To check a capacitor with a multimeter, set the dial to the capacitance setting (—| |—), safely discharge the component, and place the red probe on the anode (+) and black probe on the cathode (-). A good reading falls within the manufacturer's tolerance—typically ±20% for electrolytics—of the rated microfarad (µF) value printed on the casing. If your meter lacks a capacitance mode, you can use the resistance (Ohms) setting to observe the charge sweep, though this only reveals dead shorts or completely open failures.
Meter Setup and Safety Prerequisites (CAT Ratings)
Capacitors in HVAC systems, microwave ovens, and switch-mode power supplies store lethal electrical charges long after power is removed. A microwave high-voltage capacitor can hold 400V+ at several hundred microfarads. Always de-energize the circuit, lock out the breaker, and verify zero voltage before touching any terminals.
Before testing, you must match your multimeter's safety category to the environment. For low-voltage DC printed circuit boards (like Arduino or 12V automotive), a CAT II rated meter is sufficient. However, if you are testing HVAC run/start capacitors, washing machine motor boards, or any component tied directly to mains AC power, you must use a CAT III 600V or CAT IV 600V rated meter (such as the Fluke 87V or Brymen BM235) to protect against transient voltage spikes. For detailed safety standards, refer to the Fluke guide on capacitance measurement safety.
Meter Setup Block
- Dial Position: Set to the Capacitance symbol (—| |—). If unavailable, set to the highest Resistance range (e.g., 2MΩ or 20MΩ).
- Lead Jacks: Black lead to COM. Red lead to V/Ω/Cap (some meters require the mA/µA jack for capacitance; check your specific manual, though V/Ω is standard for modern DMMs).
- Range: Auto-ranging is standard. If using a manual ranging meter, select a range one decade higher than the expected value (e.g., use the 200µF range to test a 47µF capacitor).
The Proper Discharge Method
Never discharge a capacitor by shorting it with a screwdriver. This causes a violent spark, vaporizes metal, and can create micro-fractures in the internal foil that lead to premature failure. Instead, use a 20kΩ, 5-watt wirewound resistor attached to insulated alligator clips. Hold the resistor across the terminals for 5 seconds, then verify with your multimeter in DC Volts mode that the reading is 0.00V.
Expected Readings: Good vs. Failed Capacitors
When testing in capacitance mode, a 'good' reading is strictly numerical: it must fall within the tolerance band printed on the component. Electrolytic capacitors typically carry an 'M' tolerance code (±20%), while ceramics and films often carry 'K' (±10%) or 'J' (±5%). Below is the reference matrix for interpreting your multimeter's display.
| Rated Value | Tolerance | Good Reading Range | Shorted (Bad) | Open (Bad) | Leaky / High ESR (Bad) |
|---|---|---|---|---|---|
| 10 µF | ±20% (M) | 8.00 - 12.00 µF | 0.00 µF / Beep | OL / 0.00 nF | < 8.00 µF or unstable |
| 100 µF | ±20% (M) | 80.0 - 120.0 µF | 0.00 µF / Beep | OL / 0.00 nF | < 80.0 µF |
| 1000 µF | ±20% (M) | 800 - 1200 µF | 0.00 µF / Beep | OL / 0.00 nF | < 800 µF or slow charge |
| 0.1 µF (104) | ±10% (K) | 0.090 - 0.110 µF | 0.00 nF / Beep | OL | Unstable / fluctuating |
| 22 pF | ±5% (J) | 20.9 - 23.1 pF | 0.0 pF / Beep | OL | Reading drifts wildly |
Numerical Interpretation: If you are testing a 470µF motor start capacitor and your meter reads 445µF, the component is healthy (within the 376µF to 564µF acceptable band). If it reads 310µF, the internal electrolyte has dried out, capacitance has dropped, and the capacitor must be replaced. For a deeper understanding of how dielectric degradation affects these numbers, review the capacitor theory chapters on All About Circuits.
Step-by-Step Testing Procedures
Method A: Capacitance Mode (The Standard)
This is the most accurate method for determining if a capacitor holds its rated charge.
- Isolate the Component: Capacitors must be tested out of circuit. Parallel components on a PCB will sum their capacitance values, giving a false high reading. Desolder at least one leg of the capacitor to lift it from the pad.
- Discharge: Apply the 20kΩ resistor across the leads and verify 0V.
- Probe Placement: For polarized electrolytic capacitors, place the red probe on the anode (+) (the longer leg, or the side opposite the negative stripe) and the black probe on the cathode (-). For non-polarized ceramics or films, polarity does not matter.
- Read and Wait: Large capacitors (1000µF+) require the meter's internal circuitry to charge them to measure the time constant. The reading may take 3 to 8 seconds to stabilize. Do not remove the probes until the number stops climbing.
Method B: Resistance Mode (The Fallback)
If your multimeter lacks a capacitance setting, you can use the Ohms (Ω) setting to check for catastrophic failures (shorts and opens), though you cannot verify the exact µF value.
- Set the Dial: Turn the dial to the highest resistance range (2MΩ or 20MΩ).
- Probe Placement: Red to anode (+), Black to cathode (-).
- Observe the Sweep: A healthy capacitor will act like a momentary short circuit as it charges from the meter's internal battery. The display should start near 0Ω and rapidly climb until it hits OL (Open Line / Infinite).
- Diagnose: If the reading stays at 0Ω, the capacitor is internally shorted. If it immediately reads OL without sweeping, the internal connection is broken (open). If it climbs but stops at a fixed low resistance (e.g., 50kΩ) instead of OL, the dielectric is leaky.
Common Mistakes That Give Misleading Readings
A standard multimeter capacitance test applies a low-frequency, low-voltage AC signal. It will often report a perfect 100µF reading for a capacitor that has completely failed under high-frequency switching loads in a PC power supply. This is because standard DMMs cannot measure Equivalent Series Resistance (ESR). If you are repairing switch-mode power supplies or motherboard VRMs, a dedicated ESR meter is mandatory.
1. In-Circuit Measurements
Testing a capacitor while it is still soldered to a board is the most common beginner mistake. The multimeter measures the entire parallel network. If you probe a 10µF capacitor that is in parallel with a 0.1µF bypass cap and a semiconductor junction, your reading will be erratic and mathematically useless. Always lift a leg.
2. Body Parasitic Capacitance
The human body acts as a dielectric. If you pinch the metal probe tips and the capacitor leads with your bare fingers while testing small-value ceramics (e.g., 10pF to 100pF), your body adds 50pF to 150pF of stray capacitance to the circuit. Your meter will read wildly high or unstable. Use alligator clips or a dedicated component test jig for anything under 1nF.
3. Ignoring Dielectric Absorption
Large, high-voltage electrolytic capacitors exhibit dielectric absorption. After you discharge them to 0V, the dielectric material slowly releases trapped charges, causing a 'rebound' voltage of a few volts across the terminals minutes later. If you test the capacitor immediately after a heavy discharge cycle without letting it rest, the residual voltage can confuse the auto-ranging algorithm on cheaper multimeters, resulting in an 'OL' error or a severely under-reported µF value. Let large capacitors rest for 60 seconds after discharging before applying the meter probes.
