Capacitor Characteristics

Equivalent Series Resistance (ESR)

Equivalent Series Resistance (ESR) is the small internal resistance present inside every real capacitor. Although an ideal capacitor has zero resistance, practical capacitors contain resistive losses caused by the electrodes, dielectric material, electrolyte and internal connections. ESR is one of the most important characteristics when selecting or testing capacitors for power supplies, audio amplifiers, switch-mode power supplies (SMPS) and high-frequency circuits.

Equivalent Series Resistance

What Is ESR?

Equivalent Series Resistance represents the total internal resistance of a capacitor. Electrically, it behaves as though a small resistor were connected in series with an ideal capacitor.

The lower the ESR, the more efficiently the capacitor can charge, discharge and filter ripple currents.

Equivalent Circuit

A practical capacitor can be represented by the following equivalent circuit: 

      ESR
---/\/\/\/\----||----

The resistor represents the Equivalent Series Resistance, while the capacitor symbol represents the ideal capacitance.

What Causes ESR?

Source Description
Electrode Resistance Resistance of the metal plates and leads.
Electrolyte Resistance Present in electrolytic capacitors.
Dielectric Losses Small energy losses in the dielectric material.
Internal Connections Resistance of welds and terminals.
Ageing ESR generally increases as capacitors age.

Why ESR Is Important

  • Determines ripple current capability.
  • Affects heat generation.
  • Influences power supply performance.
  • Reduces filtering effectiveness if too high.
  • Can cause instability in voltage regulators.
  • Directly affects capacitor efficiency.

Effects of High ESR

Problem Result
Poor filtering Higher ripple voltage.
Excessive heating Reduced capacitor life.
Voltage instability Power supply malfunction.
Audio hum Poor smoothing in amplifier power supplies.
SMPS failure Difficulty starting or unstable operation.

Typical ESR by Capacitor Type

Capacitor Type Typical ESR
Ceramic Very Low
Polypropylene Film Very Low
Polyester Film Low
Silver Mica Very Low
Tantalum Low
General Electrolytic Moderate
Low-ESR Electrolytic Very Low
Supercapacitor Low to Moderate

How ESR Changes Over Time

ESR normally increases as electrolytic capacitors age. This is mainly caused by the gradual drying of the electrolyte and changes in the internal materials.

Increasing ESR is one of the most common reasons for failure in power supply capacitors.

Measuring ESR

ESR is measured using an ESR meter or an LCR meter.

  1. Disconnect power from the equipment.
  2. Discharge the capacitor safely.
  3. Connect the ESR meter.
  4. Read the ESR value.
  5. Compare the reading with the manufacturer's specifications or a reliable ESR chart.

Typical Applications Requiring Low ESR

  • Switch-mode power supplies.
  • Motherboards.
  • Graphics cards.
  • DC-DC converters.
  • Audio amplifiers.
  • LED power supplies.
  • Solar inverters.
  • Industrial controllers.

Common ESR Faults

Fault Possible Cause
High ESR Ageing or dried electrolyte.
Intermittent ESR Internal connection failure.
Excessive heating High ripple current.
Bulging capacitor Internal gas generation.
Leakage Seal failure.

Replacing Capacitors with High ESR

  • Use the same capacitance value.
  • Choose the same or a higher voltage rating.
  • Select a capacitor with equal or lower ESR where appropriate.
  • Use capacitors with suitable ripple current ratings.
  • Prefer 105°C capacitors for power supply applications.

Real-World Examples

Equipment Common ESR Problem
LCD Television Power supply fails to start.
Computer Motherboard Random resets or instability.
Audio Amplifier Excessive hum.
LED Driver Output flickering.
SMPS Poor voltage regulation.

Key Points

  • Every real capacitor has some ESR.
  • Low ESR improves efficiency and filtering performance.
  • High ESR often indicates ageing, especially in electrolytic capacitors.
  • ESR is a critical specification in switching power supplies.
  • An ESR meter is one of the most useful tools for electronics repair.

Next Lesson

Continue by learning about Ripple Current, including how AC ripple affects capacitors, why ripple-current ratings are important and how excessive ripple shortens capacitor life.

Next Lesson → Ripple Current