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2 Voltage sources in parallel..

Discussion in 'General Electronics Chat' started by dineshbabumm, Aug 5, 2006.

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  1. dineshbabumm

    dineshbabumm New Member

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    What will be the result if we connect 2 different voltage source(say a 2v and a 1v source in parallel to each other). What will be the net voltage(For both ideal and non ideal cases) if we measure voltage across the parallely connected voltage sources??

    My opinion is it will be the least of two(In my example it may be 1 v) in the ideal case as if we connect a 10 v and short it(almost similar to 0v connected in parallel with 10v),the battery gets drained and results in 0v.. But will it be the case really? What will be the result if we connect a 2v and 1v in parallel???
     
  2. dknguyen

    dknguyen Well-Known Member

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    The voltages will subtract and it will be like a short circuit of a lower voltage battery if the voltages are not equal. THe two cells simplify into one and it's like a short across the equivelant cell. Two elements connected by themselves in a single loop circuit are in parallel and in series at the same time.

    So your abstraction for case 1 is correct and it is also the same for case 2.
     
    Last edited: Aug 5, 2006
  3. akg

    akg New Member

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    the 2v batt will discharge throught the 1v batt . resulting o/p will be 1v
     
  4. dave

    Dave New Member

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  5. StopGo

    StopGo New Member

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    You would form a closed circuit out of the batteries. In situations like this you have to take into consideration the internal resistance of the batteries. Think of the batteries as a voltage source in series with a resistance. The resistance depends on the type of battery e.g. quite high for zinc carbon, very low for NiCd.

    Joining two batteries of different voltages would cause a current to flow between the batteries as follows.. I = (Vbatt2 - Vbatt1) / (Rbatt1+Rbatt2)

    i.e. your batteries will probably get hot.

    Its a waste of batteries and probably dangerous if you use batteries of low resistance like rechargeable.

    akg is right that in the end the higher voltage battery will discharge into the lower one, until they are both at the same level, but this could take some time if the internal resistance is high, or it could be near instant (hopefully not as this would probably cause your lower voltage battery to burn up / explode)
     
    Last edited: Aug 6, 2006
  6. dineshbabumm

    dineshbabumm New Member

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    Thanks for replying.. Will it subtact or will it be the least of the two batteries?? If it is the difference of two then how if we connect a 10v and a 0v in parallel(That is by shorting a 10v battery) it will result in 0v(drains the battery) and not the difference of two(should have been 10 or -10 if we go by this).. How is it possible??
     
  7. StopGo

    StopGo New Member

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    As I was trying to explain.. you have to consider that in reality each battery behaves like a voltage source in sieries with a small resistance.

    It's nonsense to think about directly connecting two voltage sources without taking into account the INTERNAL resistance.
     
  8. Hero999

    Hero999 Banned

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    If there is not internal resistanc then an infinite current will flow from the 2V source into the 1V source so the output would be 1V.

    In general connecting two voltages sources in parallel is a really stupid idea as it will always lead to lots of smoke.
     
  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It's nonsense to think about it at all! :D

    But, as usual, simple ohms law applies - as 'StopGo' says you need to consider the internal resistance of the batteries to apply it.

    But the entire question is really completely stupid! - the sort of question you get set in exams, obviously set by someone with no understanding of electronics at all!.
     
  10. Hero999

    Hero999 Banned

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    Or just to trick the studants.
     
  11. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    That's really the problem - I'm SURE that they don't purposely introduce questions that don't make sense - but considering it's going to be marked by someone comparing the answer to the one on their answer sheet, what do you put down?. Generally papers are marked by people with even less knowledge than the muppets setting the questions.

    The classic example, which I've seen more than once (and actually been given in exams twice!), is an NPN transistor, emitter to 0V, collector via a resistor to +ve, and a potential divider feeding the base. The question wants to know the voltage on the base - now we all know (or I should hope so!) that it will be around 0.7V, because the base/emitter junction is across the bottom section of the potential divider. But is this what they want?, or are they ignoring the transistor being there?, and wanting you to calculate the potential divider voltage?.

    If it's of any interest, in both exam cases I gave BOTH answers, specifiying them as 'transistor OK' and 'transistor O/C' - but did that lose me marks?.
     
  12. idarab

    idarab New Member

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    Why not just throw diodes on each + lead?

    Abe
     
  13. JimB

    JimB Super Moderator Most Helpful Member

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    A good idea, that is if we can throw those diodes far enough.:D
    This thread is four months old and should have been buried long ago.
    The original question was a theoretical scenario for the guy to get some understanding.

    Welcome to the forum, but please dont go around resurecting ancient posts.;)

    JimB
     
  14. Hero999

    Hero999 Banned

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    The higher voltage will dominate and no current will flow from the lower voltage supply, you'll also waste 0.6V-1V in the diode.
     
  15. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    OK, thread locked - it was rubbish before, and it's going nowhere now!.

    Moderator.
     
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