Kirchhoff's Law and current Sources?

In summary: In the Thevenin circuit, the voltage drop is due to the current source itself. Not exactly, as DaleSpam stated, the sourced current would create a voltage offset through the source's load. It does not apply a voltage difference, it applies a current source, the resulting voltage difference is dependent upon the load on the current source.
  • #1
dphysics
12
0
When you are using Kirchhoff's voltage law and you come across a current source, what are you supposed to include that as? Pretending that it is an ideal current source, there will be no resistance and thus no voltage drop. So, do you just ignore it in the voltage equation? (Ex: V1+V2+V3=0)

Because of the fact that you can re-arrange a resistor and voltage source in parallel to a current source and resistor in series, I'm a little confused.

Any help much appreciated.
 
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  • #2
A current requires there be an electric field, which in turn, requires there be a potential difference.
 
  • #3
dphysics said:
When you are using Kirchhoff's voltage law and you come across a current source, what are you supposed to include that as? Pretending that it is an ideal current source, there will be no resistance and thus no voltage drop. So, do you just ignore it in the voltage equation? (Ex: V1+V2+V3=0)

Because of the fact that you can re-arrange a resistor and voltage source in parallel to a current source and resistor in series, I'm a little confused.

Any help much appreciated.

That's one of the reasons I use KCL to solve circuits. I never liked KVL...
 
  • #4
Gear, would you treat the current source as a separate voltage source altogether or combine it with the surrounding resistor and use it in the equation V=IR?
 
  • #5
A current source does not project a voltage difference, just a current. You cannot replace it with a voltage source unless you re-imagine the circuit as a Thevenin equivalent circuit. Use the KCL rule for the branches that have current sources as the current source forces the current in that branch.
 
  • #6
dphysics said:
Pretending that it is an ideal current source, there will be no resistance and thus no voltage drop.
This will get you the wrong answer almost every time. Current sources typically have very high impedance. The current source supplies whatever voltage is required in order to maintain the set current.

dphysics said:
When you are using Kirchhoff's voltage law and you come across a current source, what are you supposed to include that as?
Include it as what it is, an unknown voltage that you have to solve for. In such a problem you have one more unknown (the voltage across the current source) and one more equation (the sum of the loop currents through the current source equals the set current). You still have enough equations to solve for all of your unknowns.

berkeman said:
That's one of the reasons I use KCL to solve circuits. I never liked KVL...
I agree!
 
  • #7
Born2bwire said:
A current source does not project a voltage difference, just a current. You cannot replace it with a voltage source unless you re-imagine the circuit as a Thevenin equivalent circuit.

Wouldn't there be a voltage difference to induce the current source?
 
  • #8
Gear300 said:
Wouldn't there be a voltage difference to induce the current source?

Not exactly, as DaleSpam stated, the sourced current would create a voltage offset through the source's load. It does not apply a voltage difference, it applies a current source, the resulting voltage difference is dependent upon the load on the current source.

Take a look at a MOSFET current mirror. The voltage across the sourcing transistor in a current mirror is not created (in the ideal case) by the transistor but entirely by the load connected to the mirror. In fact, with a current mirror, your load needs to create a sufficient voltage drop to force the sourcing transistor into saturation for the mirror to work properly. Actually, the current mirror itself is a Norton source, with a large but finite Norton resistance due to what is called channel length modulation. Ideally, the Norton resistance would be infinite, in which case the current mirror would not have any voltage drop associated across it (since the resistance branch would become an open circuit).

A simpler example is the Thevenin and Norton circuits. You can see that in the Norton circuit, the parallel load resistance creates the voltage drop.
 

1. What is Kirchhoff's Law?

Kirchhoff's Law, also known as Kirchhoff's Current Law (KCL), is a fundamental principle in circuit analysis that states that the sum of currents entering a node or junction in a circuit must equal the sum of currents leaving that node or junction. This law is based on the principle of conservation of charge, which states that charge cannot be created or destroyed in a circuit.

2. What is Kirchhoff's Voltage Law?

Kirchhoff's Voltage Law (KVL) is another fundamental principle in circuit analysis that states that the sum of voltage drops around a closed loop in a circuit must equal the sum of voltage sources in that loop. This law is based on the principle of conservation of energy, which states that energy cannot be created or destroyed in a circuit.

3. How do Kirchhoff's Laws apply to circuits with current sources?

Kirchhoff's Laws apply to circuits with current sources in the same way as they apply to circuits with voltage sources. KCL states that the sum of currents entering a node must equal the sum of currents leaving that node, regardless of whether those currents are coming from a voltage source or a current source. KVL states that the sum of voltage drops around a closed loop must equal the sum of voltage sources in that loop, regardless of whether those sources are voltage sources or current sources.

4. Can Kirchhoff's Laws be used in all types of circuits?

Yes, Kirchhoff's Laws can be used in all types of circuits, including series circuits, parallel circuits, and complex circuits with multiple loops and branches. These laws are fundamental principles in circuit analysis and can be applied to any type of circuit, regardless of its complexity.

5. How do you use Kirchhoff's Laws to solve circuit problems?

To solve circuit problems using Kirchhoff's Laws, you first need to identify the nodes and loops in the circuit. Then, use KCL to write equations for each node and use KVL to write equations for each loop. Finally, solve the resulting system of equations to find the unknown currents and voltages in the circuit. It is also helpful to draw a circuit diagram and label the known and unknown values to keep track of the variables in the equations.

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