Equivalent series resistance

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see Equivalent series inductance for a related discussion

Equivalent series resistance (ESR) is an effective resistance that is used to describe the resistive parts of the impedance of certain electrical components.

[edit] Overview

The theoretical treatment of devices such as capacitors and inductors tends to assume they are ideal or "perfect" devices, contributing only capacitance or inductance to the circuit. However, all physical devices are constructed of materials with finite electrical resistance, which means that physical components contain some resistance in addition to their other properties.

Q factor is quoted in inductor datasheets, which is a more convenient way (than analyzing ESR) to show the typical high-frequency deviations from the ideal in the inductor magnetics and losses.

The physical origins of ESR depend on the characteristics of the device in question. An easy way to deal with these inherent resistances in circuit analysis is to use a lumped element model to express each physical component as a combination of an ideal component and a small resistor in series, the resistor having a value equal to the resistance present in the non-ideal, physical device.

[edit] Causes of ESR

ESR is properly the real resistive component of the complex impedance Z(ω) = R + j X(ω) of the device; this complex impedance can involve several relatively minor resistances, inductances and capacitances. These small deviations from the ideal behavior of the device can become significant when it is operating under certain conditions, i.e. high frequency, high current, or temperature extremes. Different ways have been developed to represent the non-ideal behavior of electrical components, ESR being just one of them and each tailored to the typical way a component is used.

Inductors also have resistance inherent in the metal conductor, quoted as DCR in datasheets. This metallic resistance is small (typically below 1 Ω). The DC resistance is an important parameter in switch-mode power supply design. It can be modeled as a resistor in series with the inductor, therefore often leading to the DC resistance being referred to as the ESR. Though this is not precisely correct usage, the unimportant elements of ESR are often neglected in circuit discussion, since it is rare that all elements of ESR are significant to a particular application.

An inductor using a core to increase inductance will have other losses in the core, such as hysteresis and eddy current losses, in addition to wire resistance, that are also represented as a higher ESR.

In a capacitor, the metallic resistance of the leads and electrodes comprise the portion of ESR commonly quoted in datasheets. Which portion of ESR is significant depends mainly on the application. Typically quoted values of ESR for capacitors is between 0.01 and 0.1 Ω.