation -- quantitative
    -- as well as the theoretical underpinning. Can you elucidate
    that?

    Jon
     
  7. Jon Kirwan

    Jon Kirwan Guest

    No. I completely understand that behavior and it's NOT
    oscillation. Change C1 to 1p. Rerun. Looks better. Right?

    Jon
     
  8. Jon Kirwan

    Jon Kirwan Guest

    Looking back on notes I wrote a long time ago, I find
    something like worst case 5% difference in Ic for small
    signal PN2222A parts from the same reel, without the use of
    degeneration. That suggests about 1.2mV of Vbe variation.
    With 100mV of emitter degeneration, I would naively expect
    1.2% difference in Ic but my notes suggest that this was more
    like 0.6%. Suggesting a divide by 2 that I'm missed in my
    naive (without paper) analysis, which doesn't shock me.

    Emitter degeneration is pretty effective. Not just with part
    variation but also with thermal runaway.

    With LEDs soaking up most of the rail voltage, it's all just
    good.

    Jon
     
  9. Jon Kirwan

    Jon Kirwan Guest

    I have all of the mathematics required (which just happens to
    be most of the first two years of an EE degree -- I've
    checked.) I also have _some_ closed loop control theory, but
    mostly with PID -- I can do the math for that. I'm pretty
    sure I could follow anything you could write on the subject.
    I use calculus almost every single day of my life in my
    "other work."
    I'm fairly well informed about the models. I cannot say I
    know "all about" them. Which is why I'm asking for help here.
    I don't need a broad brush-off. I think I could look at
    anything you are capable of preparing and asking informed and
    precise questions where I felt I didn't understand. I can
    navigate fairly complicated math, as I do it normally in my
    work.
    I understand poles and zeros -- very simple thing
    mathematically. What I don't see is the equation showing them
    and how that equation is created from that particular
    circuit.
    Just develop the simple poles-zeros equation for me and show
    me how you constructed it. I can do the rest. I won't need
    any further help.

    Jon
     
  10. Jon Kirwan

    Jon Kirwan Guest

    Sure, that is an assumption one can make. But it doesn't help
    me learn anything at all.

    I already explained, in detail, what I can SEE that C1
    actually does do. It makes the circuit WORSE, from one
    perspective. I will CAUSE a higher than normal leading edge
    current peak. And I completely understand exactly WHY it does
    that. I can actually sit down and easily compute the time
    constant and the peak excess that will occur for various
    component values, even, and get both results right (to the
    degree that Spice agrees, anyway.) So I do understand one
    thing that C1 does do and I frankly don't like it.

    The problem for me remains, in that I still don't see the
    "other problem" nor why C1 fixes it. (It's hard to see how
    something gets fixed if you can't even see the problem in the
    first place, I suppose.)

    Anyway, I want to be able to (1) see the problem, (2) COMPUTE
    the problem, quantitatively, (3) recognize cases where the
    problem is significant enough to be worth fixing, and (4)
    understand the effect C1 has on resolving that problem in the
    circumstances where it is a significant issue. I can follow a
    good argument, if someone would just make one.

    So far, I'm clueless. And so far, John Larkin hasn't done
    anything to educate me. I'm not complaining. He owes me
    nothing. But he has wasted his valuable time on me, in
    effect, because what he has written is worse than just vague.

    By the way, right now I'm struggling with the idea of how to
    actively enable individual half-current mirror BJTs in such a
    way that I can selectively enable various combinations of
    these extended mirror sections without impacting others which
    remain "on." I'm already including emitter degeneration of 1
    Ohm (or 100mV at 100mA), which improves significantly in
    dealing with variations in Is and temperature variations with
    each BJT. And it works fine on a protoboard, so I'm liking
    that result. But if one of the BJTs has no load (infinite
    resistance in effect), it saturates like crazy and messes
    with the other BJTs. (And the degree of that problem gets
    worse with emitter degeneration, too.) So I need a cheap and
    easy way (I'm coming up with HORRIBLY EXPENSIVE and HARD ways
    I don't want to use) to be able to selectively activate an
    opposing switch (current source/sink on one side that is
    under active control and a BJT switch on the other side which
    selectively permits or blocks the programmed current.)

    I'm just not seeing a simple way. It's all way too hairy for
    my liking. (Can easily see how it might be approached with
    MOSFETs, for example, but I get BJTs at better than 2 for a
    penny and I get MOSFETs for a LOT MORE than that, even when
    on sale.) I'm looking for a BJT and discrete (I can't do
    designs with equal emitter areas on common subtrates, for
    example, and I can't afford to throw dozens of BJTs at
    everything either because I'm not doing an ASIC.)

    Fun problem.

    Jon
     
  11. Jon Kirwan

    Jon Kirwan Guest

    I'm certain that if I take this to a professor at a
    university I would get a quick and adequate explanation that
    would satisfy me and I would find that your "complexity"
    really isn't there, at all. (Doesn't mean I'm not blind --
    but I think my eyes would be quickly opened when the right
    person answers.) Anyway, I suppose I will have to do that.
    I'm interested enough. Might have to wait for late September,
    but that's okay. I don't expect or get quantitative answers
    from you. But I have appreciated some of your answers all the
    same. So don't take it the wrong way. I just don't imagine
    you have the math, is all. But you have intuition and I
    accept and respect that.
    I have simulated it. But one doesn't (and shouldn't) learn
    that way. A simulator is NOT reality. You don't hack and poke
    at a simulator to learn about reality. You use it as an
    effective tool to avoid doing those closed solutions you are
    talking about, but only when you already pretty much
    understand the theory and know why and where you are headed.
    It's a way of keeping you from missing something important
    that you already knew about, but didn't remember this time.
    Or in finding those operating points that would have you
    poking a calculator over and over again.
    It was completely useless ... to me.
    Yes. But this really isn't appropriate for the design group.
    It's too basic for that. And I imagine that when I get this
    in front of a professor in a few months, I'll find the answer
    extremely easy to understand and gather. (And he/she may just
    tell me that I'm right and it's not a problem.)

    The other effect of the capacitor, the one which forces a
    rise in peak current at the leading edge is terrible and I do
    understand that very well.
    That point is right, as far as the linked circuit goes and
    the author's discussion about thermal runaway.

    However, in my case and in the case of anyone doing LED
    strings, the point is irrelevant. There is NO runaway that
    takes place in those cases. I've done it, tried it. It
    doesn't happen. What does happen is that the feedback BJT
    gets hotter than the others and that reduces the currents
    into the LED chains. Since that feedback BJT is under closed
    loop control, there is NO RUNAWAY. So it's not an issue.

    Pulsing or otherwise.

    Jon
     
  12. Jon Kirwan

    Jon Kirwan Guest

    Speaking of which, any thoughts on this issue:
    I would plan to design the power supply to provide about 1.5V
    of headroom beyond what is needed by the LEDs at peak current
    drive. That should provide a good 1.2V for the current mirror
    side of things and 0.3V for the switch on the other end. (If
    it can be done with less headroom, so much the better.)

    The issue is this:

    To have a controlled current sink (or source) capable of
    supporting ... let's say at least 8 sinks/sources ... and
    where each sink/source can be individually enabled or
    disabled without affecting any of the others. Only BJTs as
    active devices. Only discretes, no ICs, no opamps, etc.

    My imagination fails me for a reasonable solution here. How
    about yours?

    Jon
     
  13. Jon Kirwan

    Jon Kirwan Guest

    The fundamental answer is because I want to learn how to do
    things without them. I have other reasons. But I'd like to
    know how to do the design needed to do this without
    handholding from opamp designers.

    It's like building a telescope by buying a finished mirror.
    You can do it. But you don't learn much about the various
    orders of defects, how to recognize them, how to correct for
    them... that way. There is a huge difference between an
    amateur who has fabricated their own optical pieces, learned
    how to test and correct the figures, and created their own
    final unit and someone who just goes out and buys something.

    That is the "elephant in the room" reason. But there are
    others, of course.

    Jonm
     
  14. Jon Kirwan

    Jon Kirwan Guest

    Here's the best I can do with my meager imagination, John.
    This achieves my goals, but it takes 5 BJTs per section, plus
    2 more BJTs for all sections. So the number of BJTs equals
    5*N+2, where N is the number of individually controllable
    current strings.

    It doesn't use a common current setting method, though. Each
    string is set individually through R1 (combined with the base
    drive voltage at Q1.) So I'd like to improve that detail, as
    well.

    The example uses 6V for the LED supply and assumes 3.6V for
    the microcontroller Vcc for illustration purposes. It also
    uses two LEDs in a series chain, also for illustration
    purposes.
    It's basically a diff-amp driven through a level shifter.

    I'd love to see creative improvements (WITHOUT OPAMPS OR
    MOSFETS.)

    Jon
     
  15. Jon Kirwan

    Jon Kirwan Guest

    My R2 above provides way too much gain. Should be a lot
    smaller -- orders of magnitude.
    No mosfets. I'm not fighting in that jungle, yet. I might
    start if you know where I can get them for 2 for a penny in
    10000 qty. (I give stuff away to kids and don't mind spending
    $50 to $100 for stuff I will simply give away to anyone
    interested.)

    Jon
     
  16. Jon Kirwan

    Jon Kirwan Guest

    I have some, of course. Cheapest mosfet I could find.
    Perhaps. But I buy PN2222A for less than 0.4 cents each. And
    3904/3906, if I keep my eyes open, I find at only slightly
    more. The bcx70 is something I'll look around for, too, then.
    But it doesn't seem quite cheap enough for me, today.

    I give away 50 at a time. So the difference between .00385
    and .02 is important to me.

    In any case, I want to see how this is done with BJTs.
    Okay. So...

    Jon
     
  17. Jon Kirwan

    Jon Kirwan Guest

    So I just decided to take a look (besides seeing the mosfet.)

    Looking at schematic A: I see a diff-amp with 0.2V input (I
    suppose you would tell me to use a R-divider if I were being
    cheap about this.) One of its collectors (let's call that PNP
    Q1 for now) collector is used to pull up on the base of
    another NPN (call it Q2 for now) that ties the mosfet gate.
    To get the voltage at the base of Q2 high enough that it is
    slightly or usefully active, the collector of Q1 must reach
    near to 0.65V or so. But the base is 0.2V. You see any
    problem here? I guess you offered as much thought as you felt
    I deserved.

    Jon
     
  18. Jon Kirwan

    Jon Kirwan Guest

    Explain why.
    Saturated BJT in the diff-amp?

    Jon
     
  19. Jon Kirwan

    Jon Kirwan Guest

    (The base drive will have to be significant, relative to the
    common diff-amp emitter currents. That means a stiff divider.
    Gain will also be poor, I believe.)

    Jon
     
  20. Jon Kirwan

    Jon Kirwan Guest

    I did and was summarily ignored except that he later wrote
    that I could replace the mosfets with BJTs if I wanted to.

    I'm below 200mV of headroom required for surprisingly good
    current control with BJTs only. I still need to fix it so
    that the LEDs aren't in the middle of the circuit, but can
    share a common anode or cathode. That part was just because
    it was easier to think about at the time.

    Jon
     
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