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Power Sources in Series/Parallel ?

  1. Mar 16, 2005 #1
    Power Sources in Series/Parallel....?

    hey, i've got a question. say you had one power source thats about 4v and another that's about 3v. if you hooked them up in parallel the following power source would be (now im' pretty sure im doing this right..if not..well im not an electrical engineer, which is why im coming here for such a basic question) about 7v, but what would the amps be? because i know that in parallel the voltages add up (and for loads it isn't a continuous voltage drop, it depends on resistance right?). Same for series, what would teh voltage be (and amps) if you hooked it up in series, since in series (i'm pretty sure) the voltage drop/gain is constant around the circuit.

    now for all i know this may not even be possible...so any help on this at all would be helpful.
     
  2. jcsd
  3. Mar 16, 2005 #2
    If you hook them up in series you get seven volts. If you hook them
    up in parallel, the universe would explode because the 4 volt
    supply would dump infinite power into the 3 volt supply.

    "Never cross the streams"-
    Ghostbusters
     
  4. Mar 16, 2005 #3

    Cliff_J

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    Ok, in series the voltages would add, in parallel the current will add. Make sure you keep the terms straight, its an important distinction between the two. Parallel is like how a home is wired, one bulb burns out and the others are fine, series is where everything is connected to the next peice in a row and if one bulb burns out like in a cheap christmas light all the lights in series with it go out too because no current will flow because the circuit is no longer complete.

    Lets say you have two 1.5V AA batteries that can deliver 1A of current.

    In series they can deliver 3V and 1A of current.

    In parallel they can deliver only 1.5V but 2A of current.

    If you have a 3V and a 4V source, in series they will have 7V and the current is limited by the smaller of the two current capacities.

    In parallel, you have a problem since 4 > 3 so now the 4V source will be sending current into the 3V source. You will end up with 3V and probably something getting warm or even burning up.

    If you have a simple series circuit, yes the voltage across the circuit is equal to each of the individual drops added together. Kirchoff's laws encompass that if you want to search more online.
     
  5. Mar 16, 2005 #4
    ok thanks...so im gonna go ahead and assume that if you had an ac current, transformed, then rectified, and you did that (V1 > V2) the same would happen, just in a larger form...
     
  6. Mar 17, 2005 #5

    Cliff_J

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    Yes, two sources in parallel would just plain not be a good idea if the voltages differ because its similar to a short in terms of current potential and would only be limited by the output impedance (think of that as a series resistor on the output) and this can be a very low number making impratically high currents.

    In commercial trucks or tractors there are sometimes two 12V batteries connected in parallel to increase current capacity. When replacing its typically both of them and not just one of them to keep them working efficiently. Otherwise you can end up with the new battery doing most of the work and charging the other one (and in turn discharging itself) when the equipment is turned off.

    Cliff
     
  7. Mar 17, 2005 #6
    ok cool. so you said that in series the lower current of one source would limit the overall current, how so? like what would happen, and what would the resulting current be?

    Also...after recently looking at diagrams of full wave rectifiers, is it even possible to hook these up together? (series OR parallel)
     
  8. Mar 17, 2005 #7

    Integral

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    A Full wave rectifier REQUIRES a AC input, it produces a DC output, consider that and think for a few minutes the implications of how they can be used together.
     
  9. Mar 17, 2005 #8

    Averagesupernova

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    The words 'full wave' implies feeding AC into the rectifier. I wouldn't say it requires it. A 'full wave' rectifier can be used for automatic polarity protection on a battery connection for instance.
     
  10. Mar 17, 2005 #9
    i mean can you hook up the DC leads in series or parallel
     
  11. Mar 18, 2005 #10

    Cliff_J

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    You can, but why?

    With batteries it is just a given that the cells can create only so much potential from their chemical reaction and they are arranged in a way best suited to the needs. But with an AC power source a more appropriate power supply makes more sense than cobbling together two inappropriate ones, especially when its just a matter of a larger VA transformer with a different winding.
     
  12. Mar 18, 2005 #11
    yes but i mean in order to get high voltage and current...because when you use a transformer one goes up as the other goes down....
     
  13. Mar 18, 2005 #12

    Cliff_J

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    TINSTAAFL, you will still have a simpler solution to use one source. It could be a multi-tap design if you need one of the voltages independant of the total, but still simpler than two seperate units.

    To have high voltage AND high current means high power (watts). Simply use a larger transformer to begin with and you can have both without unnecessary complications.

    A regular wall outlet is rated at 120V and 15A and their product is 1800W. That is 2.4HP and should be plenty, but there is always 240V and 30A so you have 7200W (9.6HP) available as well. Use the appropriate transformer and you can deliver that much power in pretty much any combination of volts/amps you need.
     
  14. Mar 18, 2005 #13
    What's the application?
    If you need a 4V supply and a 3V supply can't you just create the 3V supply via voltage division from the 4V supply. Or if you really need something fancy you could just cobble a simple series regulator...
     
  15. Mar 19, 2005 #14
    the 3V and 4V thign was just an example. there really is no application at this point, im just asking questions. But i was thinking more along the lines of small generators. They can only produce so much power. so if you had (for example) 2 1000W generators, how would you go about hooking them up to the same circuit (to get as much power as possible) if you had them transformed in different combinations; like if one had a high current, and low voltage, and the other was the opposite.

    What about a power transformer? (as in more than 2 coils on the transformer) what effects would that have on the ac power source, and the power produced?
     
  16. Mar 19, 2005 #15

    chroot

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    You'd have to use a boost or buck converter on one.

    - Warren
     
  17. Mar 19, 2005 #16
    what's a boost/buck converter?
     
  18. Mar 19, 2005 #17

    chroot

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    A boost converter is a device that converts a low-voltage, high-current power source into a high-voltage, low-current power source. A buck converter does the opposite.

    - Warren
     
  19. Mar 19, 2005 #18
    no see i want both as power sources for the same circuit.
     
  20. Mar 21, 2005 #19

    Cliff_J

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    infamous - if you wanted to you could put as many coils on the same magnetic core of a transformer...until you reach the point where you saturate the core with magnetic energy much like a towel can become saturated with water where it will not hold anymore even if you apply more.

    But when using more than one AC power source, the voltage and current will have a phase, meaning there is a time when it increases to a high positive point, curves down past zero to a negative low point and then back again over and over. When you combine more than one power source the volts/amps can work together or against one another in what is called constructive and destructive interference. When they add they could add like with regular math or they could completely cancel each other out like the noise cancelling headphones you can buy at a store (hence destructive interference because the headphones make a signal as big as the noise coming in but just opposite phase). With a generator this could be a complicated problem because a slight change in RPM on one or both could make the available power fluctuate.

    Converted to DC, the problem is much smaller but now how do you use the power and is it worth it to avoid the losses by converting?

    Generators come as big as is needed for the job. Even at regular retail stores you can find some that are 6000W and maybe even 10,000W and up for an entire home. There are portable semi-truck trailer units that can run an office building, hospitals and other critical facilities will sometimes have them in the basement with enough fuel for many days of operation, and some of our military bases have incredibly large generators that could be used as power stations if needed. If you think about it, all electric power is generated somewhere, its just a matter of economics. :smile:

    Cliff
     
  21. Mar 21, 2005 #20
    Here, lets start simple. What are your voltage current requirements? Are you using different voltages at different points in your circuit (IE a microprocessor running off of 5V controlloing a 12V motor)? Also, do you plan on stepping up your voltage or stepping it down? What is your input voltage (type and magnitude)? Do you really need to build your own P/S or can you accomplish your tasking using a wall wart?

    As a note, only in very rare situations will you find multiple power transformers within an applications because (as mentioned above) there is no need for more than one. You can use multiple taps, or transformers rated at higher power levels to accomplish a task.

    If you answer the question in the first paragraph, ppl will have a better idea of what you need and will be able to give advice.

    Good luck.
     
  22. Mar 21, 2005 #21
    you see at this point i'm unable to answer the first paragraph, because this is in theory. i'm trying to find out just how much power i can squeeze out of a generator if i need to. (btw..wut the heck is a wall wart?) and it will be for only one load.

    CliffJ:
    i need this generator to be really small (like teh size of an alternator, small). and all of them would be turning in a synchronized fashion, with rare and small fluxuations (hopefully)
     
  23. Mar 21, 2005 #22

    Averagesupernova

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    Hey Cliff. Just thought I'd throw this out even though it is slightly off topic. It is not at all difficult to put AC generators in parallel. They tend to self regulate their speed. Just make sure that each generator is in good working order before hooking them in parallel. Both generators need to have their governors in top notch condition. There speed is set as close together as possible. Then the phase is checked or adjusted so that they are in phase. Many times this is just accomplished by having a very slight speed error and waiting for the generators the come in phase. The switch is then thrown to hook them together.
     
  24. Mar 21, 2005 #23

    Cliff_J

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    Oh, I had guessed it to be not too difficult to couple 2 generators if designed to work as a system, but this is seemingly about ad-hoc solutions more than integrated designs. Now how the power grid can stay synch'd across such huge distances and different loads as a matter of planning (guessing) and reacting....well now we're more off topic. :smile:

    infamous - a wall wart is the transformer that plugs into the wall, typically a 2" by 2" by 2" black box with a tiny cord that leads to like a cell phone charger or what not - its ugly hanging on the outlet hence the wall wart

    A good sized automotive alternator can produce 12V and around 100A (1200W) or maybe find a heavy-duty one from a diesel truck to make up to 200A so that's 2400W.

    You need a lot of power to turn that alternator (or generator) so I don't know what you're source is but it would need at least two spare HP (746W=1HP) to get that kind of output.

    Not sure of your application, but it sounds like it needs more details presented (hint hint) to make any kind of guess as to the feasibility.
     
  25. Mar 22, 2005 #24
    yes, unfortunately i can't mention more really. check my post about "patents" in the general talk forum...see this is something that i could patent, hopefully, and gain royalties from. so before going through the fees id rather find if its even possible, so im gathering scattered info and putting it together peice by peice. and as for the alternators, you can also get car ones at about 200A (for big sound systems), and there would be about 24 of them (yeah thats right..) possibly more, not exactly alternators however...but something along those lines, producing AC electricity so that it can be manipulated, and used for the load.
     
  26. Mar 22, 2005 #25

    Cliff_J

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    Infamous - you have many challenges to bring something like this to a working prototype. It should work just as well at a small scale as a large scale other than efficiency. And the use of 24 generators doesn't seem efficient or cost effective, look at a 777 airplane that uses 2 big jets for a big increase in fuel economy over the 4 smaller jets on a 747. And a 60W scale model should be easy to build and test the feasibilty of the concept even if it doesn't account for potential scalability problems.

    You can actually get 350A alternators in the regular alternator sized case for large sound systems and bigger cases for higher capacities. But for yours, a larger single unit still seems better. Anyways, I'll use your numbers. 24 units at 200A and 14.4V (pseudo-standard cold alternator charging temp) is 69kW of power or 92.7HP. The alternators need roughly 2000RPM spin too, about 243 ft-lb of torque with a 1:1 pulley ratio.

    Yes, the alternators are wound for low voltage and high current and rectify the 3-phase AC voltage to DC. They could be wired differently to make a single phase higher AC voltage with lower current. But why not just one 69kW generator and avoid the losses (and expense) of any conversion?

    Well the patent thing is pretty well been covered, you should already have an idea that the prospects of pursuing a patent and making a profit from selling it are pretty much a few decades out of touch with the times. One company I work with has found a company in a friendly european country that is stealing one of their patented formulas for a product. After investigating the cost at 100+ thousands of dollars in legal fees and travel expenses and factoring in the yearly sales and gross profits from those sales it was determined that it would take many years to recover the money spent protecting the patent (assuming they win the court battle). Just how many days can a person justify traveling to court with lawyers in a different country? (the other company has plenty of money and lawyers to tie it up in court) Look at how poorly copyright laws are enforced overseas, you can get $10,000 pieces of software for $10. The new mantra for inventors is to bring a product to market fast enough to get enough market share to establish a brand name before the copycats arrive.

    Real big question is, what is so different about your idea from what is already out there? Unless you have some new never before used method of spinning the generators, this is not really a patentable idea or one unique enough to prove to a court that someone else has infringed on your idea. I'd guess you might even find Edison's patents referenced at the very first patent search, there was a lot of competition and discovery with Westinghouse/Tesla that resulted in many patents being issued. You may find a way to get a patent to improve on an existing design, but someone could just as easily improve yours and pay you nothing while you're stuck with 10 or 15 grand in legal fees to get the patent. Or throw lawyers at you and wait until you run out of money from paying lawyers to fight them in court with counter-suits. Microsoft was quite good at this in the 80s/90s and it was a big deal when a small company like the one that made doublespace actually won a settlement against them.

    Not trying to be a buzzkill, but trying to establish some of the reality of production and legal protections (and costs of those protections) that go with it. There was a show on the TechTV called "Invent This!" and it was truly sad to see so many people invest their life savings into ideas they were sure were going to work. There were only a few success stories and in those the people did it all: invent, design, produce, market, and distribute their product.
     
    Last edited: Mar 22, 2005
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