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Electrical Engineering Trivia Thread

Discussion in 'General' started by Prince Charon, Apr 9, 2019.

  1. Prince Charon

    Prince Charon Just zis guy, you know?

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    There was an odd derail on the Sanctimonious Assholery on QQ thread, relating to electrical engineering. Clearly, this thread is needed to prevent such derails.
     
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  2. vyor

    vyor Oh that's cute

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    I ARRIVE
     
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  3. NuclearBird

    NuclearBird Herald of Emissions

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    Let me start this off in the wake of a disastrous Energistics midterm test:

    Never forget the difference between phase voltage and line voltage.
     
  4. vyor

    vyor Oh that's cute

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    Ouch.

    For that matter, when looking at VRMs and transformers, do not assume wattage in == wattage out, you will lose energy to heat and if you don't, you're probably doing something wrong(or you're magic).
     
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  5. NuclearBird

    NuclearBird Herald of Emissions

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    Don't forget the Z=R+jX impedance and the wnding layout, including grounding of different types.

    Usually the resistance is negligible at nominal conditions, leaving jX as a reactance on the line.
    Winding type (Star-Star, Delta-Delta. Star-Delta) determines behaviour when the three-phase system isn't fully symmetrical (usually due to a short circuit). There is a mtehod to solve this issue, especially when transformers are involved, but it would take too long to explain.
    Expanding on the previous point: grounded star windings are great when dealing with high voltage (120kV) systems.
    Get used to dividing and multiplying by sqrt(3).
    There is capacitance on everything, unless it's declared negligible.

    Now for something outside of Energistics, yet extremely important for Electrical Engineering as a whole:
    You solve for

    z = exp(sT)

    where T is the Sampling Period. Reducing it to a logarithm might sound appealing, but some analysis shows that once you start testing for equivalencies in stability criterions you find that things diverge wildly. Instead, when you take

    z=exp(sT/2)/exp(-sT/2) ~ (1+sT/2)/(1-sT/2)

    and "approximate" the numerator and denominator in the above way, then test each side for how stability on one side shows on the other.
    You find that no matter which side you start on, once you start with a stable s=a+jß on the continuous side where a<0 (Real component is negative) or a z=r*exp(j*Th) where r<1 (magnitide less than one) you get a result on the other side of the equation that indicates stability and vice versa.

    Testing for z when s->0+ (s approaches 0 from a higher value) you get a convergence to +1 from a higher value. This is for cases where Re{s}>0
    Testing for z when s->0- (s approaches 0 from a lower value) you get a convergence to +1 from a lower value. This is for cases where Re{s}<0

    Why is this important? You see, testing a system for stability in discrete and continuous complex frequency domain is easy: Determine the poles of the system, the roots of the denominator polynomial.

    For continuous time systems, f(t) -> F(s), then test for every single pole (root). If real component Re{pi}<0 for every i=1...n where n is the order of the polynomial, then the system is surely stable.

    For discrete time systems, f[k] -> F(z), then test for every single pole (root). If the magnitude of pi, |pi|<1 for every i=1...n where n is the order of the polynomial, then the system is surely stable.
    Despite the fractional polynomial being a mere approximation the equivalence holds perfectly outside of (1,0) on the complex plane, despite having used a fairly crude approximation of an exponential function.
     
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  6. molemole

    molemole Versed in the lewd.

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    For those wanting to do DSP, one word: Don't. Outside defense, there are rather few jobs in pure DSP, and most image processing and speech is run by deep learning guys with CS degrees.

    If you do do DSP, then by God read Shannon's theorem and the original derivation. If you know that well, and you know linear algebra decently well then 80% of DSP is pretty much just reading and minor understanding.
    All stuff I wish I knew before leaving with a EE undergrad, hopefully someone can make use.
     
  7. vyor

    vyor Oh that's cute

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    The only real modern jobs in DPS are the designers for the damn things, and that ain't a big field.
     
  8. molemole

    molemole Versed in the lewd.

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    More hardware than anything else these days. The issue is that outside of a few firms like Microsoft and MERL, most Silicon Valley companies don't believe in using traditional signal processing for data conditioning. And most DSP for telecoms has been commodified, so the big market for DSP guys is more hardware/VLSI design than algorithm design and 'pure' DSP.
    That will change once the system realizes that asking a CS graduate with no knowledge of sampling to code speech processing systems is suboptimal, but that'll take time. Until then, not much present.

    DSP is damn near everywhere, at least in the mathematical sense. Telecoms? Check. Information retrieval? Check. Image systems? Check. Robotics and localization/SLAM? Check (mostly to check data integrity). Hardware? Of course, most specialist chips are built to run things like the Viterbi algorithm. Speech processing? Hell yes, but there's little market. Etc, etc, but it's become commiditized enough that one doesn't need a DSP engineer when one can hire a CS generalist, use some libraries and get more utility out of the guy. Sure, the marginal gains in performance from the DSP specialist will be gone, but who cares?
     
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  9. NuclearBirb

    NuclearBirb A mysterious birb.

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    I'm always upset that tesla got fucked over and we lost wireless electricity and AC usable in everything
     
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  10. vyor

    vyor Oh that's cute

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    You can't use AC for some things, like using it with lightbulbs is how you get flickering and harsh headaches.
     
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  11. LordVile

    LordVile Not too sore, are you?

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    What?
     
  12. NuclearBird

    NuclearBird Herald of Emissions

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    First. You can technically use a high power radio transmitter to beam energy, but it would be a system less efficient than most governments today.

    Second. Flickering as a disturbance is mostly confined to 5-8Hz IIRC, and most outlets provide power at 50 or 60Hz.
     
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  13. vyor

    vyor Oh that's cute

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    Not exactly true. You notice it at 5-8hz fairly easily, but you know how fluorescent tubes can cause headaches? That's why. A very high frequency flicker.


    I know my stuuuuuf
     
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  14. NuclearBird

    NuclearBird Herald of Emissions

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    Huh. They didn't mention that in the energistics curriculum, just the 5-8Hz bit.
     
  15. vyor

    vyor Oh that's cute

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    Well, it's only LEDs and Fluorescent tubes that get the flickering; incandescent bulbs don't see the same effect. But ya.
     
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  16. molemole

    molemole Versed in the lewd.

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    Also for the wireless transfer bit: Somewhat niche, but power control systems often 'dump' reactive power along the way at stepdown centers, at least in India. For long-range HVDC links for instance energy gets siphoned off for power factor improvement and control before conversion (usually with condensers of some sort, or a reactive load). Which cannot be done with wireless transfer unless at great expense, and so long-range power transmission is supremely inefficient.
     
  17. NuclearBird

    NuclearBird Herald of Emissions

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    On the matter of dumping reactive power: They only taught us that phi adjustments can be made by injecting capacitive current into the power line. Never heard it described as dumping power.

    As for wireless transmission having difficulty with reactive power: You could probably alleviate the issue by matching the impedance of the converters on the sending/receiving end, the latter because some reactive power is needed for nominal operation of certain devices, at least on an industrial scale.
     
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  18. molemole

    molemole Versed in the lewd.

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    At a very crude level, transformers are a consumer of power and a reactive load (inductive) in and of themselves (thanks to coil imperfections). Balancing that with capacitive loads makes for power factor improvement, albeit a slapdash approach that hasn't been in use for a while outside small microgrids in villages.
     
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  19. NuclearBirb

    NuclearBirb A mysterious birb.

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    I was in conspiracy theory heals documentary guys, not in modern electrical engineering. My knowledge in that boils down to grounding exists, lightning goes up, and if you touch too much you die
     
  20. Prince Charon

    Prince Charon Just zis guy, you know?

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    On the other hand, devices to convert AC to DC are not expensive, and could be attached to a receiver, or a lamp.
     
  21. vyor

    vyor Oh that's cute

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    I mean, ya but still.

    Lightning usually goes down.
     
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  22. Prince Charon

    Prince Charon Just zis guy, you know?

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  23. NuclearBird

    NuclearBird Herald of Emissions

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    The direction of lightning mostl depends on the distribution of charge carriers between the clouds and the ground, but the info shared above is accurate as far as I can tell.

    As for AC to DC converters (it's too early to make that joke), there remains the problem of phase shifitng and warping of the current under load due to the nonlinear characteristics of diodes and controlled transistors, especially in switching mode. A transformer stage could alleviate this, but then you'll have to match that to the power supply. This mostly concerns industrial applications, where three phase machines are used, along with the requisite three phase rectifiers.

    Of course, you don't actually need to do all that much if you're feeling cheap and lazy. Just put a split capacitive load between the two input wires and ground the middle. Should smooth out the warped AC current.
     
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  24. vyor

    vyor Oh that's cute

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    To a point anyway, you still get some impressive output ripple.
     
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  25. NuclearBird

    NuclearBird Herald of Emissions

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    No shit. For most applications it remains acceptable, though. If you're really concerned about ripple, you could use an LC filter to suppress it.
     
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  26. LordVile

    LordVile Not too sore, are you?

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    A device which works in Watts does not have the same sensitivity to ripple as most chips.

    Also, is their any way for getting proper power transformers with all the taps for different voltages instead of ordering a custom one?
    I tried to build a current limited +/-15v power supply for recharging batteries and lab bench stuff.
    But damn near no body sells single transformers above 12v.
    I will need to tear down a washing machine at this rate.
     
  27. vyor

    vyor Oh that's cute

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    wut

    No, just... no.
     
  28. NuclearBird

    NuclearBird Herald of Emissions

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    British scientists are hard at work in their task of decyphering whatever you just said.

    Did you mean chips that consume power in the range of 1xN Watts?
     
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  29. LordVile

    LordVile Not too sore, are you?

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    Yes, exactly.
    An LED already has a regulated current source if you use DC, low frequency power ripple should be compensated for by the driver.
    It's not as if it's logic ICs or communication equipment.
    we know he frequency of the ripple so putting a bypass capacitor to create a LC filter at the frequency should give us 20dB/decade especially if the transformer inductance is taken into consideration.
     
  30. molemole

    molemole Versed in the lewd.

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    Less EE trivia and more EE-application-trivia, I have heard that UMich Ann Arbor has hired several new faculty for computer vision/image processing. Funding has been granted from the Big Three for autonomous vehicles. For those who want to do a PhD in the area, it might be worth it to apply.
     
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