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Sinking and Sourcing Current Supplies?

chopnhack

Apr 28, 2014
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Does this diagram illustrate sinking and sourcing supplies? My text uses conventional current supply and states that:

1.5.3 A S S O C I A T E D V A R I A B L E S C O N V E N T I O N
Equation 1.4 implies a specific relation between reference directions chosen for
voltage and current.
Definition of terminal variables v and i for the resistor that defines the positive flow of current (flow of positive charge) is directed in at the resistor terminal assigned to be positive in voltage. This convention, referred to as associated variables, is generalized to an arbitrary element in Figure 1.20 and will be followed whenever possible in this text. The variables v and i are called the terminal variables for the element. Note that the values of each of these variables may be positive or negative depending on the actual direction of current flow or the actual polarity of the voltage.
Definition of the terminal variables v and i for a two-terminal element under the associated variables convention.
Associated Variables Convention Define current to flow in at the device terminal assigned to be positive in voltage.

From this I understand that current flows from the more positive to the negative. When looking at the diagram below, the top rail would be the more positive in relation to the bottom rail that has been grounded.

1. What would prevent both sources from flowing to the top?
2. Why was the 2A supply chosen when the 10A is a higher source?

The example gives an answer of 2A + (7-0)/1Ω + (7-0)/7Ω - 10 = 0 satisfying KCL .



upload_2014-9-8_19-36-9.png

As always, thanks in advance!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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You can think of this in a simple way.

  1. 10A has to get from one end of the current source back to the other.
  2. All of the elements are in parallel so the currents must add up to 10A (this is simply a restatement of KCL)
  3. 2A goes through the 2A current source.
  4. The remainder goes through the two resistors.
So now you know the total current flowing through the two resistors (call them R1 and R2, and the currents I1 and I2)

You know that the unknown voltage V = I1 * R1 and V = I2 * R2, and that I1 + I2 = 10 - 2.

With some manipulation you can determine the voltage.

However, more simply, if you determine the effective resistance of the two resistors in parallel then you know both the resistance and the current and can determine the voltage directly.
 

Laplace

Apr 4, 2010
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1. What would prevent both sources from flowing to the top?
Current sources adjust their generated voltage to whatever is necessary to force the specified current to flow in the specified direction through the circuit. In this case both current sources generate e=7V.

2. Why was the 2A supply chosen when the 10A is a higher source?
It would appear that both current sources were selected to be part of this problem definition.
 

Harald Kapp

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1. What would prevent both sources from flowing to the top?
The arrows as indictaed show both currents flowing counter clockwise (that's the advantage of having arrows in place). Therefore as 10A flow into the top node, 2A flow out of it (see Steve's answer).
 
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