To test a capacitor, set your digital multimeter (DMM) to the capacitance mode (marked with a capacitor symbol or 'F'), safely discharge the component, and place the red probe on the anode (+) and black probe on the cathode (-). A good reading will fall within ±20% of the rated microfarad (µF) value printed on the casing. If the meter reads 'OL' (Over Limit) or zero, the capacitor has failed open or shorted, respectively.
Meter Setup and Safety Category Requirements
Capacitors in mains-powered appliances, switching power supplies, and HVAC systems can store 300V to 400V DC long after the power is disconnected. Never assume a capacitor is safe just because the device is unplugged. Always verify and discharge before handling.
When working on mains-connected equipment, your multimeter must have the correct safety category rating. For testing capacitors on the load side of an appliance plug or a benchtop power supply, a CAT II 1000V rated meter is required. If you are testing HVAC contactor boards or hardwired motor circuits, you must step up to a CAT III 600V rated meter (such as the Fluke 117 or Klein Tools MM400) to protect against transient voltage spikes that can arc across the meter's internal gaps.
Meter Setup Block
- Dial Position: Rotate to the Capacitance setting (⊣⊢ or 'F'). If your meter lacks capacitance mode, you can only test for dead shorts using the Continuity (sound wave) or Ohms (Ω) settings, but you cannot verify the component's actual health.
- Lead Jacks: Insert the black lead into the COM jack. Insert the red lead into the V/Ω/Hz/F jack. (Note: Some older or specialized benchtop meters have a dedicated 'Cx' or 'CxLx' jack; consult your specific manual).
- Range Selection: Most modern DMMs are auto-ranging. If using a manual-ranging meter, select the next highest range above the capacitor's rated µF (e.g., select the 200µF range to test a 100µF capacitor).
Step-by-Step Probe Placement and Discharge Procedure
According to Fluke's official testing guidelines, proper isolation and discharge are mandatory before the probes ever touch the terminals.
- Isolate and Discharge: Turn off power and verify zero voltage. Discharge the capacitor using a purpose-built capacitor discharge tool or a high-wattage resistor (a 20kΩ, 5W ceramic resistor is ideal for most hobby and appliance caps). Never use a flathead screwdriver to short the terminals; the massive instantaneous current can vaporize internal foil connections and create hidden micro-shorts.
- Remove from Circuit: Desolder or physically disconnect at least one leg of the capacitor from the PCB. Testing a capacitor while it is still soldered in-circuit will yield false readings due to parallel impedance paths.
- Probe Placement (Electrolytic): Identify the polarity stripe (cathode/-). Place the red probe on the long leg (anode/+) and the black probe on the short leg (cathode/-). Reversing polarity on an electrolytic cap during a capacitance test can sometimes yield slightly inaccurate readings or trigger a meter error, though modern DMMs are generally tolerant of brief reverse-bias measurement.
- Probe Placement (Ceramic/Film): Polarity does not matter. Place one probe on each leg.
- Read and Hold: Wait 2 to 5 seconds. The multimeter outputs a small known AC or DC test current to charge the capacitor and calculate the time constant. Wait for the digits to stabilize.
Expected Readings: Good vs Bad Capacitor Values
A capacitor's health is judged against its printed rating and manufacturing tolerance (typically ±20% for standard aluminum electrolytics, and ±10% or ±5% for film/ceramic). As detailed in SparkFun's capacitor tutorial, dielectric degradation usually manifests as a drop in total capacitance before a catastrophic short occurs.
| Rated Value | Tolerance | Good Reading (Pass) | Bad: Degraded (Dry) | Bad: Shorted | Bad: Open |
|---|---|---|---|---|---|
| 10 µF | ±20% | 8.0 µF – 12.0 µF | < 7.5 µF | 0.00 µF (or Continuity Beep) | OL (Over Limit) |
| 470 µF | ±20% | 376 µF – 564 µF | < 350 µF | 0.00 µF | OL |
| 35 µF (HVAC Run) | ±6% | 32.9 µF – 37.1 µF | < 30.0 µF | 0.00 µF | OL |
| 0.1 µF (104 Ceramic) | ±10% | 0.09 µF – 0.11 µF | < 0.08 µF | 0.00 µF | OL |
Four Mistakes That Cause Misleading Readings
Even with a high-end benchtop meter, user error can mask a failing component. Avoid these common bench and jobsite pitfalls:
- Testing In-Circuit: Capacitance in parallel is additive ($C_{total} = C_1 + C_2$). If you test a 10µF capacitor while it is still soldered next to a 0.1µF bypass cap and an inductor, your meter will read the combined impedance of the entire local circuit, often showing a falsely 'good' or wildly inflated number.
- The Screwdriver Discharge: Shorting a 400V, 100µF power supply cap with a screwdriver creates a current spike exceeding 100 amps for a few milliseconds. This physically tears the microscopic aluminum foil inside the component. The capacitance might read 'normal' on your DMM, but the internal equivalent series resistance (ESR) will be ruined, causing the cap to overheat and fail under load.
- Touching the Probes on Small Values: The human body has a stray capacitance of roughly 50pF to 100pF. If you are testing small ceramic capacitors (e.g., 22pF or 100pF) and your fingers touch the metal probe tips or the component leads, your body will parallel the circuit, doubling or tripling the reading. Use alligator clips or a dedicated component test socket for sub-nanofarad values.
- Ignoring Dielectric Absorption: Large electrolytic capacitors exhibit 'memory.' After you discharge them to 0V, the chemical dielectric layer slowly releases trapped charge back into the terminals over several minutes. A cap you verified as 'dead' 10 minutes ago can rebuild up to 10% of its original voltage, delivering a painful shock if you grab the leads barehanded.
Frequently Asked Questions
How to test a capacitor without a multimeter?
Without a meter, you are limited to visual inspection and destructive 'smoke' testing, neither of which is recommended for precision electronics. Look for physical failure markers: a domed or bulging top vent on aluminum electrolytics, black crusty electrolyte leakage around the base rubber seal, or a burnt smell. Some old-school technicians use a series incandescent bulb test across mains voltage to check for shorts, but this is incredibly dangerous, violates modern safety practices, and will not tell you if the capacitor has lost its microfarad capacity due to drying out.
How to test an HVAC dual run capacitor?
HVAC dual run capacitors (commonly rated 35/5µF or 45/5µF) have three terminals: C (Common), Herm (Hermetic Compressor), and Fan. You must test them as two separate capacitors sharing a common ground. First, safely discharge all three terminals to the metal casing. Set your meter to capacitance. Place your probes across C and Herm to read the compressor side (e.g., 35µF). Then place your probes across C and Fan to read the fan motor side (e.g., 5µF). If the Herm side reads 28µF (a 20% drop), the compressor will struggle to start and draw high amperage, even if the Fan side reads a perfect 5µF.
Can a capacitor test good but still be bad?
Yes. A standard multimeter only measures total capacitance (the ability to store charge). It does not measure Equivalent Series Resistance (ESR) or leakage current. In high-frequency switching power supplies (like PC ATX supplies or LED drivers), a capacitor can retain 95% of its rated µF but develop a high ESR due to dried electrolyte. This high ESR prevents the capacitor from filtering high-frequency ripple current, leading to voltage spikes and system crashes. To catch this, you must use a dedicated ESR meter or an oscilloscope to measure the AC ripple voltage across the capacitor's terminals while the circuit is energized.
What does 'OL' mean when testing a capacitor?
'OL' stands for Over Limit (or Open Line on some older meter displays). In capacitance mode, an 'OL' reading means the meter's test current cannot flow into the component to charge it. This indicates an internal open circuit—the microscopic wire connecting the terminal pin to the internal foil roll has snapped, or the internal fuse has blown. An 'OL' reading is a definitive failure; the capacitor is dead and must be replaced.
