Twocoil

Twocoil is a term used in electronics to describe a pair of magnetically coupled coils. It is a simple two-winding element that underpins many devices and systems, including transformers, inductive sensors, and wireless power links. While not a formal standard term, it serves as a convenient shorthand for studying or describing two closely linked inductors.

The behavior of a twocoil system is governed by mutual inductance M between the coils and their self-inductances L1 and L2. The voltages induced in each coil can be expressed as v1 = L1 di1/dt + M di2/dt and v2 = M di1/dt + L2 di2/dt. The coupling coefficient k, defined as M / sqrt(L1 L2), ranges from 0 to 1 and indicates how effectively the coils share magnetic flux. Winding direction and orientation determine whether the coupling is constructive (additive) or destructive (subtractive), influencing transfer efficiency and phase.

Common applications include power transfer between coils in wireless chargers, the transformer stages in power supplies, and inductive sensing where the presence or position of a target alters the mutual coupling. Additionally, twocoil configurations are used in RF systems, impedance matching networks, and magnetic resonance sensing, where coil geometry and spacing affect performance.

Design considerations cover core materials, air gaps, the number of turns, wire gauge, and parasitic elements such as inter-winding capacitance. Frequency also plays a crucial role: higher frequencies allow smaller coils but increase losses and stray capacitance, while lower frequencies require larger geometries. Practical design often balances coupling strength, efficiency, size, and tolerances for alignment.

See also: transformer, mutual inductance, inductive charging, RF coil, wireless power transfer.