A good capacitor reads within its printed tolerance band—typically ±5% to ±20% of its microfarad (µF) rating—and shows an Equivalent Series Resistance (ESR) well below 1 ohm for standard power-supply electrolytics. If your digital multimeter (DMM) reads 'OL' (open loop) immediately, the capacitor has failed open. If it reads near 0 µF or 0 ohms and stays there, it has failed shorted.
The Direct Answer: What a Good Reading Looks Like Numerically
Before you touch a probe to a component, you need to know what number you are looking for. Capacitors are manufactured with specific tolerance codes printed on their jackets. A reading outside this mathematical window means the dielectric has degraded, even if the meter doesn't show a hard short or open.
- J Tolerance (±5%): A 100µF capacitor is good if it reads between 95µF and 105µF.
- K Tolerance (±10%): A 100µF capacitor is good if it reads between 90µF and 110µF.
- M Tolerance (±20%): The most common for aluminum electrolytics. A 100µF capacitor is good if it reads between 80µF and 120µF.
Standard DMMs measure capacitance by applying a low-frequency AC or pulsed DC test voltage and calculating the time constant. While this confirms the bulk capacitance, it will not reveal high-frequency ESR degradation—a common failure mode in switch-mode power supplies (SMPS). For power supply filtering, a capacitance reading of 95µF on a 100µF cap might look 'good' on a DMM, but if the ESR has spiked to 5 ohms, the capacitor will still cause excessive ripple and circuit failure.
Meter Setup, Safety, and CAT Ratings
Testing capacitors, especially large motor-run or mains-filter capacitors, carries a severe shock hazard if the component is not properly discharged. A 400V SMPS filter capacitor can hold a lethal charge for days after power is removed.
Never short a large capacitor with a screwdriver; this causes explosive arcing and damages the internal foil. Always discharge using a 10kΩ, 5W power resistor attached to insulated probes for 5 to 10 seconds. If you are testing in-circuit on a mains-powered board (like an AC motor run cap or SMPS input filter), your meter must be rated CAT III 600V or CAT II 1000V (e.g., Fluke 117 or Brymen BM235) per IEC 61010 safety standards. Never use a CAT I hobby meter on mains-derived circuits.
Meter Setup Block
Configure your digital multimeter exactly as follows before testing:
- Dial Position: Set to the Capacitance mode (marked with the
—||—symbol). If your meter lacks this, set it to Ohms (Ω) for a basic short/open check. - Lead Jacks: Black lead into
COM. Red lead into theV/Ω/Hz/CAPjack (some meters have a dedicatedmA/µAorCAPjack; consult your manual). - Range: Use Auto-ranging if available. If manual, set the range one decade above the expected value (e.g., select the 200µF range to test a 100µF capacitor).
- Zeroing: Touch the probes together and press the 'REL' (Relative) or 'Zero' button to null out the internal capacitance of your test leads, which can add 50pF to 100pF of error.
Step-by-Step Testing Procedure
Follow this sequence to ensure accurate readings and avoid damaging your meter's internal protection circuits.
- Isolate the Component: Remove the capacitor from the circuit board entirely. If desoldering both legs is impossible, lift at least one leg out of its pad. Testing in-circuit will result in parallel impedance from surrounding components, rendering the reading useless.
- Discharge Safely: Apply your 10kΩ bleeder resistor across the terminals. Verify the voltage is 0V using the DC voltage mode on your DMM before switching to capacitance mode.
- Probe Placement: Touch the red and black probes to the capacitor leads. For capacitance mode, polarity does not matter. However, if you are checking for leakage current using the Ohms mode, place the red probe on the anode (+) and black on the cathode (-).
- Wait for Stabilization: Large electrolytic capacitors (1000µF+) take several seconds to charge via the meter's internal test voltage. Wait until the display stops climbing and the reading stabilizes.
- Compare to Spec: Check the final stabilized number against the tolerance math outlined in the first section.
Expected Readings: Good vs. Bad Capacitors
Use this reference table to interpret your DMM display. These values assume a standard bench test on an isolated component.
| Test Mode | Good Reading | Bad: Open | Bad: Short | Bad: Degraded |
|---|---|---|---|---|
| Capacitance (µF) | Within ±5% to ±20% of printed rating | Reads 'OL' or 0.00 immediately | Reads near 0µF and stays | Reads >20% below rating (e.g., 60µF on a 100µF cap) |
| Resistance / Ohms (Ω) | Climbs steadily to 'OL' (open loop) | Reads 'OL' immediately (no charge) | Reads 0Ω to 5Ω and stays | Stops climbing at a low value (e.g., 10kΩ), indicating high leakage |
| ESR (Ohms)* | Typically < 1.0Ω for power electrolytics | Reads 'OL' | Reads 0.0Ω | > 2.0Ω (requires dedicated ESR meter) |
*Note: Standard DMMs cannot accurately measure high-frequency ESR. A dedicated ESR meter is required for switch-mode power supply diagnostics. See Fluke's guide on capacitor testing for more on ESR limitations.
Frequently Asked Questions
How to check a capacitor with a digital meter that lacks a capacitance setting?
If your DMM only measures voltage, current, and resistance, you can use the Ohms (Ω) mode to check for catastrophic failure, though not for degradation. Set the meter to its highest resistance range (e.g., 2MΩ). Touch the probes to the capacitor leads. A good capacitor will show a low resistance initially, then the number will steadily climb as the cap charges from the meter's internal battery, eventually reaching 'OL' (open loop). If it immediately reads 'OL', the capacitor is open (dead). If it reads a low, stable resistance (like 4Ω) and never climbs, the capacitor is shorted. This method cannot tell you if a 100µF cap has degraded to 40µF; it only finds hard shorts and opens.
Why do my small ceramic capacitor readings fluctuate or read higher than expected?
This is almost always caused by body capacitance and parasitic lead capacitance. When testing small ceramic or film capacitors in the picofarad (pF) range, touching the metal probe tips with your fingers adds your body's capacitance (roughly 50pF to 150pF) in parallel with the component. Furthermore, standard test leads add about 100pF of their own. To get an accurate reading on a 22pF ceramic capacitor, you must use the meter's 'REL' (Relative) function to zero out the leads while they are disconnected, and then hold the capacitor by its insulated body or use alligator clips so your skin does not bridge the test points.
Can I check a capacitor with a digital meter without removing it from the PCB?
Generally, no. A standard digital multimeter applies a low-frequency test signal. If the capacitor is soldered into a circuit, the meter will measure the combined parallel impedance of the capacitor, nearby bypass caps, and transformer windings, resulting in a wildly inaccurate and usually much higher reading. You must lift at least one leg of the capacitor from the PCB pad to isolate it. The only exception is if you are using a specialized high-frequency ESR meter, which uses a 100kHz AC signal that passes through parallel low-impedance paths (like transformer coils) without being affected by them, allowing for in-circuit degradation checks.
What safety category (CAT rating) is needed for testing HVAC or mains capacitors?
When testing motor-run or motor-start capacitors in HVAC systems, pool pumps, or mains-connected power supplies, you are working in an environment with high available fault currents. You must use a meter rated CAT III 600V minimum. A CAT III rating ensures the meter's internal clearances and arc-flash protection can survive a transient voltage spike (like a motor kicking back or a lightning strike on the grid) while you are probing the terminals. Never use a cheap, unrated hobby multimeter for these measurements; the internal shunt can vaporize, causing severe burns or blindness.
