To check capacitors with a multimeter, set your digital multimeter (DMM) to the capacitance mode (marked with the -| |- symbol), ensure the capacitor is fully discharged, and place the red probe on the anode (+) and black probe on the cathode (-) for polarized types. A good reading falls within ±20% of the printed microfarad (µF) rating on the casing. If your meter lacks a capacitance setting, you can use the resistance (Ohms) mode to check for internal shorts and basic charge retention, though this will not yield a precise µF value.

Safety First: Discharging and CAT Ratings

WARNING: Mains and High-Voltage DC Hazard
Capacitors in mains-powered appliances (microwaves, HVAC units, PC power supplies) can store lethal charges for weeks after being unplugged. A 400V 100µF capacitor holds enough energy to cause severe shock or ventricular fibrillation. Never assume a capacitor is dead just because the device is unplugged.

Before testing, you must safely discharge the capacitor. For low-voltage DC circuits (under 50V), briefly shorting the leads with an insulated screwdriver is acceptable for small values. For mains-voltage or high-capacitance electrolytics, use a high-wattage bleeder resistor (e.g., a 20kΩ 5W ceramic resistor) connected across the leads for 10 to 30 seconds to dissipate the energy without damaging the capacitor's internal dielectric.

Safety Category (CAT) Requirements: If you are probing a board connected to mains voltage (even if powered off, if it remains plugged in or hardwired), your multimeter and test leads must be rated for the environment. According to Fluke's safety guidelines on CAT ratings, use a minimum of CAT III 600V for fixed appliance wiring and distribution boards, or CAT IV 600V if measuring at the service entrance or outdoor HVAC disconnect. For isolated, unplugged low-voltage DC boards (like a 12V Arduino project), a CAT II meter is sufficient.

Meter Setup and Probe Placement

DMM Configuration for Capacitance

  • Dial Position: Set to Capacitance (-| |-). If your meter has a dedicated capacitance jack, use it; otherwise, use the standard V/Ω jack.
  • Lead Jacks: Black lead to COM. Red lead to V/Ω/Hz (or the dedicated Cx/µF jack on older or budget meters like the DT830B).
  • Range Setting: Auto-ranging is preferred. If using a manual ranging meter, start at the 200µF or 2mF range and step up if the display shows 'OL' (Over Limit).
  • Zeroing (Crucial for small caps): Touch the probe tips together and press the REL (Relative) or Zero button. This subtracts the inherent capacitance of your test leads (typically 0.1nF to 0.5nF), which will otherwise ruin your readings when testing small ceramic capacitors.

Probe Placement: For polarized capacitors (aluminum electrolytic, tantalum), place the red probe on the anode (+) and the black probe on the cathode (-). The cathode is usually marked with a contrasting stripe or a minus sign on the casing. For non-polarized capacitors (ceramic, film, mica), probe placement direction does not matter.

Step-by-Step Testing Procedure

  1. Isolate the Component: Desolder and lift at least one leg of the capacitor from the PCB. Measuring in-circuit will result in parallel circuit ghost readings, as the meter will measure the combined capacitance of the target capacitor and all parallel traces/components.
  2. Discharge: Apply your bleeder resistor across the leads. Verify it is discharged by switching your DMM to DC Voltage mode and confirming a reading of < 0.1V.
  3. Zero the Meter: Short the probes and press REL to nullify lead capacitance.
  4. Measure Capacitance: Apply the probes to the leads. Wait 3 to 5 seconds for the meter's internal charging circuit to stabilize and display the final µF or nF value.
  5. Measure Resistance (Leakage Check): Switch the dial to Ohms (Ω), select the highest range (e.g., 20MΩ), and apply the probes. Watch the display: it should start at a low resistance and steadily climb until it reads 'OL' (Open Loop/Infinite). If it stays at a low, fixed resistance, the capacitor has high internal leakage and is failing.

Expected Readings: Good vs. Bad Values

Manufacturers typically specify a tolerance of ±20% for standard electrolytic capacitors and ±10% for film or precision ceramics. As noted in this guide on capacitance and reactance, a capacitor that has drifted more than 20% from its nominal value will alter the timing constants or filter cutoff frequencies of the circuit, leading to erratic behavior.

Capacitor Type Printed Rating Good Capacitance Reading Bad / Failed Reading Resistance (Leakage) Behavior
Aluminum Electrolytic 470µF 25V 376 µF to 564 µF < 350 µF, or reads 'OL' Climbs steadily to 'OL'
Ceramic Disc (104) 0.1µF (100nF) 50V 0.08 µF to 0.12 µF Reads 'OL' (open) or 0 (short) Instant 'OL' (no charge time)
Metallized Film 4.7µF 400V AC 4.2 µF to 5.1 µF < 3.8 µF (dielectric loss) Climbs steadily to 'OL'
Tantalum (Polarized) 10µF 16V 9.0 µF to 11.0 µF Reads 0 Ω (dead short) Climbs to 'OL' (if not shorted)

Common Mistakes That Give Misleading Readings

Even with a high-end bench meter, operator error can make a dead capacitor look good, or a good capacitor look dead. Avoid these bench pitfalls:

  • Measuring In-Circuit: This is the most common mistake. Parallel components (especially other capacitors and low-resistance inductor coils) will skew the reading. A 10µF cap might read as 45µF because the meter is measuring the entire power rail's decoupling network. Always lift one leg.
  • Touching the Metal Probe Tips: When testing for leakage in Ohms mode, holding the metal tips with your bare fingers introduces your body's resistance (typically 10kΩ to 100kΩ) in parallel with the capacitor. The meter will never reach 'OL', leading you to falsely diagnose the capacitor as having high internal leakage.
  • Ignoring Dielectric Absorption: If you short a large capacitor to discharge it, the dielectric material can 'relax' and regenerate a small voltage over the next few minutes. If you immediately switch to Ohms mode, this residual voltage can confuse the meter's test current, yielding erratic resistance readings. Wait a minute after discharging before testing resistance.
  • Missing the ESR Failure Mode: A standard DMM capacitance test applies a low-frequency DC charge. A dried-out electrolytic capacitor might still show a perfect 470µF capacitance on a DMM, but fail in a high-frequency switching power supply due to high Equivalent Series Resistance (ESR). If a circuit is failing but the DMM says the caps are 'good', you need a dedicated 100kHz ESR meter to check the internal health of the electrolyte.

Frequently Asked Questions About Capacitor Testing

Can I check a capacitor with a multimeter without desoldering it?

Technically you can place probes on it, but the reading will be useless for diagnostic purposes. In-circuit testing measures the parallel combination of the capacitor and the surrounding circuit traces. While an in-circuit test can sometimes confirm a dead short (if the reading is exactly 0.00 µF or 0 Ω), it cannot confirm a good capacitance value or detect subtle dielectric degradation. For reliable data, you must isolate at least one leg of the component from the PCB.

Why does my multimeter show 'OL' immediately when testing a capacitor?

An immediate 'OL' (Over Limit or Open Loop) in capacitance mode means one of three things: First, the capacitor is internally open (the lead has detached from the internal foil, a common failure in cheap electrolytics). Second, your meter's manual range is set too low (e.g., trying to read a 1000µF cap on the 200µF scale). Third, you are testing a very small ceramic capacitor (like a 10pF) that is below the minimum resolution threshold of your specific DMM, which typically bottoms out around 0.1nF to 1nF.

How do I test a dual run/start capacitor on an HVAC compressor with a multimeter?

HVAC dual capacitors have three terminals: C (Common), FAN, and HERM (Compressor). Ensure the 240V disconnect is pulled and locked out before touching anything. Discharge the terminals using a 20kΩ 5W resistor. Set your DMM to capacitance. To test the fan section, place probes on C and FAN. To test the compressor section, place probes on C and HERM. Compare the µF readings to the data plate on the side of the can (e.g., 45µF + 5µF). If either reading is more than 6% below the rated value (the standard HVAC tolerance limit per Fluke's HVAC testing guidelines), the entire can must be replaced, even if the other half tests fine.

What is the difference between testing with a multimeter vs an ESR meter?

A standard multimeter measures static capacitance (the ability to store a DC charge) and DC leakage. An ESR (Equivalent Series Resistance) meter injects a high-frequency AC signal (usually 100kHz) to measure the internal resistive losses of the capacitor without needing to desolder it from the board. As electrolytic capacitors age and their internal electrolyte dries out, their ESR spikes, causing them to overheat and fail in switching power supplies, even while their static µF capacitance remains perfectly normal. For motherboard and power supply repair, an ESR meter is mandatory; for general audio crossovers and timing circuits, a DMM capacitance test is sufficient.