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A capacitor is “healthy” if, once safely removed and discharged, it measures within ±10–20 % of its name-plate value on the multimeter’s capacitance range and shows a rising-to-infinite resistance on the Ω range—any large deviation, constant low Ω or constant ∞ Ω indicates short, leak or open failure respectively [1][2].
Preparation
• Isolate and discharge: clip a 1 kΩ–10 kΩ/1–5 W resistor across the leads for 3–30 s (large cans may need longer). Verify < 1 V remains before touching [3].
• Remove from circuit whenever possible—parasitic paths can mislead readings by ≥30 % [4].
Capacitance-mode test (most accurate)
a) Dial to “C ⊢⊣” or “μF”.
b) Plug the part into meter sockets or clip probes (observe polarity for electrolytics).
c) Wait for the value to settle; compare with the printed rating and its tolerance band.
– Good: within spec (typically ±10 % film, ±20 % aluminium electrolytic).
– Bad: < 80 % rated, meter shows “OL”, erratic value or huge dispersion.
Why it works: “The DMM charges the capacitor with a known current, measures the resulting voltage, then calculates capacitance” —Fluke [1].
Resistance/continuity test (basic health check, all meters)
a) Select the highest Ω range (≥1 MΩ).
b) Probe the part; watch the reading for 10–30 s.
– Good (> 1 µF): starts low (< 100 Ω) then climbs steadily toward “OL” as the cap charges.
– Short: stays near 0 Ω, continuity beeper latched.
– Open/small value: frozen at “OL” instantly.
c) Reverse the probes; a good polarized capacitor reproduces the same “charge curve” in the opposite direction.
Optional DC-leak/retention test
Charge to ≤ 50 % of rated voltage, remove the source, then measure decay. A drop < 10 % after 60 s is typical; faster loss implies leakage [5].
Interpretation quick-look
| Meter mode | Good behaviour | Short | Open | Leaky |
|---|---|---|---|---|
| Capacitance | ±10–20 % of label | — / OL | 0 – few nF | Value OK but see Ω test |
| Ω (high range) | Rising → OL | Stays 0–10 Ω | Stays OL | Stabilises at mid-Ω (tens–hundreds kΩ) |
| Continuity | Single chirp | Continuous beep | Silent | Brief beep, then silence |
• Field data show that aluminium electrolytics cause ≈ 40 % of power-supply failures in IT equipment [6]; hence rapid multimeter screening of bulk caps remains a frontline maintenance practice.
• Modern mid-range DMMs now reach 40 mF ranges and auto-detect polarity, shrinking the gap to dedicated LCR meters [7].
• “High ESR with nominal capacitance is the silent killer” —Keysight [2]. ESR meters and impedance analysers (< 100 kHz) are therefore gaining popularity for in-circuit checks.
Common pitfalls: forgetting residual charge, misreading tolerance code (e.g., “K” = ±10 %), measuring tiny (pF) ceramics with long test leads (adds tens of pF stray).
• High-voltage capacitors (> 400 V) in SMPS or motor drives can store lethal energy; local regulations (e.g., OSHA 29 CFR 1910.147) require lock-out/tag-out and proof of zero energy before servicing.
• Never apply meter probes to “live” power capacitors – catastrophic arc-flash risk.
• Affordable handheld meters still lack ESR accuracy below 0.05 Ω; a niche for low-cost impedance spectroscopy exists.
• Ageing models for polymer electrolytics under high ripple remain scarce; longitudinal studies would aid predictive maintenance.
• Discharge, isolate, and visually inspect every capacitor first.
• Use capacitance mode for the numeric value; ±10–20 % = OK.
• Use high-range Ω test for shorts, opens and leakage—watch the resistance climb.
• Confirm suspect electrolytics with an ESR check; they top failure statistics in power electronics.
• Follow safety standards, especially above 50 V; the stored energy can be lethal.
Properly applied, a common multimeter detects the majority of capacitor faults in under a minute, saving time and preventing downstream circuit damage.
Sources
[1] Fluke Corporation – “How to Measure Capacitance with a Digital Multimeter” (2023)
[2] Keysight Technologies – Application Note 5992-3987EN “Testing Capacitors with a Hand-Held DMM” (2022)
[3] Tameson – “Capacitor Multimeter Testing Guide” (2024)
[4] wikiHow – “5 Ways to Test a Capacitor” (accessed 2024-06-05)
[5] Electrical4U – “Leakage Current Test of Capacitor” (2023)
[6] B. Pecht et al., IEEE Trans. Reliability, “Failure Analysis of SMPS Capacitors”, vol 70 no 1, 2021.
[7] Extech Instruments – DMM Comparison Sheet, rev. 2024.
