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Parallel circuit of current sources

  1. Mar 16, 2017 #1

    BAJ

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    Assume I have a parallel circuit of current sources with 5 current sources generating 1mA each. Therefore from both the terminals I will get 5mA total. Now if I add a battery to one of the terminals where current(i) will flow into my circuit. If i = 50mA, will I get 55mA current from another terminal of my circuit??
     
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  3. Mar 17, 2017 #2

    Baluncore

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    Welcome to PF.
    Fixed current sources regulate a current flowing from a more positive node of the circuit to a more negative terminal. They do not fix the voltage, that is decided by the way external components respond to the sourced current.

    If there are five 1mA sources in parallel, you will get the equivalent of a 5mA current source.

    How do you “add a battery” to “one terminal”. You need to draw a couple of circuit diagrams showing the sources with and without the battery. Are you going to be charging that battery or is it an arbitrary fixed voltage source.

    I can see no way that your statement could be true. So my answer is probably No.

    But produce your circuit diagrams, (drag and drop them onto the edt window), so we can work out exactly how you modify the circuit when you add the battery.
     
  4. Mar 17, 2017 #3

    BAJ

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    Actually by adding a battery I meant connecting the battery in a way that it is able to deliver the current to one terminal. However here is my proposed circuit. Proposed_Circuit_Diag.jpg
     
  5. Mar 17, 2017 #4

    FOIWATER

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    Replace the five current sources by a single 5 mA current source in series with a battery and resistor.

    What would be the value of 'i' then?
     
  6. Mar 17, 2017 #5

    BAJ

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    55mA. Isn't it?
     
  7. Mar 17, 2017 #6

    FOIWATER

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    If you have a single loop and you know the value of current through any one component, isn't that the value of the current through out the circuit?

    In this circuit the current source dictates the current. The voltage source would dictate the voltage level.

    The ideal current source allows any voltage to be developed across it. The ideal battery allows any amount of current to flow through it.

    That's the way I would look at this, some one else may have more insight.
     
  8. Mar 17, 2017 #7

    BAJ

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    But wont the resultant current add up although the current source is adding current to the circuit?
     
  9. Mar 17, 2017 #8

    FOIWATER

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    No the current source's value will determine total circuit current is my understanding of this. The voltage developed across the resistor will then be determined by ohm's law, and the voltage developed across the current source will be determined by the value of the battery voltage.

    The amount of current through the battery is dictated by the current source is what I am saying.

    The issue lies when we say "the battery supplies 50mA and the current sources supply 5mA.." That wouldn't happen.
     
  10. Mar 17, 2017 #9

    BAJ

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    "No the current source's value will determine total circuit current is my understanding of this." ----- didn't understand. can u explain a bit??
     
  11. Mar 17, 2017 #10

    BAJ

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    And moreover I don't want to consider it as equivalent of 5mA in series with the battery. Because if you consider only the parallel circuit and the current source, then both pts A and B will have 5mA current. Now when battery delivers 50mA current in pt A, then definitely backflow is going to happen for which the pt B will have a current reading of 55mA (theoretically). It looks correct, no?
     
  12. Mar 17, 2017 #11

    FOIWATER

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    Sure.

    Have you ever read about what constitutes a current source? like you already know batteries and generators are examples of voltage sources right? what is an example of a current source?

    have you studied transistors yet? This is something that comes up in very basic transistor circuits. Transistors are examples of current sources. The current through a transistor is a function of its amplification factor, and the base current. Not the voltage tied to the collector of the transistor.

    Take a basic transistor circuit:

    3KeMDx1403029052.jpg

    This is quite similiar to your circuit. You have a battery, Vcc, you have a resistor, and you have a current source. The current through the battery, resistor, and current source is affected only by the base current of the transistor. The voltage supply is a constant, and the current it delivers is a function of the current source. The current source is drawing current from the battery. They cannot be different..

    If you have a circuit with no battery, you assume the current source is able to draw its current from an in-explicit source of energy that has been absorbed into the current source symbol.. but this is how I understand it..

    Someone may come along and set us both straight, though.
     
  13. Mar 17, 2017 #12

    FOIWATER

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    The battery can't deliver 50mA in pt A. It can't deliver more than the current sources are drawing.
     
  14. Mar 17, 2017 #13

    BAJ

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    Thanks for your quick explanation, cleared the matter more. However consider this, the current sources are not drawing current from the battery, they are simply graphite placed parallely like the current sources drawing current from an electrolytic solution. And I have experimented earlier (without attaching the battery) that orienting 5 graphite blocks in this way can fetch you ~1mA of current. Now if I subject this parallel circuit to an external power supply, like the diagram I have provided, what will be the value of 'i' in this scenario.

    I am a Biotech engineer, only have basic knowledge on electrical, sorry for my ignorance.
     
  15. Mar 17, 2017 #14

    FOIWATER

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    No apologies required ! I love the discussion..

    The current sources would drop sufficient voltage to limit the current supplying capabilities of the voltage source to levels dictated by the current sources themselves. The current sources don't 'pick-up' the current injected by the battery and add it to their own current like that. They deliver what they deliver, and they drop voltage to limit the current through the battery to their own levels.

    That may be wrong but it is my understanding.

    I can say with certainty that the current in a series circuit has to be a constant. Different sources will not contribute different current levels that 'add up' that way.
     
  16. Mar 17, 2017 #15

    FOIWATER

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    You see, the current injected by the battery is determined itself by the current source. Current sources behave that way. That is, they drop voltage supplied externally (in your case the battery) and they just supply current.

    If this makes sense and you are looking for a way to remember it from here on out, remember:

    Current sources determine current levels.
    Voltage sources determine voltage levels.

    Current sources will drop any amount of voltage to ensure they continue to supply their current (ideally)
    Voltage sources will sink any amount of current to ensure they supply their voltage (ideally)
     
  17. Mar 17, 2017 #16

    BAJ

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    But as I explained the components I am calling current sources are not conventional current sources, they draw current from an electrolysis of a solution, not from the battery. Or are you saying, if I attach the parallel circuit with battery, these graphites will draw current from the battery?

    I agree to that in series, current has to be constant. But "The current sources would drop sufficient voltage to limit the current supplying capabilities of the voltage source to levels dictated by the current sources themselves." .... can u explain a bit more?
     
  18. Mar 17, 2017 #17

    FOIWATER

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    Sure.

    Let me answer your question with a question. In your series circuit, what is the voltage drop across the current source?

    Would you agree that, if the current source drops voltage, any voltage, that it limits the amount of current that the battery injects?

    My understanding is that the current source will drop whatever voltage is necessary to ensure the current through the whole loop is determined by it alone. The batteries ability to inject current is commandeered by the current source.

    Consider this: Consider replacing the current source with a resistor. For sure the battery would not be able to supply as much current then. Same is true for the current source, right? it has to drop some voltage?

    That means the current source DOES in fact influence the batteries ability to source a current. Question then becomes, how much does it influence it? answer is, it influences it infinitely. The current source is a supplier of current and a dropper of voltage. It will drop whatever is required to supply its current.

    It's the boss when it comes to current !
     
    Last edited: Mar 17, 2017
  19. Mar 17, 2017 #18

    BAJ

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    The voltage drop across the current source must be determined by the circuit it is connected to. In my case, it may be ~50mV or something, I forgot actually. And yes I agree to your 2nd sentence too if the resistance in the circuit is considered constant. If the resistance is variable then it will be different matter.
     
  20. Mar 17, 2017 #19

    FOIWATER

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    Right. So my advice would be.... make the circuit ! Make the circuit and see if the current is limited to the value of the current supply.

    Ensure in reality that the current source is rated for at least the voltage of the battery, though ! and vice versa for current ! The discussion here has been 100% theoretical using 'ideal' components.

    In reality, there are levels to the voltages and currents components can accept.. as you know
     
  21. Mar 17, 2017 #20

    BAJ

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    Ok sure. Thanks for all of your explanations. It helped to get a clear understanding. "It's the boss when it comes to current !" ------ I will definitely remember this :)
     
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