amateur EE confusions and questions

Discussion in 'General Electronics Chat' started by fez, Jul 18, 2010.

  1. fez

    Thread Starter Member

    Dec 6, 2009
    47
    1
    Hello, I'm new here, and hopefully I'll stay for a while. I'm going into my junior year at college, EE major. So far I've studied basic circuit theory, basic digital logic and a signals & systems course, along with some labs.

    I'm hoping to set up my my own workshop, I haven't had a lot of hands-on work. But I have some amateur questions I'd like answered. They probably have very simple answers but they are bothering me.

    1. During the labs I've come across ICs with similar, but not the same, names. For instance, the UA741 op amp and the LM741 op amp. Is the number "741" supposed to give any information? If they are the same op amps then why do they have the different codes UA and LM, what do these codes mean?

    2. Apparently there are lots of different op amps out there. How does one find just the right op amp one needs for a situation? Or is it just a matter of experience, i.e. learn to use different op amps, remember their attributes and then pick out the appropriate ones by memory when you build a project?

    3. Batteries... What exactly is the potential, relative to the earth ground, of, say, a 9V battery's -ive terminal? A battery supplies an emf, it produces a voltage across it but I never know what potential, relative to the earth ground, its terminals are at.

    Thanks a lot!
     
  2. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    LM is used by National Semiconductor (NSC) for many of their analog ICs.
    So the LM741 is the NSC version.
    uA would be used by different people I'm not sure who started it but Texas Instruments make a uA741. Most TI op amps start with OPA

    Some companies use the same prefixes because they feel like it I guess.
    Many chips, especially old ones like the 741, are made by many manufacturers.

    You pick op amps based on specifications you need to decide. Your application will require a gain, a bandwidth, an offset, a linearity, output swing capability, current source/sink ability, package size, temperature range, pinout, input impedance, amount of noise, parameter drift, power consumption.... some or all may be important. You just need to look at the parameters that matter to you and you will generally narrow down the options to a handful of chips.
    Manufacturers now offer selection tables where you filter based on parameter ranges you specify. For example:
    http://www.national.com/cat/index.cgi?i=i//10

    A battery that isn't connect to earth can have any potential with respect to earth. If it's not connected there is no relationship. One of the reasons for grounding household wiring is to keep the AC outlet within that 120V rms of the earth. It could be 120 Vrms between the prongs yet a few hundred volts above earth especially if there was some kind of a fault somewhere.
    A voltage source only defines the voltage between its terminals, it tells you nothing of its absolute potential in the environment.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    I will try to answer the questions.

    1) The 741 is the base number giving the type of opamp.
    The UA and LM are prefixes, different manufacturers can have different prefixes.

    2) About opamps, there are a lot of parameters needed to determine what opamp to use.
    There are the powersupply range, if the opamp has rail to rail capability, the input impedance , the output current, the bandwidht.
    I have attached a PDF with a lot of information on it.
    I also attached the "opamps for everyone" PDF.

    3) battery circuits are floating from the earth. For the simplicity the - connection of the battery is concidered as the gound connection.

    Bertus
     
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    Not that its important or anything, but National's "LM" stands for Linear Monolithic. I don't know if they still use it or not but "DM" was Digital Monolithic. I knew a bunch more but that was 30 or 40 years ago.
     
    sage.radachowsky likes this.
  5. fez

    Thread Starter Member

    Dec 6, 2009
    47
    1
    I see. Thank you very much! I would ask more but I need to catch on the relevant information first. I'll be back!

    EDIT: come to think of it, I do have some further question for right now!

    4. We've all used AA batteries. If isolated batteries have floating potentials are their terminals (though of course fixed in between them) how come we never get electric shocks when touching the terminals. If my finger touches the +ive terminal, which for all I know may be at, say, 378.312 V, we have a p.d. of 378.312 V between the terminal and the earth, and my body is connecting the two! Why do I not receive a shock?

    5. I just checked out the national semiconductor's web page (the one provided by Ghar above). There's a whole variety of products there, with lots of divisions and groupings. Is there any book or webpage I can be guided to which summarizes all EE products, old to modern, along with their various types and categories? For instance, I have an idea (but no exact definition) of what a "high precision op amp" and a "micropower op amp" may be, but no idea for what a "rail to rail" op amp is, or what "fully differential" amplifiers/op-amps are. And what the heck is a "general purpose" op amp anyway?

    6. So HOW do different manufacturers produce the same product? For instance, how do NSC and TI produce the same op amp, the 741? Is the 741 from both exactly the same, or are there differences of any kind?
     
    Last edited: Jul 19, 2010
  6. sage.radachowsky

    Member

    May 11, 2010
    241
    38
    That is really interesting to know, for historical reasons.

    I am curious why Linear sometimes calls their chips LTCxxxx and sometimes LTxxxx.
     
  7. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    The voltage differences that let you feel a shock are in the hundreds of volts. Painful shocks are in the thousands.

    Insulators don't conduct very well but they do conduct. Over time charge does bleed off and things equalize. Humid air is more conductive than dry air, which is why you get less static shocks with high humidity situations. Static safe areas recommend a humidity between 50% and 80% or something similar to that, the 80% limit coming from corrosion and condensation reasons.

    The free space capacitance (capacitance to the environment) of something like a door knob is in the picofarad range, say 10 pF. The approximation is you get about 100 pF per meter of radius of an object.
    A value for the resistivity of air I found on Wiki would put it at 3x10^14 Ohm-meters, so let's just say you get a resistance on the order of 10^14 ohms to something conducting in the area.

    The time constant of such a system is about 3000 seconds or about an hour so it would naturally discharge completely in about a day if you left it alone. I just got a good idea for an experiment :)
    Totally rough calculation with a ton of assumptions but it's an idea.

    Going back to pain there is very little stored energy in a 10 pF capacitor except at very high voltages. The shock is over without you noticing it most of the time.

    Anyway, what can actually raise the potential of something above Earth? You need to induce charge on the thing which doesn't happen all that often and things are rarely entirely isolated electrically. Anything with moisture won't be an amazing insulator.

    A lot of these are marketing terms without strict definitions.
    "High precision" might mean very low offsets and input current with great impedance and gain.
    "Micropower" means very low power consumption, all op-amps consume power in their biasing.
    Rail to rail means the output can swing from almost the negative supply to almost the positive supply. Something like the 741 can only go within 2V or so from either (so +/-5V supplies gives you +/-3V output) and there's a similar constraint on input voltage. Some are rail to rail output only while others have both rail to rail input and output.
    You'll figure out the terms by just browsing the product line and getting experience with practical electronics.
     
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