Capacitors are one of the most common sources of failure in electronics — power supply filter capacitors bulge and lose capacitance, motor-run capacitors degrade and cause starting problems, and coupling capacitors go leaky and corrupt signals. Knowing how to test a capacitor quickly and accurately saves hours of troubleshooting. This guide covers every method from a basic multimeter continuity check to a full LCR meter analysis, including in-circuit testing.
Always discharge capacitors before testing
Step 1: Visual Inspection
Start with your eyes. Many failed capacitors announce themselves physically before any electrical test is needed:
- Bulged top: The vent score on the top of an electrolytic capacitor is designed to expand when internal pressure builds. A dome-shaped or cracked top means the electrolyte has vented — the capacitor has failed. Replace it immediately.
- Electrolyte residue: Brown or black residue around the base of a capacitor on the PCB is electrolyte that leaked through the bottom seal. The capacitor is failed and the surrounding PCB traces may be corroded.
- Burn marks: Scorch marks indicate the capacitor overheated — from overcurrent, reverse polarity, or overvoltage. Failed.
- Split or cracked body: Ceramic, film, and tantalum capacitors can crack under mechanical stress. A cracked capacitor is unreliable even if it still tests within spec.
If the capacitor passes visual inspection, proceed with electrical testing.
Method 1: Multimeter Capacitance Mode
Modern digital multimeters include a capacitance measurement function — usually the symbol F or a range labelled nF/μF. This is the quickest check but it only tells you the capacitance value; it does not test ESR, leakage, or dissipation factor.
- Power down the circuit and discharge the capacitor completely.
- Desolder at least one lead, or remove the capacitor from the circuit entirely. In-circuit readings are almost always wrong because parallel paths affect the measurement.
- Set the multimeter to the capacitance range that covers the expected value. Auto-ranging meters select this automatically.
- Connect the probes — observe polarity for electrolytics (positive probe to positive lead marked with the longer leg or the side opposite the stripe).
- Read the display and compare to the marked capacitance and tolerance (typically ±20% for electrolytics, ±5% or ±10% for film caps).
| Reading | What It Indicates | Action |
|---|---|---|
| Within tolerance (±20% electrolytic, ±5–10% film) | Capacitance value is good | Test ESR separately if in a critical application |
| 10–30% low (e.g. 68 μF instead of 100 μF) | Electrolyte drying / partial failure | Replace in any power supply or audio application |
| More than 30% low or near zero | Open circuit — capacitor has failed open | Replace immediately |
| Overrange or very high value | Short circuit — plates have contacted | Replace immediately; check for PCB damage |
| Fluctuating or unstable reading | Intermittent internal connection | Replace |
Method 2: Resistance Mode (Leakage Check)
A capacitor should block DC. If it passes significant DC current, it has a leakage fault — the dielectric has degraded or broken down. A multimeter in resistance mode can detect this, though it is a qualitative test rather than a precision measurement.
- Discharge the capacitor and set the multimeter to its highest resistance range (typically 20 MΩ or 200 MΩ).
- Connect the probes — observe polarity for electrolytics.
- Watch the display: the resistance should start low (the meter charges the capacitor) and rise steadily toward "OL" (open loop / overrange) as the cap charges. This charging behaviour confirms the capacitor is not short-circuited.
- If the resistance settles at a finite value (e.g., 100 kΩ or 1 MΩ) instead of rising to overrange, the capacitor has DC leakage. The lower the settled value, the worse the leakage.
Polarity matters for this test on electrolytics
Method 3: ESR Testing — The Most Important Test for Electrolytics
Equivalent Series Resistance (ESR) is the internal resistance of a capacitor — it represents the combined resistance of the electrolyte, the foil, the leads, and the oxide layer. In new, healthy electrolytic capacitors, ESR is typically a few milliohms to a few ohms depending on size and type. As the electrolyte dries out or degrades, ESR rises — sometimes to tens of ohms — while the capacitance value may still appear correct on a basic meter.
High ESR is responsible for:
- Power supply ripple voltage increasing (cap can't charge/discharge fast enough)
- Capacitor overheating from I²R losses at the ripple frequency
- Audio amplifier distortion and oscillator instability
- Motor-start capacitor failure — the motor hums but won't start
ESR can be measured in-circuit with a dedicated ESR meter, without desoldering. Most ESR meters apply a very low-voltage AC signal at 100 kHz — at this frequency, the capacitive reactance of a large electrolytic is essentially zero, so the instrument reads almost pure ESR. This is one of the fastest diagnostic tools in electronics servicing.
| Capacitor size | New ESR (typical) | Replace above |
|---|---|---|
| 1 μF – 10 μF, 50 V | 5 – 15 Ω | 20 Ω |
| 47 μF – 100 μF, 25 V | 1 – 5 Ω | 8 Ω |
| 220 μF – 1000 μF, 25 V | 0.2 – 1 Ω | 2 Ω |
| 2200 μF – 4700 μF, 25 V | 0.05 – 0.3 Ω | 0.5 Ω |
| Motor-run film cap, 1 μF – 50 μF | < 0.5 Ω at 50 Hz | 1 Ω at 50 Hz |
Method 4: LCR Meter (Most Complete Test)
An LCR meter gives you the complete picture: capacitance, ESR, dissipation factor (D), and quality factor (Q) all in one measurement. For incoming inspection or precision work, this is the only acceptable method.
- Discharge the capacitor and remove from circuit.
- Run open and short compensation on the test fixture.
- Select the test frequency: 100 Hz for large electrolytics (this is the ripple frequency they'll see), 1 kHz for film caps, 1 MHz for small ceramics.
- Connect the capacitor — observe polarity for electrolytics.
- Read Cs (series capacitance) and D (dissipation factor). For a healthy electrolytic, D should be below 0.1. For film capacitors, D should be below 0.01. For good ceramic caps, below 0.05.
Testing Motor-Run and Motor-Start Capacitors
Single-phase AC motors use capacitors to create a phase shift for starting and running. These capacitors are subject to both high voltage (typically 250–440 V AC) and mechanical vibration, and they fail more often than almost any other component in industrial equipment.
Symptoms of a failed motor capacitor: motor hums loudly but won't start; motor starts if given a manual push; motor draws high current; capacitor body is swollen or shows white powder residue.
To test a motor-run capacitor with a multimeter:
- Disconnect power completely. The capacitor is connected directly to mains voltage — treat it as live until you have confirmed it is discharged.
- Discharge through a 10 kΩ, 5 W resistor held across the terminals for 30 seconds.
- Measure capacitance and compare to the marking. Motor-run capacitors are typically rated at ±5% — a 25 μF capacitor reading below 22 μF should be replaced.
- Measure insulation resistance from each terminal to the case using a 500 V insulation tester. Should exceed 100 MΩ. Low insulation indicates the capacitor is unsafe and must be replaced.
In-Circuit Testing Limitations
Testing capacitors in-circuit without desoldering is possible but unreliable. Parallel components create parallel paths that appear as additional capacitance or reduced resistance. An ESR meter is the most reliable in-circuit tool because the ESR measurement is dominated by the lowest-impedance path — the capacitor being tested — and most circuit components in parallel have much higher impedance at 100 kHz.
Even with an ESR meter, any reading that seems unusually low should be confirmed by desoldering one lead. In-circuit ESR testing is a screen, not a definitive test.