To check a bad capacitor, set your digital multimeter to the capacitance mode (⊣⊢), safely discharge the component using a high-wattage bleeder resistor (e.g., 5W 100Ω for large electrolytics, 1kΩ for small ceramics), and connect the probes to the leads. A reading within ±10% to ±20% of the printed nominal value indicates a healthy part. If the meter reads "OL" (open circuit), "0.00" (shorted), or a value significantly lower than the rated microfarads or nanofarads, the capacitor has failed. For accurate in-circuit testing, you must desolder at least one leg to prevent parallel components from skewing the measurement.
Visual Failure Modes: What a Bad Capacitor Looks Like
Before you even touch a multimeter, a visual inspection will often confirm a dead capacitor. Different dielectrics fail in distinctly different ways, and recognizing these physical symptoms is a core bench skill.
Aluminum Electrolytic Failures
- Bulging and Doming: The top vent (the cross-scored aluminum disc) pushes upward. This is caused by internal gas generation as the liquid electrolyte breaks down due to heat, overvoltage, or simply reaching the end of its operational lifespan (typically 2,000 to 10,000 hours at rated temperature).
- Leaking and Crusting: A brownish, sticky residue or white, powdery crust at the base or top of the can. The electrolyte has boiled out, meaning the capacitor has lost its ability to form the internal oxide dielectric layer. It is effectively dead.
- Sleeve Shrinkage: The plastic outer sleeve shrinks, cracks, or turns brittle. While not an electrical failure on its own, it indicates the part has been running far above its ambient temperature rating, accelerating internal degradation.
Ceramic (MLCC) and Film Failures
- Flex Cracking: Multi-Layer Ceramic Capacitors (MLCCs) are brittle. If the PCB flexes during assembly or operation, a hairline fracture will form, usually parallel to the PCB edge and near the solder pad. Visually, this looks like a tiny crack in the ceramic body. In modern 2025/2026 high-density boards using 0402 or 0201 packages, these are nearly invisible to the naked eye and require a microscope.
- Short-Circuit Charring: When an MLCC cracks internally, the layers short out, causing localized extreme heat. Look for a tiny scorch mark or a blown hole in the side of the component.
- Film Capacitor Swelling: Metallized polypropylene (CBB) capacitors used in motor-run applications or audio crossovers can physically expand, split their plastic outer shell, or melt their internal potting compound when subjected to high AC ripple currents or voltage spikes.
Decoding Capacitor Markings and Codes
Knowing what the markings mean is critical when ordering replacements. While aluminum electrolytics print their values directly (e.g., 470µF 25V), ceramic and SMD components use standardized EIA codes that trip up many beginners.
The 3-Digit EIA Code (Through-Hole Ceramics)
The first two digits are the significant figures, and the third digit is the multiplier (number of zeros), yielding a value in picofarads (pF).
- 104 = 10 × 10^4 pF = 100,000 pF = 100 nF = 0.1 µF (The most common bypass capacitor in electronics).
- 103 = 10 × 10^3 pF = 10,000 pF = 10 nF.
- 471 = 47 × 10^1 pF = 470 pF.
A trailing letter indicates tolerance: J = ±5%, K = ±10%, M = ±20%, Z = +80%/-20%.
SMD Tantalum and Electrolytic Markings
SMD tantalum capacitors are typically yellow or black rectangular prisms. A printed band or bar on one end indicates the Anode (Positive). This is the exact opposite of through-hole aluminum electrolytics, where the printed stripe indicates the Cathode (Negative). Reversing a tantalum capacitor will cause it to short, overheat, and literally catch fire. Always verify polarity against the datasheet before soldering.
| Component Type | Marking Style | Polarity Indicator | Example Decoding |
|---|---|---|---|
| Through-Hole Electrolytic | Printed text (µF and V) | Stripe = Negative (-) | "220µF 16V" |
| SMD Tantalum | Printed text + Bar | Bar = Positive (+) | "47 6V" (47µF, 6.3V) |
| Through-Hole Ceramic | 3-Digit EIA Code | Non-polarized | "104K" (100nF, ±10%) |
| SMD MLCC Ceramic | Usually unmarked | Non-polarized | Requires schematic or measurement |
Capacitor Types: Which to Use and How to Substitute
When the exact replacement part is out of stock, you must understand the underlying construction and dielectric properties to substitute safely. According to the TDK MLCC Technical Guide, swapping dielectrics blindly can lead to catastrophic circuit failure due to temperature coefficients and voltage derating.
| Type | Construction / Dielectric | Tolerance & Tempco | Typical Use Case |
|---|---|---|---|
| Aluminum Electrolytic | Foil and liquid electrolyte | ±20%, High ESR, High Temp drift | Bulk DC filtering, power supply smoothing |
| MLCC (X7R / X5R) | Multi-layer ceramic | ±10% to ±20%, Moderate voltage derating | General decoupling, bypass, mid-freq filtering |
| MLCC (C0G / NP0) | Multi-layer ceramic (Class I) | ±5%, Zero tempco, highly stable | RF circuits, precision timing, PLL loops |
| Metallized Film (CBB) | Polypropylene / Polyester film | ±5% to ±10%, Low dielectric absorption | Audio crossovers, motor run, high-voltage snubbers |
| Tantalum | Sintered tantalum powder | ±10% to ±20%, Low ESR, compact | Space-constrained DC filtering, medical devices |
The Golden Rules of Safe Substitution
- Voltage Rating: ALWAYS use a replacement with an equal or higher voltage rating. A 50V cap can safely replace a 25V cap, but a 16V cap installed in a 24V rail will experience dielectric breakdown and explode. Note that MLCC ceramics suffer from severe DC bias derating; a 10µF 6.3V X5R ceramic may only provide 2µF of actual capacitance at 5V DC.
- Capacitance Value: For bypass and bulk filtering, a value ±20% higher is usually acceptable. For timing circuits (like a 555 timer oscillator) or active filters, you must match the exact value, or use a C0G/NP0 ceramic to prevent frequency drift as the room temperature changes.
- ESR (Equivalent Series Resistance): Never replace a "Low-ESR" or polymer capacitor in a switching power supply (SMPS) with a standard general-purpose electrolytic. The standard part will overheat due to high AC ripple current and fail within weeks. As noted in the Cornell Dubilier Application Guide, ripple current ratings are just as critical as capacitance in power applications.
- Temperature Rating: Always substitute up. A 105°C rated electrolytic can replace an 85°C part, but putting an 85°C part in a 105°C environment will cut its lifespan by 75%.
Frequently Asked Questions
How to check a bad capacitor without a multimeter?
If you lack a multimeter with a capacitance function, rely on visual inspection and functional substitution. Look for the physical symptoms detailed above: bulging tops, crusty electrolyte leaks, or charred ceramic bodies. For non-polarized ceramic or film capacitors in low-voltage circuits, you can sometimes use an ohmmeter (or a simple continuity tester) to check for a dead short, which indicates a failed dielectric. However, you cannot verify if the capacitor has lost its capacitance (gone "open" or drifted low) without a dedicated capacitance meter or an ESR (Equivalent Series Resistance) meter.
How to check a bad capacitor in circuit?
Testing a capacitor while it is still soldered to the PCB is highly unreliable for capacitance measurements. The multimeter will measure the parallel combination of the capacitor and every other component connected to that net, yielding a falsely high reading. To test in-circuit, you must use an ESR Meter. An ESR meter injects a high-frequency AC signal (typically 100 kHz) that passes through the capacitor's reactance but measures the resistive losses of the internal foil and electrolyte. If the ESR reads higher than the manufacturer's spec (e.g., > 0.5Ω for a 1000µF cap), the capacitor is bad, even if its capacitance reads normally. If you only have a standard multimeter, you must desolder and lift at least one leg of the capacitor off the pad to isolate it from the circuit.
Can a bad capacitor cause a power supply to fail completely?
Yes, absolutely. A failed capacitor can cause a power supply to exhibit three distinct failure modes depending on how it died. If the capacitor fails shorted, it will draw massive current, blowing the primary fuse or tripping the supply's Over-Current Protection (OCP), resulting in a completely dead unit. If it fails open or loses capacitance on the primary side, the rectified DC bus will have massive voltage ripple, causing the PWM controller to brown out or shut down via under-voltage lockout (UVLO). If a secondary-side filter capacitor degrades (high ESR), the power supply may turn on but will exhibit severe output noise, causing the connected logic (like a router or motherboard) to randomly reboot or fail to boot, as documented in standard Fluke troubleshooting procedures.
How to check a bad start or run capacitor on an HVAC compressor?
Motor run and start capacitors (usually large, silver or black metallized film cylinders) degrade over time due to constant AC stress. To check them, disconnect power, safely discharge the terminals with a 20kΩ 5W resistor, and disconnect the wires. Set your multimeter to capacitance. A healthy 45µF run capacitor should read between 40.5µF and 49.5µF (the standard ±10% tolerance). If it reads below 35µF, the motor will struggle to start, draw excessive locked-rotor amperage (LRA), and trip the thermal overload. Visually, if the top dome is pushed up or the terminal spades are melted/discolored, replace it immediately. Never substitute a "run" capacitor with a "start" capacitor; start capacitors are designed for intermittent duty and will overheat and explode if left in the circuit continuously.
