Capacitors and inductors as used in electric circuits are not ideal components with only capacitance or inductance. However, they can be treated, to a very good degree of approximation, as being ideal capacitors and inductors in series with a resistance; this resistance is defined as the equivalent series resistance (ESR). If not otherwise specified, the ESR is always an AC resistance, which means it is measured at specified frequencies, 100 kHz for switched-mode power supply components, 120 Hz for linear power-supply components, and at its self-resonant frequency for general-application components. Additionally, audio components may report a "Q factor", incorporating ESR among other things, at 1000 Hz.

Overview

Electrical circuit theory deals with ideal resistors, capacitors and inductors, each assumed to contribute only resistance, capacitance or inductance to the circuit. However, all components have a non-zero value of each of these parameters. In particular, all physical devices are constructed of materials with finite electrical resistance, so that physical components have some resistance in addition to their other properties. The physical origins of ESR depend on the device in question.

One 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 ESR. The ESR can be measured and included in a component's datasheet. To some extent it can be calculated from the device properties.

Q factor, which is related to ESR and is sometimes a more convenient parameter than ESR to use in calculations of high-frequency non-ideal performance of real inductors, is quoted in inductor data sheets.

Capacitors, inductors, and resistors are usually designed to minimise other parameters. In many cases this can be done to a sufficient extent that parasitic ESR can increase enough to cause circuit malfunction and even component damage, although measured capacitance may remain within tolerance. While this happens with normal aging, high temperatures and large ripple current exacerbate the problem. In a circuit with significant ripple current, an increase in ESR will increase heat accumulation, thus accelerating aging.

Electrolytic capacitors rated for high-temperature operation and of higher quality than basic consumer-grade parts are less susceptible to become prematurely unusable due to ESR increase. A cheap electrolytic capacitor may be rated for a life of less than 1000 hours (6 weeks) at 85 °C. Higher-grade parts are typically rated at a few thousand hours at maximum rated temperature, as can be seen from manufacturers' datasheets. If ESR is critical, specification of a part with higher temperature rating, "low ESR" or larger capacitance than is otherwise required may be advantageous. There is no standard for "low ESR" capacitor rating.

Polymer capacitors usually have lower ESR than wet-electrolytic of same value, and stable under varying temperature. Therefore, polymer capacitors can handle higher ripple current. From about 2007 it became common for better-quality computer motherboards to use only polymer capacitors where wet electrolytics had been used previously.

The ESR of capacitors larger than about 1 μF is easily measured in-circuit with an ESR meter.

{| class="wikitable"

|+ Typical values of ESR for capacitors

|-

! Type !! 22μF !! 100μF !! 470μF

|-

| Standard aluminum ||align="right"| 7–30Ω ||align="right"| 2–7Ω ||align="right"| 0.13–1.5Ω

|-

| Low-ESR aluminum ||align="right"| 1–5Ω ||align="right"| 0.3–1.6Ω

|-

| Solid aluminum ||align="right"| 0.2–0.5Ω

|-

| Sanyo OS-CON ||align="right"| 0.04–0.07Ω ||align="right"| 0.03–0.06Ω

|-

| Standard solid tantalum ||align="right"| 1.1–2.5Ω ||align="right"| 0.9–1.5Ω

|-

| Low-ESR tantalum ||align="right"| 0.2–1Ω ||align="right"| 0.08–0.4Ω

|-

| Wet-foil tantalum ||align="right"| 2.5–3.5Ω ||align="right"| 1.8–3.9Ω

|-

| Stacked-foil film ||align="right"| < 0.015Ω

|-

| Ceramic ||align="right"| < 0.015Ω

|}

See also

  • Capacitor plague
  • Polymer capacitor
  • Dissipation factor
  • RC circuit
  • Output impedance
  • Equivalent series inductance (ESL)

References

  • Determining the Equivalent Series Resistance (ESR) of Capacitors
  • Application note of ESR of capacitors
  • "Capacitors: Equivalent Series Resistance (ESR)", General Atomics Engineering Bulletin, pp.&nbsp;2–9
  • ESR Calculator

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