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Most books say that to connect sources in series, the current sources will be equal in magnitude. I really don't understand the meaning of this statement. What happens if you connect current sources of different value in series? And what is the difference between a voltage and current source because both of them produce current?

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  • $\begingroup$ For ideal current sources you can't do it, you will get a contradiction. Since however no current source is ideal you have to take into account internal resistance $\endgroup$ Commented Mar 30, 2015 at 16:29
  • $\begingroup$ But what if you do? What contradiction $\endgroup$
    – Socre
    Commented Mar 30, 2015 at 16:30
  • $\begingroup$ Do you mean what if you do put two ideal sources? This is impossible, ideal current sources cannot exist by saying "what if we do" has no physical meaning since it cannot be done. $\endgroup$ Commented Mar 30, 2015 at 16:32
  • $\begingroup$ Let me give you a situation, What if lets say a 10 year old boy who doesn't know nothing about a current source pressed the 'red button' which accedentally turned on 2 current sources connected in series and some how calibrated to the same value. Then what would happen $\endgroup$
    – Socre
    Commented Mar 30, 2015 at 16:40

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A current source will change the voltage across its terminals so that a certain current will flow. A perfect current source has "infinite" differential impedance - that is, regardless of the voltage you apply to it, it will always let the same current flow.

Now if you have two such sources, they will "fight to the death". Let's say one source tries to establish a current of 1 A in the circuit and the other is configured for 500 mA. As the second senses the "too big" current coming in from the first, it will lower the output voltage - going negative in an attempt to reduce the current. Meanwhile the first will continue cranking up the voltage, trying to force more current through the second.

As I said - "to the death". They cannot both have their way, and in the case of any real current sources, one of them will stop performing to specification.

This is why we put current sources in parallel, and voltage sources in series (the same argument I used above applies, mutatis mutandis, when you try to put different voltage sources in parallel).

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  • $\begingroup$ So one would burn out, but which one, the one with the higher value or lower $\endgroup$
    – Socre
    Commented Mar 30, 2015 at 16:46
  • $\begingroup$ @Socre There is no way of knowing which one burns out - "the weaker one". This will depend on the way the individual sources are constructed. In reality there is no such thing as an "ideal" current source - and to the extent they are not ideal they will adjust (and likely come up with some intermediate value of the current; or oscillate). $\endgroup$
    – Floris
    Commented Mar 30, 2015 at 16:49

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