Coupled circuits

(electricity) Two or more electric circuits so arranged that energy can transfer electrically or magnetically from one to another.

Two or more electric circuits are said to be coupled if energy can transfer electrically or magnetically from one to another. If electric charge, or current, or rate of change of current in one circuit produces electromotive force or affects the voltage between nodes in another circuit, the two circuits are coupled.

Between coupled circuits there is mutual inductance, resistance, or capacitance, or some combination of these. The concept of a mutual parameter is based on the loop method of analysis. A mutual parameter can be one that carries two or more loop currents; such a network has conductive coupling because electricity can flow from one circuit to the other.

Also, there can be purely inductive coupling, which appears if the magnetic field produced by current in one circuit links the other circuit. A two-winding transformer is an application of inductive coupling, with energy transferred through the magnetic field only.

It is also possible to have mutual capacitance, with energy transferred through the electric field only. Examples are the mutual capacitance between grid and plate circuits of a vacuum tube, or the capacitive interference between two transmission lines, as a power line and a telephone line, that run for a considerable distance side by side.

There are several ways to show the relative polarities of inductive coupling. Figure 1 shows two coils wound on the same core. Current flowing into the upper end of coil 1 would produce magnetic flux upward in the core, and so also would current flowing into the upper end of coil 2. For this reason the upper ends of the two coils are said to be corresponding ends. Dots are placed on a diagram at the corresponding ends of coupled coils. Such dots are shown in Fig. 1, though they are not needed in this figure. Dots are also shown in Fig. 2, where they give the only means of identifying corresponding ends of the coils shown.

Polarity of coils.

Mutual inductance L₁₂. Letters a, b, c, and d label terminals.