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How to power my circuit.

Discussion in 'Electronic Basics' started by Daniel Pitts, Aug 8, 2012.

  1. Daniel Pitts

    Daniel Pitts Guest

    Hello,

    So far, all of my circuits have had the power supplied by an Arduino
    (compatible) board. I'm going to try to change the setup here a little,
    and design a circuit that has just an ATTiny in its place, but now I
    have to think about power supply.

    The circuit I'm envisioning is going to have three Texas Instruments
    "TLC5916" constant-current sink's, powering one column of a multiplexed
    8x8 RGB led matrix. The circuit will also have an ATTiny85 (or ATTiny84,
    depending on a few things), and a 74HC238 (to select the row on the LED
    matrix).

    Now, if I'm reading the specs right, I think I can power the whole thing
    on 3 AA batteries (4.5 volts). But if I wanted to have more robust power
    handling, I'm not sure what I want to do. I'd like to try to keep the
    circuit cheap, and potentially have it powered by a wall socket or USB.
    I also don't have the time/money to etch my own PCB, so something that I
    can plug into my protoboard is preferable.

    Is it just me, or is deciding on a power supply a relatively difficult
    problem, compared to other aspects of working on digital circuitry?

    Thanks in advance for any suggestions!
     
  2. Guest

    Just use a 5V wall wart. You can either get a mating connector for your board
    or just cut the end off and solder the ends to your circuit. The ones with
    switching regulators are probably the best bet because you'll be sure they're
    regulated. These are generally specified as 120/240V. Linear or T/R (junk)
    will be 120V only (in the US).
     
  3. Rich Webb

    Rich Webb Guest

    s/deciding on a power supply/analog design/ -- Yes, although it can be a
    lot of fun.

    If you don't need portability then krw's suggestion of a regulated wall
    wart is spot on and by far the simplest. Do ensure that the output is
    actually regulated, though; some are not more than a step-down
    transformer with a diode bridge.
     
  4. Les Cargill

    Les Cargill Guest

    Most tend to be switchers these days because switchers can be lighter,
    which matters in containerized shipping.

    Heck, there's an external "laptop" power supply with my wife's new
    cheap desktop.
     
  5. Daniel Pitts

    Daniel Pitts Guest

    Okay, that all sounds good for now, and that's the approach I'll take.
    Although, how do I verify if it is actually regulated or not?

    In any case, I would like to know more about how to work on this kind of
    thing in the future. For instance, I need to replace my sprinkler
    system panel, which uses 24V for the valve controls, but I'll probably
    end up using 5V or 3.3V ICs for most of the rest of the circuit.

    Also, some idea's I had for projects require higher voltages. I'd be
    interested in creating a Geiger Counter, for instance. I'd also like to
    play around with EL wire for a project. Both of those require much
    higher voltages.

    Basically, I'm self-taught here, and would like some guidance on how
    best to learn when/why to use various methods of power supply, and how
    to design/build those.

    Thanks again for all the advice,
    Daniel.
     
  6. Rich Webb

    Rich Webb Guest

    Generally (one expects that there may be exceptions) all you would need
    to do is to check the unloaded terminal voltage. If it's (pulls random
    wall-wart out of drawer) labeled "output 6 VDC 100 mA" and the no-load
    voltage is 8 V then it's not really regulated. At its rated load it
    should drop the extra 2 V internally (i.e., its output impedance is
    about 20 ohms) so whatever it was intended to be plugged into would get
    close to 6 V.

    What else is handy... a "7.5 V 1000 mA" that measures 9.5 V unloaded and
    a "12 V 400 mA" that's a spanking 18.5 V. There's also a "3.6 V 1 A"
    that's reading 3.4 V. The last one is noticeably smaller and lighter
    than the traditional transformer + bridge style. It's probably a tad low
    since it isn't seeing its recommended minimum load (prob 100 mA or so).

    A random 7 to 9 V wall wart paired with a simple 7805 linear regulator
    on your breadboard might be a better start. You'll start to become
    acquainted with filter caps and (possibly) heat-sinks but also another
    step towards a DIY project.
    Wouldn't hurt to pick up a copy of "The Art of Electronics" and its
    companion workbook but you could start with some of the manufacturer
    application notes at, say, National and TI (now joined at the hip) or
    even the Wikipedia entry on voltage regulators.
     
  7. Guest

    Several families of CMOS will work on wide voltage ranges, as well. I like to
    use these, when possible, because having one P/N simplifies BOMs. Brighness
    variation can be reduced significantly by driving LEDs with a current source,
    at the cost of some complexity. I've had to do whenever driving LEDs directly
    from a battery.
    Boost regulators will cost power, twice (higher voltage for the ballast
    resistor to dump and the inefficiency of the boost regulator). It's better to
    use a current source, if "constant" brightness is needed.
     
  8. Guest

    I can believe that but even at 90% efficiency, you're throwing away >10% of
    your batteries' capacity *and* (Vboost-Vbatt)*Iled.
    It certainly is. You're both boosting to a higher voltage *AND* dissipating
    more in the ballast resistor because of it. To light an LED, current is the
    only thing that matters. You're throwing away everything above battery
    voltage, plus the 11% of the total for the boost regulator. The current
    source is nothing more than a voltage variable resistor so it dissipates
    *less* power than a resistor at high voltage (assuming the current is set to
    light the LED at the minimum voltage).
    Different discussion. Wall warts are even simpler.
     
  9. Daniel Pitts

    Daniel Pitts Guest

    Actually, I'm planning on using the TLC5916 from TI, which is a
    constant-current sink 8bit shift register, for exactly this purpose.
    That way brightness will be consistent.
     
  10. Guest

    Geez. With the current source the current *does* its thing, without burning
    the extra power.
    ..6V overhead is all that's needed.
    What LED? We used blues down to 3V and they were plenty bright to be seen
    outside during full sunlight (the product is used outside).
    Nonsense. It works. BJT + FET
    Simply wrong, as shown.
     
  11. Guest

    ....and you just said it couldn't be done.
     
  12. Guest

    But it's Maxim. Good luck actually getting them.
     
  13. JW

    JW Guest

  14. Guest

    This week.
     
  15. Guest

    You don't need 3.6V unless you want to put your eye out.
    See above.
    You CANNOT gain efficiency by boosting the voltage and then pissing it away in
    a ballast resistor. A current source is simply a variable resistor.
    No, because it's *NOT* boosted to your 5V.
    No.
     
  16. Guest

    A 3.6V LED is only 3.6V at its rated current. Unless you're lighting a room,
    it's *not* needed. It'll be quite visible at 1/4 (or 1/10) of its rated
    current. At 1/4, we found that blues were quite visible in full sunlight.
    Running them "hotter" is nuts.
    We now know you're an idiot.
    Utter nonsense.
    You're still paying 11% for the boost.
     
  17. Daniel Pitts

    Daniel Pitts Guest

    I've already got my TL5916 (I'm using a 74HC238 to multiplex). Also, if
    I recall, the MAX7219 was much more expensive for what I needed.

    I got my TL5916 for 88 cents, and the 74HC238 for 25 cents. Where the
    MAX7219 is over $10 from the same supplier ($13 if I wanted DIPS, which
    I do for now).

    So to power 192 LEDs (8x8xRGB) I could spend spend over $30 using
    MAX7219, or I could spend just 3*.88+.25=$2.89. I think a 90% cheaper
    solution is the better solution, especially for a hobby project.
     
  18. Guest

    You don't want to see the LED? Read the fucking thread before you make more
    of an ass of yourself.
    You really are trying to be right at all costs, even by being wrong. Are you
    really on Obama's campaign team?
    Of course you would. You've already demonstrated that you're clueless.
    If you're going to all that trouble, just use a damned wall wart and be done
    with it.
     
  19. Jon Kirwan

    Jon Kirwan Guest

    I've got some very nice, 16x16 RGB arrays. Each already has 6
    ICs for driving all that. Separate supplies for each color,
    turn-pot resistors for adjusting white balance by setting the
    100% current source levels, and pwm capability within that
    range from 0/256 to 255/256 of the 100% current level for
    each color of each RGB LED. Used mostly to make outdoor TV
    sets, driven by NTSC or some other TV broadcast code.

    These bricks have huge heat sinks behind each one to
    dissipate the power. Be aware of the potentials for that
    aspect when you start talking about hundreds of LEDs.

    Jon
     
  20. Guest

    READ THE THREAD, MORON.
    I bet you could understand a simple thread if you really worked at it.
    Look in the fucking mirror, moron.
     
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