What is Stray Inductance?

Stray inductance

Stray inductance is unintended and unwanted inductance in a circuit.

Inductance does not exist only within inductors. In fact, any wires or component leads that have current flowing through them create magnetic fields. When these magnetic fields are created, they can produce an inductive effect. Thus, wires or components leads can act as inductors if they are long enough. Such effects are often present within circuits (for example, between conductive runs of wire traces or components with long leads such as capacitors), even though they are not intended. This unintended inductance is referred to as stray inductance, and it can result in a disruption of normal current flow within a circuit.

Designers of circuits try to minimize stray inductance as much as possible. They do this by keeping the leads of electronic components very short and grouping components in such a way to eliminate capacitive coupling.

A good example of this is a capacitor. When buying a capacitor fresh from a manufacturer, the leads of the capacitor are usually quite long.

Capacitor with long leads

The anode and the cathode leads can extend down for several inches on capacitors, as can be seen from the illustration above.

However, plugging a capacitor with the leads this long into a circuit can create some issues in the circuit.

When you have capacitors that have very long leads close to each other, these leads are, essentially, wires. When you have wires close to each other in circuits, they can produce an inductive effect. Even a small amount of wire can have considerable inductance. When you have this, the inductance can serve to impede or block high-frequency signals. This is because inductance or a device that acts as an inductor has high impedance to high-frequency signals. Thus, it is difficult for high-frequency signals to pass through a circuit that exhibits inductive qualities. When you add unwanted inductance into a circuit, the circuit can now act (again unwanted) to block high-frequency signals. Thus, a capacitor, which is a device that has low reactance at high frequencies, may now impede high-frequency signals from passing through. If this is radio circuit or an audio circuit, entire ranges of frequencies may be blocked, which of course can cause poor transmission of numerous frequencies. This is why it is desired that no stray inductance should exist in a circuit.

Therefore, this is the reason capacitor leads must be kept short, less than 1.5mm in length ideally, to effectively stop inductive effects, which can limit a capacitor's ability to pass high-frequency signals.

Capacitor with short leads

Surface mount capacitors are even better to use in circuits because their lead-less terminals being placed directly on the power plane of a circuit stops almost all inductance. This is because surface mount components have no leads, but just terminals. This will limit the inductance that capacitors can emit and will allow them to pass high frequency signals better in a circuit.

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