Equivalent Series Resistance (ESR): Capacitor Loss Metric
Equivalent series resistance (ESR) represents the resistive component that appears in series with an ideal capacitor. Manufacturers specify ESR in milliohms or ohms at a given frequency and temperature, because losses increase the effective impedance and generate heat under AC ripple currents.
Designers convert ESR into power dissipation using I²R. Comparing that heat to the component’s ripple-current rating ensures electrolytic, polymer, or MLCC devices run below their maximum core temperature.
Definition and Measurement
ESR consolidates electrode resistance, lead resistance, and dielectric loss angles into a single series element. LCR meters measure impedance Z at a test frequency (often 100 kHz) and derive ESR from the real part of Z. Some datasheets also publish dissipation factor tanδ, where ESR = tanδ / (ωC).
Bridge-based measurement techniques—such as the Schering bridge—originated in the early 1900s to characterise insulation. Modern impedance analyzers automate balancing, but the same concept applies.
Historical Perspective
ESR gained prominence during the switch-mode power supply boom of the 1980s and 1990s. Higher frequencies exposed hidden losses in electrolytic capacitors, leading to failures dubbed “capacitor plague.” Manufacturers responded with low-ESR polymers and multilayer ceramics, and engineers began specifying ESR explicitly rather than relying on capacitance alone.
Today’s automotive and aerospace programs often require ESR verification across temperature sweeps, ensuring reliability in vibration and thermal cycling environments.
Concepts and Relationships
Frequency Dependence
ESR typically decreases with frequency up to a minimum point before parasitic inductance dominates. Manufacturers plot impedance and ESR versus frequency so designers can pick devices near their self-resonant frequency for decoupling.
Thermal Considerations
Elevated temperatures increase electrolyte conductivity but accelerate aging. Using Ohm’s law calculators to estimate ripple current at worst-case load steps prevents runaway heating and extends service life.
Quality Factor and Bandwidth
Resonant tanks combine inductance, capacitance, and ESR. The quality factor Q equals reactance divided by ESR. Low ESR yields high Q, narrow bandwidth, and efficient energy storage, while high ESR damps oscillations—useful in snubber networks.
Applications
Power Conversion
Buck and boost converters rely on low-ESR capacitors to minimize output ripple. When evaluating EMI filters, ESR informs damping choices that suppress resonant spikes.
Signal Integrity
Digital systems use MLCCs with sub-10 mΩ ESR to shunt high-frequency transients. ESR directly affects impedance targets in power-distribution networks modeled with the LC resonance calculator.
Reliability Screening
Maintenance crews monitor ESR drift to detect dry-out in electrolytic capacitors before catastrophic failure. Increasing ESR causes voltage droop and heat, making predictive replacement possible.
Importance
ESR is the key indicator that bridges ideal component values with real-world performance. It determines efficiency, noise, and thermal stress in almost every circuit containing capacitors.
By tracking ESR versus frequency, temperature, and lifetime, engineers avoid unexpected outages and meet stringent automotive, medical, or aerospace standards.