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Power sources in series?

  1. Nov 16, 2014 #1
    When two batteries/power supplies are connected in series their current must be equal while the voltage is increased. Why is that so? Why wouldn't current increase equally as voltage increasing when connected in series?

    Just wanted to understand the theory behind this.
  2. jcsd
  3. Nov 16, 2014 #2


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    If the current was not equal, then there would be an accumulation of charge.

    What the current is depends on the load. Given negligible internal resistance and a simple resistive load, the current would be larger than that for a single power source - but the current through both power sources would still be the same.
  4. Nov 16, 2014 #3

    Well if the load's resistance is equal to 1 Ohm, and there is a 5W power supply and another 10W power supply connected in series.
    The 5W supply's current is 2.23A at 2.23V, while the 10W supply is 3.16A at 3.16V, the current through both source would be equal to what?
    I'm confused to which value would it be equal to(for it to be the same), if charge accumulates then current is equal to 2.23+3.16?

    Also, wouldn't the accumulation of charge go against (KCL)?
    Last edited: Nov 16, 2014
  5. Nov 16, 2014 #4
    Power supplied by voltage source to a circuit is P= V⋅I . V is voltage between terminals of the source and I current through the source. The powers supplied by sources are P1=V1⋅I1 and P2=V2⋅I2. Since current through the sources put in series must be same, how much power in equation Ptot=P1+P2 gets generated by each source depends on voltages of the sources during opearation. If one voltage source generates twice more power than other, it must have twice higher voltage.
  6. Nov 16, 2014 #5


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    To work out this sort of problem you have to approach it in the right order or you get nonsense. You do not get power supplies that are '10W or 5W'. You normally specify a power supply in terms of the volts it will supply and then the current will depend on the resistance of the load. The 10W or 5W rating refers to the maximum power it will supply (i.e. there will be a limit to the current it can supply before it fails or limits itself).
    So: you have to add together the voltages of the two series-connected supplies. Then you work out the current that the load will pass with that voltage. It is always possible that the value of current you get from your calculation will exceed the current that one or both supplies can deliver and that means you cannot work out what will happen without more knowledge of the details of the power supply spec. It's no longer an ideal situation.
    If you apply Kirchhoff to this sort of problem, you can't go wrong. (Well - you know what I mean!)
  7. Nov 16, 2014 #6

    The current is way beyond the budget! What to do now?
    I know that if two power source with "
    unequal" current are in series the circuit would immediately fail and major damages would occur. Therefore, was told to avoid it.
  8. Nov 17, 2014 #7


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    Both supplies, if in series, would, of course, have the same current through them. You can't fight K1!
    What to do now? Major re-think, imo. If two supplies have the same nominal op volts then you can contemplate putting them 'almost' in parallel, as long as you have a low value balancing resistor to take care of any fine voltage difference between them. Without that resistor, only the one supply would actually be supplying any current.
    You need to specify the actual problem in detail, I think, if you want any really useful advice about this situation. It's not straightforward.
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