Capacitor Maintenance

Testing Capacitors

Capacitors can fail due to age, excessive temperature, overvoltage, high ripple current or manufacturing defects. Testing a capacitor helps determine whether it is still suitable for use or needs replacement. Depending on the type of capacitor and the equipment available, testing may include measuring capacitance, equivalent series resistance (ESR), leakage current or observing circuit behaviour.

Testing Capacitors

Safety First

  • Always disconnect power before testing.
  • Discharge large capacitors using a suitable resistor before handling them.
  • Never short high-voltage capacitors with a screwdriver.
  • Wear eye protection when working with high-energy power supplies.
  • Observe capacitor polarity during testing and replacement.

Visual Inspection

Many faulty capacitors can be identified without using test equipment.

Observation Possible Problem
Bulging top Internal gas pressure from capacitor failure.
Leaking electrolyte Seal failure or overheating.
Burn marks Overheating or overvoltage.
Cracked casing Mechanical or thermal damage.
Loose terminals Physical damage.

Testing with a Digital Multimeter

Many digital multimeters include a capacitance measurement function.

  1. Disconnect the capacitor from the circuit if possible.
  2. Fully discharge the capacitor.
  3. Select the capacitance measurement mode.
  4. Connect the probes to the capacitor terminals.
  5. Compare the measured value with the rated capacitance.

A reading within the manufacturer's specified tolerance usually indicates a healthy capacitor, although this test does not measure ESR.

Testing with an ESR Meter

An Equivalent Series Resistance (ESR) meter measures the capacitor's internal resistance. This is one of the most effective methods for diagnosing ageing electrolytic capacitors.

ESR Result Condition
Very low Usually good.
Slightly increased Normal ageing.
High ESR Capacitor is deteriorating.
Extremely high or open Capacitor has failed.

Many ESR meters can test capacitors while they remain in the circuit, provided there are no parallel components that significantly affect the reading.

Testing with an Ohmmeter

If a capacitance meter is unavailable, a basic resistance test can provide a simple indication of capacitor condition.

  1. Discharge the capacitor.
  2. Select a high resistance range.
  3. Connect the probes to the capacitor.
  4. The resistance should start low and gradually increase as the capacitor charges.
Reading Possible Condition
Resistance increases to infinity Likely normal.
Constant low resistance Short-circuited capacitor.
Infinite resistance immediately Open capacitor.

This method provides only a rough indication and is not suitable for precision testing.

Testing with an Oscilloscope

An oscilloscope allows ripple voltage and noise to be observed directly in a working circuit.

Typical indications include:
  • Excessive ripple on a power supply output.
  • Oscillation caused by faulty decoupling capacitors.
  • Poor filtering in switch-mode power supplies.
  • Unstable regulator output.

Testing in Circuit

Capacitors can often be tested without removing them from the PCB using an ESR meter. However, capacitance measurements made in circuit may be affected by parallel components.

If the reading is doubtful, remove one lead from the circuit or desolder the capacitor completely for a more accurate measurement.

Common Capacitor Faults

Fault Symptoms
Reduced Capacitance Increased ripple, weak bass, timing errors.
High ESR Heating, unstable power supply, poor filtering.
Leakage Current Battery drain, timing problems.
Short Circuit Fuse blows or power supply overload.
Open Circuit No filtering or signal coupling.

Typical Symptoms in Equipment

Equipment Possible Capacitor Problem
Audio Amplifier Hum, distortion or reduced bass.
Television Failure to start or unstable picture.
Computer Power Supply Random shutdowns or failure to boot.
LED Driver Visible flicker.
Motor Controller Erratic operation.

When to Replace a Capacitor

  • Capacitance is outside the specified tolerance.
  • ESR is significantly higher than expected.
  • The capacitor is bulging or leaking.
  • The casing is cracked or damaged.
  • The equipment shows symptoms consistent with capacitor failure.

Repair Tips

  • Replace with the same capacitance unless a design change is justified.
  • Use an equal or higher voltage rating.
  • Choose 105°C capacitors for power supplies.
  • Use low-ESR capacitors where originally specified.
  • Observe polarity on electrolytic capacitors.
  • After replacement, inspect solder joints and clean any flux residue.

Key Points

  • Always inspect capacitors visually before taking measurements.
  • ESR testing is one of the best methods for evaluating electrolytic capacitors.
  • Capacitance, ESR and leakage current together provide the best assessment of capacitor health.
  • Many capacitor faults can be diagnosed without removing the component from the circuit.
  • Replacing faulty capacitors promptly improves equipment reliability.

Next Lesson

Continue by learning about Capacitor Failures, including common failure modes, their causes and how to prevent them.

Next Lesson → Capacitor Failures