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Datasheets and pin ordering

Discussion in 'Electronic Basics' started by Abstract Dissonance, Feb 9, 2006.

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  1. I just blew a adj regulator cause in the data sheet the pins are orders
    reading left to right 1 3 2 ;/ WTF is that?

    1 = In, 3 = GND, 2 = Out..

    Why not just enumerate them like everyone learns in elementary school as 1,
    2, 3,

    1 = In, 2 = GND and 3 = Out?

    Another issue is how to know which one is the first pin? On the same data
    sheet as above they show the "top down" view but that could mean 1, 2, 3 or
    3, 2, 1 considering what you call top and bottom... they do say "bottom"
    next to it though. Does this mean the bottom is the "side" with the leads
    comming out?

    Another sheet completely omits the "top" or "bottom" reference and I guess
    leaves it up to you to guess? (this is from a different regulator and the
    pins in and out pins are actually flipped as to the first regulator(unless I
    had the first one wrong but seemed to work in that it adjed the voltage
    approximately correct(except it went below the 1.25V limit down to .9V for
    some reason ;/)

    Why can't they just make it plain and simple and label the pins on the
    "front" view(which is obvious cause you put the text on it)?

  2. heres the two sheets I'm talking about:

    These look ok

    And this one looks the best(least ambiguous)

    Is there any place I can get good datasheets instead of relying on luck to
    get the pins right? Whats the reason for this crap? Laziness, stupidity, or
    they really want you to screw up some components to get it right so they can
    make more money?

    The motorola doc is the only one that seems like its any good. It seems the
    graphic design engineers don't realize that pin order is important?

  3. Guest

    A trip to the optician would seem in order, or maybe your monitor is no
    good, or your just plain careless. Just as well its just a cheap
  4. PeteS

    PeteS Guest

    I really don't see your issue - what regulator are you using?

    For instance, this is a standard datasheet from On Semi (nee Motorola

    It's perfectly clear for physical pin order. Perhaps you are confusing
    schematic pin order and physical pin order.

    The two need have no relationship to each other except the function of
    a pin number is the same.

    Standard regulator datasheet schematics usually show input on the left,
    adjust or gnd in the centre and output on the right. If the pins
    assigned happen to be (as with the LM317T)
    Input = 3, Adj = 1, Output = 2, then the schematic would show (left to
    right) 3,1,2 although the physical graphic shows clearly the pin
    numbering and positions.


  5. ? I have the two data sheets?
    yes, and its perfectly obvious what the pin order is... is it perfectly
    obvious to you what the pin order is on the two data sheets I've shown?
    yes, but at the time I was wiring up the regulator I didn't expect the pin
    order to be randomly determined...

    Maybe there is a reason why you would order the pins(which is what you are
    doing when you label them) out of order but I don't see it.

    Sure its my fault for not noticing that it was 1 *3* 2 But seems kinda
    stupid to me why you would do something like that when there is no need. I
    suppose it came after the fact though when they drew the graphic and
    probably had to have it match up with the schematic that was lableled that

    Thats not the real issue... its just strange to me that it would be done
    like that for no reason that I can see as it just makes it less intuitive.
    It would be like when counting a group of 10 objects to start 8, 3, 5, 2, 9,
    4, 0, 1, 7, 6... maybe there is good reason to do it but if your just
    counting them surely you would do it in the "logical" order.

    The main thing that gets me, which both you and bozo fail to see is that

    On Page one in the OUtline Dimensions box

    orders the pins 1,2,3 like one expects but that it does it using a 2D
    perspective from the "top" or "bottom"... this is completely ambigously as
    the top looks exactly like the bottom except for the pins. Its entirely
    possible that they mean the bottom since it shows the pins and if it were
    the top they would not show then... but this is somewhat ambigous to someone
    what does know what the bottom or top is(as if you get it wrong you flip the
    pin order from 1,2,3 to 3,2,1 and obviously you'll screw something up).

    It could be me though... my issue is specifically with that pdf S78LXX and
    not with the ones you have given as it is obvious. Maybe its clear to you
    but if so then why. I'm not asking what the real pin order is for the part
    but how to determine the pin order given that specific datasheet(a big
    difference). If you can do it unambiguously then there is probably something
    I'm missing... if you can't then theres a problem with that
    datasheet(atleast probably).

  6. hmm... maybe you would be right but since you don't seem to offer any reason
    to why I am wrong makes me think that you are just BS'ing.

    If you are going to say something like that then tell me why, on the 2
    sheets I gave, where it tells you the pin order unambiguously else I have to
    believe you

    First page under Outline Dimensions...

    given that datasheet determine the pin order unambiguously. If you can't
    then maybe there is something wrong with your eyes? If you assume the
    graphic i the lower left corner is looking at the bottom then I feel that is
    a big assumption(atleast for someone that is not used to looking at these
    types of graphics).

    Maybe there is some standard think similar to how DIP's have the little
    circle or cutout in the corner to determine the first pin but I don't see
    that here.

    Anyways... like I said.. if you are going to say that shit then back it up
    with some factual evidence instead of just trying to say something personal
    else I gotta take your argument as crap.

    (and use the damn datasheet for the S78LXX that I gave and not some other
    one where its obvious. My problem is not with determing the pin order but
    with determing the pin order from that specific datasheet.).

    I eagerly wait your response,
  7. oh, nevermind... sorry... didn't know you were from aol.
  8. Deefoo

    Deefoo Guest

    I agree with you that datasheets do not always show clear pin-outs,
    escpecially with discrete devices, but in your case I think it is quite
    clear. As you say yourself, since they show the pins you should look at the
    side where the pins are. What is top or bottom is completely irrelevant. And
    in this case, since they don't even mention top or bottom, they do not
    create any confusion, so it is actually a very clear datasheet.

  9. huh?

    If you look from the bottom and read the pins from left to right then you
    have, for example, In Gnd Out but if you read from the top then you have Out
    Gnd In(reading from left to right again).

    If you write the word Apple on a transparent piece of papper then it does
    matter which side you read from... One side makes perfect sense and the
    other doesn't.

    The hole point rests on if the pins shown are on the bottom or top. If it is
    a "solid" in that they don't show the the outline(which is says outline
    dimensions) then ofcourse thats the bottom...

    but if "faces" are transparent then you can't tell what is bottom or top by
    that graphic. Now it maybe obvious that all faces are considered
    non-transparent but I didn't know this. I'm sure that datasheets are not
    written geared towards the amateur so that could be my problem... but simply
    showing an isometrix perspective solves that problem(which some data sheets
    seem to do and its completely obvious what is what(I suppose in some cases
    it could be confusing if the packaging is completely symmetric).

    Anyways, I just feel they could have done a little better job. Now that I
    know the pins are shown cause its *suppose* to represent the bottom then its
    not a big deal. It just seems kinda stupid that looking at it one has to(if
    they are not familiar with the "rules") have to "guess"(i.e., assume
    something that might not be true). There are ways to be completely
    unambiguous and thats by including an isometric perspective. I guess I
    shoudln't bitch though. (not that I'm really bitching but just trying to
    find out whats going on and to make sure.)

    Actually I took a course in graphic engineering many years ago so I should
    probably not have had any problem. Although I do remember using dashed
    lines for elements that were "behind".

  10. Deefoo

    Deefoo Guest

    From a mechanical point of view these drawings are completely clear. They
    show the device from all its important sides by simply rotating it. There is
    no reason whatsoever to assume that the device is transparent, especially
    since no mention is made of top or bottom or whatever.

  11. Chris

    Chris Guest

    Hi, Jon. Sorry, but Ich Bin Ein AOLer, too. Teenage daughters, you
    know. They'd just die without AOL. :-(

    A couple of things come to mind here. First, the two data sheets
    you're talking about _do_ happen to be perfectly correct, even if the
    first isn't as clear as possible.

    The first says "BOTTOM VIEW" in caps right next to the drawing of the
    part. It's customary when only giving one view on a drawing to specify
    the view, which they have done. The business of "1, 3, 2" is a bit of
    a "gotcha", and it's definitely not as clear as, say, the National
    Semiconductor datasheet:

    where they actually have arrows going from pin descriptions to pins on
    the bottom view.

    The second S78LXX data sheet is perfect. They give the standard three
    regular views, number the pins on the bottom view, and give the pin
    numbers on the functional schematic. I would suppose you might not be
    too familiar with technical drawing. That's not really critical, but
    you might want to just wander through a high school technical drawing
    book when you get a chance, and get up to speed. The pinout is just
    bass-ackwards from the TO-220 pinout you're familiar with.

    Look. I've smoked both the TO-92 78LXX (plugged it in backwards) as
    well as an LM317CT (circuit layout IN-ADJ-OUT-BOOM). When I did that
    with the LM317, though, they were going for about 10 times as much in
    today's dollars.

    The problem with the drawings is that they aren't what you expect.
    That can be a problem with datasheets. Very rarely, you'll find an
    error. Sometimes, you'll find something that isn't very clear. That's
    more common.

    But you need some perspective here. You're a hobbyist, trying to get
    up to speed on some pretty basic stuff. The semiconductor
    manufacturers do know what they're doing. If they didn't, they'd be
    out of business.

    Even as a technician or engineer, the rule of thumb is to look for your
    mistake first -- almost always, you'll save a lot of time that way.
    And you really have to look at each datasheet as an opportunity to
    learn. Clear your mind, get a cup of coffee if it helps, and take the
    time to really read and understand the information there. It will be
    well worth the time. Then, if something isn't clear, see if the part
    is second-sourced, and try to get another datasheet. If it's single
    source, send an email to the app engineers at the manufacturer. They
    do have people who are paid to help. Just neglect to mention that it's
    a hobbyist one-off, and make sure to read the datasheet first to avoid
    looking foolish.

    If it helps, I've found there's a slight correlation between the length
    of a .pdf file and the clarity of the datasheet (not including those
    which have been scanned). And I find National Semiconductor datasheets
    are generally (not always) put together with more care than most.
    Either that, or I'm more accustomed to their way of describing things.
    But I'll look there first if I can.

    What I'd like to know, though, is what you're doing playing with TO-92
    packages with your power supply, anyway. They're small, their thermal
    resistance is around 160 to 180 degrees C per watt, and they aren't
    really made for attaching a good heat sink. You're cranking current,
    and you're going to need TO-220 or TO-3 packages here.

    Let's assume your 25.2VCT transformer has an unregulated 34V, and
    you're using your LM317 to put out 3.3V at .5A that means your power
    dissipation will be

    (34V - 3.3V) * .5A = 15.35 watts
    [Note: Power in a pass element = volts dropped * amps passed]

    Your little TO-92 package will glow like a nite lite. Isn't going to

    If I could offer you a little free advice. Go to Surplus Sales of

    and take a look at their Triple Deltron Power Supply (PS) 10656X, that
    has two 12 - 16 vdc outputs @ 2.5 amps each and a 5 vdc output @ 8
    amps. It costs $35 in single quantities. The two 12 - 16 vdc supplies
    are adjustable, and all the power supplies have independent current
    limit adjusts on board. If you mount this supply under your bench, you
    can be set up with a mongo +5V and a +/-12V supply (as well as a 24V
    supply for the industrial stuff, if you go from the +12 to the -12),
    which should get you 80% of the way toward where you want to go. Just
    use a 5 amp 3AG fuse (not slo-blo) in series, and mount that with an
    industrial power switch in a jiffy box.

    If you're dying for a variable supply too, get a 28VDC @ 1A linear open
    frame ((PS) IHB28-1) for $25, set the on-board current limit for 1/2A,
    and place an LM317CK in series to give you your 1.25V-to-24VDC variable
    supply. Don't forget to put 10uF caps at the input and output of the
    regulator. And also, get a good heat sink capable of dissipating more
    than 15 watts without cooking the LM317CK IC.

    Linear open frames are typically based on a standard LM723 circuit (not
    shown in datasheets -- from what I understand, someone tried to
    copyright the circuit a long time ago, without success). They have
    very low ripple, great line and load regulation, and they're pretty
    much indestructible, as long as you keep them fairly cool. It might
    also be a good idea to run a couple of small muffin fans on the open
    frame linears to help with heat dissipation. They also have remote
    sensing, which means they can compensate for line drops to the load.

    If you want, you can also put 0.1 ohm 1% resistors in line with each
    V+out, if you put the V+sense on the other side of the resistor. You
    can then use your DMM and a 4PDT break-before-make switch to read
    current exactly with an autoranging DMM on whichever supply you choose
    (100mV/amp, to the resolution of the meter you're using).

    You'll have the electronics for the whole shebang for less than $75.
    That way, you can spend your electronics time actually learning,
    instead of preparing to learn. A good bench power supply is a
    necessity for electronics, so you might as well just get something good
    on the bench right now, and play when you have the toy. You're walking
    into getting stuck in a chicken and egg problem.

    Good luck
  12. Heh, its obvious to you ;) Its not 100% obvious to me and leaves something
    like 3% ambiguity(being resonable)... while, say, the isometric view leaves
    0% ambiguity. Maybe not a big difference in your opinion but is in mine(not
    really but).

    Although you cannot be at 0% since on another datasheet they did label the
    view as bottom. If it was not ambiguous to the person who wrote that they
    would not have done it. Obviously they must have thought to themselfs(even
    if for a nanosecond) that they should write it(hence there was some
    ambiguity). That or they could have just did it for fun. What if they wrote
    top instead? Would there still be no confusion?

    My point is that it might make complete sense to you but thats simply
    because you probably have more experience dealing with them... take some avg
    joe's and ask them and I'm sure you won't get 100% positive results. This
    leads me to the conlusion that they should show the isometric view as
    virtually unambiguous. Maybe there are overiding reasons not to do this but
    thats a different story. I was mainly interested in how to interpret it and
    since I don't really doubt you I will assume you are right(obviously you are
    since I didn't destroy my regulator(which I actually tested before I posted
    anyways)). Just trying to be absolutely clear on whats going on so if I get
    another datasheet and it does the same thing I can be confident that I won't
    screw up my component.

  13. I looked at your datasheets and I do agree that the 132 numbering thing
    is a bit odd, but there is nothing unclear about the pictures. Standard
    drafting (or is it draughting) technique is to draw non-hidden features
    with solid lines and hidden features with dashed/broken lines. Since
    the leads don't protrude thru the top of the part, it should now be
    clear to you that the bottom was pictured. The bottom is the side that
    would normally be placed against a circuit board. The top would be the
    portion that would normally be visible when the part is mounted to a
  14. Well, I'm in complete agreement. You've the benefit of looking with fresh
    eyes and can see the laziness and crass stupidity that turns up too often in
    these things.
    All they really need is a labelled perspective drawing but that'd then be
    regarded as non-technical, hence passe.
  15. well its obvious now but it wasn't obvious at 3AM ;) The datasheet should
    take those types of considerations into account. Specially the 1 3 2. I
    swear I first saw 1 2 3.

  16. I'm glad someone agree's with me ;) Its not that Theres anything
    technically wrong with what they did but seems that they could make it
    alittle easier on some of us. Its good to see some of these datasheets
    using the perspective view so it's very easy to get it at a glance so one
    can get on with the real work(instead of trying to make sure the the pin
    ordering is correct).

  17. ehsjr

    ehsjr Guest

    It is unambiguous. Each drawing is rotated 90 degrees. The
    vertical diagram, when rotated 90 degrees to the horizontal,
    must end up with its flat side down, or its flat side up.
    Picture it: to rotate from the vertical position shown
    to horizontal, how could it end up with the pins facing away
    from you *and* lying on its round side as shown? Impossible
    with only a 90 degree rotation.

    Logically, if you don't know about engineering drawing,
    how could you see the pins if they were facing away from you?

    I suppose it's something like a doctor reading an xray:
    he or she would see things on it that are obvious that I
    would miss. Or, if someone posted the engineering drawing
    of a nuclear submarine or a pocket watch or whatever, you
    wouldn't be able to read it unless you were trained/experienced
    in that field.


    If you can't
  18. ehsjr

    ehsjr Guest

    Oh hell - talk to us after you've correctly identified
    the pin 1 end of an IC, and correctly identified where
    it goes on the board, and then plugged it in backwards.
    Three times in a row.

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