Parasitic capacitance

In electrical circuits, parasitic capacitance, or stray capacitance is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. All actual circuit elements such as inductors, diodes, and transistors have internal capacitance, which can cause their behavior to depart from that of 'ideal' circuit elements. Additionally, there is always non-zero capacitance between any two conductors; this can be significant at higher frequencies with closely spaced conductors, such as wires or printed circuit board traces.

The parasitic capacitance between the turns of an inductor or other wound component is often described as self-capacitance. However, self-capacitance of a conductive object is a different phenomenon, referring to the capacitance of the object without reference to another object.

When two conductors at different potentials are close to one another, they are affected by each other's electric field and store opposite electric charges like a capacitor. Changing the potential v between the conductors requires a current i into or out of the conductors to charge or discharge them.

where C is the capacitance between the conductors. For example, an inductor often acts as though it includes a parallel capacitor, because of its closely spaced windings. When a potential difference exists across the coil, wires lying adjacent to each other are at different potentials. They act like the plates of a capacitor, and store charge. Any change in the voltage across the coil requires extra current to charge and discharge these small 'capacitors'. When the voltage changes only slowly, as in low-frequency circuits, the extra current is usually negligible, but when the voltage changes quickly the extra current is larger and can affect the operation of the circuit.

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