Capacitor Characteristics

Leakage Current

An ideal capacitor blocks direct current (DC) completely after it becomes fully charged. In reality, every capacitor allows a very small amount of DC current to pass through its dielectric. This unwanted current is known as leakage current. Leakage current is usually extremely small in high-quality capacitors, but it becomes increasingly important in precision electronic circuits, battery- powered equipment, timing circuits and long-term energy storage applications.

Capacitor Leakage Current

What Is Leakage Current?

Leakage current is the small DC current that continues to flow through a capacitor even after it has reached its fully charged state.

It occurs because the dielectric material is not a perfect insulator. Every real dielectric has a very high—but finite—resistance.

Equivalent Circuit

A practical capacitor can be represented as an ideal capacitor with a large resistance connected in parallel. 


          Leakage Resistance
             │
             ├──/\/\/\/\/──┐
             │             │
-------------┴------||-----┴------------

The parallel resistance represents the imperfect insulation of the dielectric. Higher insulation resistance results in lower leakage current.

What Causes Leakage Current?

Cause Description
Dielectric Imperfections Microscopic defects allow a small current to flow.
Moisture Humidity can reduce insulation resistance.
Ageing The dielectric may deteriorate over time.
High Temperature Leakage current generally increases with temperature.
Over-voltage Electrical stress can damage the dielectric.

Leakage Current by Capacitor Type

Capacitor Type Typical Leakage
Ceramic Extremely Low
Silver Mica Extremely Low
Polypropylene Film Very Low
Polyester Film Very Low
Tantalum Low
Electrolytic Moderate
Supercapacitor Relatively High

Why Leakage Current Matters

  • Reduces battery life.
  • Causes stored charge to disappear over time.
  • Reduces timing accuracy.
  • Increases power consumption.
  • Affects precision analogue circuits.
  • Limits long-term energy storage.

Factors Affecting Leakage Current

Factor Effect
Temperature Higher temperature usually increases leakage.
Applied Voltage Leakage often increases near the rated voltage.
Capacitance Larger capacitors generally have higher leakage.
Age Older capacitors often develop more leakage.
Dielectric Material Different materials have different insulation properties.

How Leakage Current Is Measured

  1. Apply the rated DC voltage to the capacitor.
  2. Allow sufficient charging time.
  3. Measure the remaining DC current with a sensitive ammeter or leakage tester.
  4. Compare the measured value with the manufacturer's specification.

Leakage current is usually measured in microamperes (µA) or nanoamperes (nA).

Insulation Resistance vs Leakage Current

Characteristic Leakage Current Insulation Resistance
Measured In µA or nA MΩ or GΩ
Desired Value Low High
Relationship Higher leakage means lower insulation resistance. Higher insulation resistance means lower leakage.

Common Faults

Fault Possible Cause
Excessive leakage current Ageing or dielectric damage.
Electrolyte leakage Seal failure.
Short circuit Dielectric breakdown.
Poor insulation resistance Moisture or contamination.
Intermittent leakage Mechanical damage.

Applications Requiring Low Leakage

  • Precision timers.
  • Sample-and-hold circuits.
  • Medical equipment.
  • Battery-powered electronics.
  • Memory backup circuits.
  • Measurement instruments.
  • Space and aerospace electronics.
  • Energy storage systems.

Real-World Examples

Equipment Importance of Low Leakage
Digital Multimeter Maintains measurement accuracy.
RTC Backup Circuit Extends backup operating time.
Medical ECG Equipment Reduces measurement errors.
Solar Data Logger Minimises battery drain.
Precision Oscilloscope Improves analogue accuracy.

Key Points

  • No real capacitor is a perfect insulator.
  • Leakage current is the small DC current that flows through the dielectric.
  • Good capacitors have high insulation resistance and very low leakage current.
  • Leakage current increases with temperature, ageing and dielectric damage.
  • Low leakage is essential in precision and battery-powered electronic equipment.

Next Lesson

Continue by learning about Ripple Current, including how alternating current flowing through a capacitor generates heat and why ripple-current ratings are critical in power supply design.

Next Lesson → Ripple Current