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Home / Technical Articles / Back To School – Let’s Calculate Current I3
Back To School - Let's calculate I3
Back To School – Let’s calculate I3

Techniques for solving DC circuits

Referring to the following circuit, calculate direct current I3:

  1. Using Kirchhoff’s laws;
  2. Using nodal analysis;
  3. Applying Thévenin’s theorem at nodes A and B.

Ok, let’s dive into calculation…

For start, let’s try to remember two Kirchhoff’s Laws //
Kirchhoff’s Current Law – The algebraic sum of currents at a node is zero.
Kirchhoff’s Voltage Law – The algebraic sum of voltages around a closed circuit loop is zero.

– KCL stands for Kirchhoff’s Current Law
– KVL stands for Kirchhoff’s Voltage Law

Currents I1, I2 and I3

The following equation can be written:

Deriving from KCL at node A

Substituting values, we obtain:

Uab

Finally,

Direct currents I1, I2 and I3

Thévenin’s voltage UTh at nodes A and B can be easily calculated by disconnecting the right part of the circuit:

Calculate Thévenins voltage

Thévenin’s voltage UTh at nodes A and B

Thévenin’s equivalent resistance RTh is the resistance “seen” from nodes A and B, when all generators are deactivated (in our case, only E1 is present):

Thévenin’s equivalent resistance RTh

The left side of the circuit can now be substituted by its Thévenin equivalent, in order to calculate current I3:

The left side of the circuit can now be substituted by its Thévenin equivalent

This single-mesh circuit can be easily solved using KVL (Kirchhoff’s voltage law):

This single-mesh circuit can be easily solved using KVL (Kirchhoff’s voltage law)

Readers should note that UAB0 ≠ UAB:

UAB = UTh – RTh · I3 = 8.333 – 4.166 · 0.041 = 8.163

Comparing the three methods, we can conclude that Thévenin’s theorem is very powerful, in particular when a single current value is needed.


Another Kirchhoff’s Laws Worked Example (VIDEO)


Thevenin’s Theorem. Example with solution (VIDEO)

Reference // Fundamentals of electric power engineering – Ceraolo, Massimo, Davide Poli.

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

3 Comments


  1. venkatesha
    Aug 10, 2015

    I want more information about electrical engineering.


  2. nitin
    May 04, 2015

    Nostalgic knowledge


  3. HESFORD QUAYE-LARBI
    Apr 13, 2015

    Very good materials for Electrical Engineering practice.

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