Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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"-
  4. Mar 16, 2005 #3


    User Avatar
    Science Advisor

    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


    User Avatar
    Science Advisor

    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.

  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


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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
    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


    User Avatar
    Science Advisor

    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


    User Avatar
    Science Advisor

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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


    User Avatar
    Science Advisor

    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:

  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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Power Sources in Series/Parallel ?
  1. Series and parallel (Replies: 1)

  2. Series or Parallel? (Replies: 2)