Source transformation

Finding a solution to a circuit can be somewhat difficult without using tricks or methods that make the circuit appear simpler. Circuit solutions are often simplified, especially with mixed sources, by transforming a voltage into a current source, and vice versa. This process is known as a source transformation, and is an application of Thevenin's theorem and Norton's theorem.

Process

Performing a source transformation is the process of using Ohms Law to take an existing voltage source in series with a resistance, and replace it with a current source in parallel with the same resistance. Since source transformations are bilateral, one can be derived from the other. Source transformations are not limited to resistive circuits however. They can be performed on a circuit involving capacitors and inductors, as long as the circuit is first put into the frequency domain. In general, the concept of source transformation is an application of Thevenin's theorem to a current source, or Norton's theorem to a voltage source.

Specifically, source transformations are used to exploit the equivalence of a real current source and a real voltage source, such as a battery. Application of Thevenin's theorem and Norton's theorem gives the quantities associated with the equivalence. Specifically, suppose we have a real current source I, which is an ideal current source in parallel with an impedance. If the ideal current source is rated at I amperes, and the parallel resistor has an impedance Z, then applying a source transformation gives an equivalent real voltage source, which is ideal, and in series with the impedance. This new voltage source V, has a value equal to the ideal current source's value times the resistance contained in the real current source $V=I\cdot Z$. The impedance component of the real voltage source retains its real current source value.

In general, source transformations can be summarized by keeping two things in mind:

• Ohm's Law
• Impedances remain the same

Example of a Source Transformation

Source transformations are easy to perform as long as you are familiar with Ohms Law. Basically, if you have a voltage source in series with an impedance, you divide the voltage by the impedance to find the value of the equivalent current source in parallel with the impedance. The converse is if you start with a current source, and want to find the equivalent voltage source, you multiply the value of the current source with the value of the impedance. A visual example of what is being done during a source transformation can be seen in Figure 1.

Remember:

$V=I\cdot Z$

$I={\cfrac {V}{Z}}$

Figure 1. An example of a DC source transformation. Notice that the impedance Z is the same in both configurations.

See also

• Ohms Law
• Thevenin's theorem
• current source
• voltage source
• electrical impedance