To check if a capacitor is good, safely discharge it, remove it from the circuit, and measure both its capacitance and Equivalent Series Resistance (ESR) using a digital multimeter (DMM) with capacitance mode or a dedicated ESR meter. A healthy capacitor reads within ±20% of its rated value and shows an ESR below the manufacturer's maximum threshold (typically under 1Ω for large electrolytics). Visually, it must have no bulging, crusty electrolyte leaks, or scorched epoxy.
Relying solely on a capacitance reading is a classic bench mistake. A 1000µF electrolytic might read exactly 1000µF on your meter but still fail to filter a switching power supply because its internal ESR has skyrocketed from 0.05Ω to 5Ω. To properly diagnose and replace passive components, you need to understand failure modes, decode physical markings, and know the strict rules for substitution.
Visual and Physical Failure Modes
Before you even touch your meter, perform a visual inspection. Different dielectric materials fail in distinct, recognizable ways.
- Aluminum Electrolytics: These are the most common failure points in power supplies and motherboards. Look for a domed or bulging top. The cross or "K" scored into the aluminum can is a pressure relief vent; if it has popped open, the capacitor is dead. Check the base for brown, crusty electrolyte leaks, which indicate the internal paper has dried out and vented.
- Multilayer Ceramic Capacitors (MLCCs): Ceramics rarely bulge. They typically fail as dead shorts due to mechanical stress (board flexing) causing internal micro-cracks. Visually, look for hairline cracks on the PCB near the component, or a scorched/blackened epoxy coating on the capacitor body.
- Film Capacitors (Polypropylene/Polyester): Usually found in motor run circuits and audio crossovers. They tend to fail open. Visual clues include melted plastic casing, a bulging side seam, or a burnt smell near the leads.
Step-by-Step Testing: Capacitance and ESR
Testing in-circuit is unreliable for capacitance because parallel components will skew the reading. In-circuit testing is only acceptable if you are using a dedicated ESR meter to hunt for dead shorts or massively degraded electrolytics on a populated board. For accurate diagnostics, desolder the part.
- Discharge: Connect your bleeder resistor across the leads for 10-30 seconds. Verify with a DC voltmeter that the voltage reads 0V.
- Measure Capacitance: Set your DMM to capacitance mode (usually denoted by -||-). Connect the probes. If the reading is more than 20% below the rated value, the electrolyte has dried out; discard it.
- Measure ESR: Connect an ESR meter (which typically injects a 100kHz AC signal to bypass the capacitive reactance). Compare the reading to the acceptable thresholds below.
Typical Maximum ESR Thresholds (at 100kHz)
| Capacitance | Voltage Rating | Max Acceptable ESR | Typical Good ESR |
|---|---|---|---|
| 10 µF | 50V | < 5.0 Ω | 1.5 - 2.5 Ω |
| 100 µF | 25V | < 1.5 Ω | 0.4 - 0.8 Ω |
| 1000 µF | 16V | < 0.3 Ω | 0.05 - 0.1 Ω |
| 4700 µF | 35V | < 0.2 Ω | 0.02 - 0.05 Ω |
For a deeper understanding of how internal resistance affects filtering, refer to the All About Circuits guide on ESR.
Decoding Physical Markings and Codes
When you pull a part and need a replacement, you must accurately read the markings. Through-hole electrolytics are straightforward, but ceramics and SMD parts use condensed codes.
Ceramic 3-Digit Codes
Most ceramic capacitors use a three-digit code based on picofarads (pF). The first two digits are the base value, and the third is the multiplier (number of zeros).
- 104: 10 × 10,000 pF = 100,000 pF = 100 nF = 0.1 µF. (This is the most common bypass cap you will ever see).
- 473: 47 × 1,000 pF = 47,000 pF = 47 nF.
- 221: 22 × 10 pF = 220 pF.
Tolerance and Temperature Coefficients (Tempco)
Look for a letter following the capacitance code. J means ±5%, K means ±10%, and M means ±20%. More importantly, look for the dielectric class code (e.g., C0G, X7R, Y5V). According to DigiKey's capacitor selection guidelines, X7R is the standard for general bypass and decoupling because it remains stable across temperature changes. Y5V and Z5U are highly unstable and lose up to 80% of their capacitance when voltage or temperature shifts; avoid substituting these into timing or filter circuits.
Capacitor Types, Selection, and Safe Substitution
Knowing which type of capacitor to use for a specific job prevents premature failure. When the exact OEM part is out of stock, follow the substitution hierarchy: Voltage UP, Capacitance MATCH, ESR DOWN.
| Type | Construction / Dielectric | Tolerance | Tempco Stability | Typical Use Case |
|---|---|---|---|---|
| MLCC (Ceramic) | Alternating metal/ceramic layers | ±5% to ±20% | High (C0G) to Low (Y5V) | High-frequency bypass, decoupling, RF tuning |
| Aluminum Electrolytic | Etched aluminum foil, liquid/polymer electrolyte | ±20% | Moderate (Degrades with heat) | Bulk power supply filtering, low-frequency audio coupling |
| Film (Polypropylene) | Metalized plastic film, wound or stacked | ±1% to ±10% | Very High | Motor run/start, audio crossovers, high-voltage snubbers |
| Tantalum | Tantalum pentoxide dielectric | ±10% to ±20% | High | Space-constrained low-ESR filtering (medical/aerospace) |
Safe Substitution Rules
- Voltage Rating: You can always substitute a higher voltage rating (e.g., using a 25V cap in place of a 16V cap). Never go lower. Note that electrolytics rated significantly higher than the circuit voltage may not "form" the oxide layer properly, but for standard hobby/DIY voltage margins (e.g., 25V in a 12V circuit), this is perfectly fine.
- Capacitance Value: Stay within 20%. For power supply bulk filtering, going slightly higher (e.g., 1200µF instead of 1000µF) is generally acceptable, but do not exceed the inrush current limits of your upstream rectifier diodes or fuses.
- ESR Requirements: Never substitute a "Standard" electrolytic for a "Low ESR" or "Polymer" capacitor in a switching mode power supply (SMPS). The high ripple current in an SMPS will boil the liquid electrolyte in a standard cap within weeks. Always match or beat the original ESR spec.
Frequently Asked Questions
How to check if a capacitor is good without removing it from the board?
You cannot accurately measure capacitance in-circuit because parallel traces and components create a combined parallel capacitance that skews your DMM reading. However, you can use a dedicated ESR meter in-circuit. ESR meters inject a high-frequency (100kHz) AC signal that passes through the capacitor's reactance, measuring only the resistive losses. If an in-circuit electrolytic shows an ESR of >5Ω, it is degraded and should be desoldered for replacement. If you suspect a ceramic capacitor is shorted in-circuit, a standard DMM in resistance mode will read near 0Ω across the power rails.
How to check if a motor start or run capacitor is good?
Motor capacitors (typically metalized polypropylene film in metal or plastic cans) fail by losing capacitance, causing the motor to hum but not spin, or to trip the breaker. Disconnect power, discharge the terminals with a 20kΩ 5W resistor, and disconnect the spade wires. Set your DMM to capacitance. A 40µF run capacitor should read between 36µF and 44µF (±10%). If it reads significantly lower, or if your meter shows "OL" (open loop), the internal film has fractured. Visually inspect the top dome; if the pressure interrupter has popped up, the capacitor is permanently failed. For more on motor circuits, see the Electronics Tutorials capacitor guide.
How to check if a tiny SMD ceramic capacitor is good?
Testing surface-mount MLCCs (like 0603 or 0402 sizes) with standard DMM probes is physically difficult and inaccurate due to probe parasitic capacitance. First, perform a visual inspection under a magnifying lamp for micro-cracks near the solder pads—this is the #1 failure mode for SMD ceramics. To test electrically, you must desolder it. Use tweezers to hold it and measure with a DMM. Note that multimeters often cannot accurately measure values below 100pF due to the capacitance of the test leads themselves. If testing sub-100pF SMD caps, you need an LCR meter with a dedicated SMD tweezer probe fixture.
