Millman's theorem

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In electrical engineering, Millman's theorem (or the parallel generator theorem) is a useful method to simplify the solution of a circuit. It is named after Jacob Millman, who proved the theorem. A similar method, known as Tank's method, had already been commonly used before Millman's proof.

[edit] Explanation

Figure 1

Millman's theorem gives a very quick method to compute the voltage at the ends of a circuit made up of only branches in parallel.

Let e_k be the voltage generators and a_m the current generators.

Let R_i be the resistances on the branches with no generator.

Let R_k be the resistances on the branches with voltage generators.

Let R_m be the resistances on the branches with current generators.

Then Millman states that the voltage at the ends of the circuit is given by:

$v=\frac{\sum_{}\frac{\pm e_{k}}{R_{k}}+\sum_{}\pm a_{m}}{\sum_{}\frac{1}{R_{k}}+\sum_{}\frac{1}{R_{i}}}$

It can be proved by considering the circuit as a single supernode. Then, according to Ohm and Kirchhoff, the voltage between the ends of the circuit is equal to the total current entering the supernode divided by the total equivalent conductance of the supernode.

The total current is the sum of the currents flowing in each branch.

The total equivalent conductance of the supernode is the sum of the conductance of each branch, since all the branches are in parallel. When computing the equivalent conductance all the generators have to be switched off, so all voltage generators become short circuits and all current generators become open circuits. That's why the resistances on the branches with current generators do not appear in the expression of the total equivalent conductance.