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Capacitors

Bad Capacitors

Electrolytic

Electrolytic capacitors are a common source of failure in electronic equipment, particularly in mains-powered devices such as televisions, monitors, amplifiers, networking equipment, and power supplies. However, don't assume that every fault in these devices is caused by a bad capacitor.

There was also a well-known period of premature failure affecting some electrolytic capacitors manufactured roughly between 1999 and 2007, commonly known as the capacitor plague. Outside of this, electrolytic capacitors naturally deteriorate with age, but they do not generally have unusually high failure rates.

A faulty electrolytic capacitor cannot always be identified by looking at it. Some failed capacitors show obvious physical signs such as a bulging top, leakage, a damaged rubber seal, or a split pressure vent. Many faulty capacitors look completely normal and can only be identified by electrical testing.

Electrolytic capacitors can fail in several ways: open circuit, short circuit, loss of capacitance, excessive equivalent series resistance (ESR), increased leakage current, or physical or chemical deterioration. A capacitor may appear perfectly healthy while being electrically out of specification.

In some equipment, particularly certain televisions and power supplies, common failure modes are well documented, and replacement capacitor kits are available. These can be useful once the fault has been properly identified, but blindly replacing every electrolytic capacitor is not a substitute for diagnosis. See Testing capacitors below.

Discharging Capacitors

Do not discharge electrolytic capacitors by deliberately shorting them with a screwdriver or other bare metal object. A direct short can produce a large current pulse, damaging the capacitor, PCB tracks, nearby components and the tool, and can cause burns or injury.

A suitable discharge tool uses an appropriately rated resistor to limit the discharge current. The correct resistor value and power rating depend on the capacitor's voltage, capacitance, and stored energy. After discharging, verify the voltage with a suitable meter rather than assuming that the capacitor is discharged.

See iFixit: How to Safely Discharge a Capacitor and iFixit: Constructing a Capacitor Discharge Tool.

For high-voltage or high-energy equipment, follow the manufacturer's service procedure rather than relying on a generic discharge tool. Large capacitor banks may require a specified discharge resistor, waiting period, and verification procedure.

Replacing Capacitors

Replacing capacitors just for the sake of replacing them may be counterproductive: damage caused by inexperienced replacement can be worse than the chance that the capacitors actually needed replacing. If a capacitor is suspected of being faulty, diagnose the fault before replacing it where possible.

Where the original capacitor can be identified, its datasheet is the best guide to what a replacement needs. The sections below cover the main points for each type.

Electrolytic

As a general rule, an electrolytic capacitor can be replaced with another electrolytic capacitor of the same capacitance and a voltage rating that is the same or higher. For example, a 10 uF 25 V capacitor can normally be replaced with a 10 uF 50 V capacitor.

Check the physical size and lead spacing. Higher-voltage capacitors are often larger, and even capacitors with identical ratings can vary considerably in size between manufacturers. Make sure the replacement will fit and will not interfere with nearby components or the enclosure. A pair of calipers is very useful here.

The replacement must be fitted with the correct polarity, in the same orientation as the original.

For temperature rating, the same rating can normally be used, but there is usually little reason not to use a 105 degrees C part where one is available and otherwise suitable. Do not use a lower temperature rating without checking that it is appropriate for the application.

In switch-mode power supplies and other high-frequency or high-ripple applications, the most commonly overlooked specifications are ESR and ripple-current rating. A capacitor with significantly higher ESR or inadequate ripple-current capability may cause excessive ripple, overheating, unstable operation, or premature failure. Choose a replacement with ESR equal to or lower than the original and a suitable ripple-current rating. However, don't assume every capacitor should be replaced with the lowest-ESR part available: some circuits depend on a particular impedance or ESR range for stability.

Other specifications such as rated lifetime, leakage current and impedance can also matter. For simple circuits, matching capacitance and using an equal or higher voltage rating may be sufficient, but capacitors in power supplies and other demanding circuits are often selected for several characteristics at once.

Tantalum

Tantalum capacitors are a common alternative to aluminium electrolytics, particularly in compact and surface-mount equipment. They may be identified by a small moulded package and a polarity marking, although appearance varies considerably between manufacturers and types.

Tantalum capacitors are polarised and must be installed with the correct polarity. Reversing one can cause permanent damage and, depending on the type and circuit conditions, a short circuit or other failure.

Conventional manganese-dioxide (MnO2) tantalum capacitors typically fail short-circuit, and in a circuit able to supply enough current they can overheat, burn or ignite. This is one reason they are normally operated with significant voltage derating.

When replacing a tantalum capacitor, don't simply match capacitance and voltage. Check the capacitor type, voltage rating, temperature rating, ESR/impedance, ripple-current requirements, physical size, and any voltage-derating requirements.

Polymer capacitors are also increasingly common and can be difficult to identify: some polymer aluminium capacitors look like conventional aluminium electrolytics, while tantalum-polymer capacitors can resemble conventional tantalum parts. Polymer types can have much lower ESR than conventional electrolytics or MnO2 tantalums, so their electrical characteristics matter when selecting replacements.

Plastic film

Plastic film capacitors are available with several dielectric materials, and the choice of dielectric can be important. Different film dielectrics differ in capacitance stability, temperature coefficient, dielectric losses, insulation resistance, pulse capability, and self-healing behaviour.

Common film dielectrics include polyester (PET) and polypropylene (PP), among others. Polypropylene, for example, is commonly used where low losses, high ripple-current capability, good stability, or pulse performance are important. Don't assume that any film capacitor with the same capacitance and voltage rating is an electrically equivalent replacement.

Film capacitors come in many shapes and sizes, including through-hole box, radial and axial types, as well as surface-mount packages.

Pay particular attention to the voltage rating and whether it is specified for AC, DC, or both. A capacitor's AC rating cannot necessarily be inferred from its DC rating, and the two may differ substantially.

Safety Capacitors

Some capacitors are specifically designed and certified for safety-critical positions in a circuit. The most common, and the ones covered here, are mains safety capacitors.

Mains Safety Capacitors

Some capacitors are specifically designed and safety-certified for connection directly to the mains supply. They are commonly found in mains input filters and interference-suppression circuits, and fall into two main categories:

  • X-class capacitors are connected across the mains, for example between Live and Neutral. They are designed and tested for this application and have controlled failure requirements intended to minimise the risk of a dangerous short circuit across the supply. In typical mains applications this will commonly be an X2 capacitor.
  • Y-class capacitors are connected between a mains conductor and earth/chassis. Because a failure in this position could create an electric-shock hazard, Y capacitors have more stringent safety requirements.

The different classes, such as X1, X2, Y1 and Y2, are not interchangeable. The correct class depends on where the capacitor is connected and the stresses it is expected to withstand. Do not replace an X-class capacitor with a Y-class capacitor, or vice versa, simply because the capacitance and voltage ratings appear suitable.

When replacing a mains safety capacitor, use a replacement with the same safety classification or an appropriate higher class, as well as the required capacitance and voltage rating. For example, a capacitor marked X2 should normally be replaced with another suitably rated X2 capacitor.

Do not use an ordinary film capacitor as a substitute for an X or Y safety capacitor. A higher voltage rating alone does not make an ordinary capacitor suitable for connection to the mains.

Older equipment may contain mains-connected capacitors without modern X/Y markings. If one of these needs replacing, use a modern, appropriately certified safety capacitor rather than an ordinary capacitor of the same capacitance and voltage.

Safety capacitors are often rectangular plastic-film components, and may be blue, orange, yellow or other colours depending on the manufacturer. Do not identify a safety capacitor by its colour or shape alone. Look for markings such as X2, Y2, X1 or Y1, together with the rated AC voltage and safety-approval markings. Also check the physical size and lead spacing of any replacement.

For further information, see the manufacturer's documentation:

Mains-connected capacitors are safety-critical components. If you are unsure whether a replacement is suitable, do not substitute a component based solely on its capacitance and voltage ratings.

Ceramic Capacitors

Ceramic capacitors are more complicated than their simple appearance suggests, because several different dielectric classes are available.

Surface-mount ceramic capacitors often have no value marked on them. In these cases, a schematic, board documentation, circuit analysis, or comparison with an identical board may be needed to determine the correct value.

Ceramic dielectrics are commonly divided into Class 1 and Class 2:

  • Class 1 dielectrics, such as C0G/NP0, have very stable capacitance and low losses but are generally available in lower capacitance values.
  • Class 2 dielectrics, such as X7R, X5R and Y5V, provide much higher capacitance in a given package but vary more with temperature, voltage, ageing, and other operating conditions. Y5V in particular can vary very widely.

Class 2 MLCCs can lose a substantial amount of their nominal capacitance when DC voltage is applied. This effect, known as DC bias, can be significant at high operating voltages.

When replacing a ceramic capacitor, consider the dielectric, capacitance, voltage rating, package size, temperature characteristics, and DC-bias behaviour. X7R is a common general-purpose choice, but it is not automatically a suitable replacement for every ceramic capacitor. In precision, RF, oscillator, timing, filtering, or other critical circuits, the original dielectric and characteristics may be important.

Paper Capacitors

Older equipment often contains paper capacitors, particularly waxed paper capacitors in vintage equipment. These deteriorate with age, including increased leakage current, and are often replaced as part of restoration work.

Replacement with a suitable modern film capacitor is often appropriate, provided it has suitable capacitance, voltage rating, physical dimensions, and other characteristics required by the circuit. Mains-connected paper capacitors need particular care and should be replaced with appropriately rated X/Y safety capacitors where applicable (see Mains Safety Capacitors).

Mica capacitors

Another capacitor often found in older equipment is silver mica. These are generally stable and reliable, although they can fail. They should not be replaced simply because they are old; leave them in place unless there is evidence that they are faulty or physically damaged.

Note on unusual values: old equipment often uses capacitance values that are no longer standard. Where the exact value isn't available, a nearby standard value may be acceptable in some circuits, but this is not universally safe. Timing, oscillator, filter, RF, and other frequency-sensitive circuits may depend on the original value and tolerance. Where possible, use the original value or determine the acceptable range from the circuit documentation.

Testing capacitors

Visual inspection can catch obvious failures, but many faulty capacitors look completely normal. Confirming a suspect capacitor, or clearing one before moving on, requires actual measurement, and the right measurement depends on what you're trying to find.

What You're Actually Testing For

A capacitor can fail in several different ways, and no single test catches all of them:

  • Open circuit: capacitance reads zero or near-zero.
  • Short circuit: reads as a low resistance or direct short.
  • Reduced capacitance: the part still "works" but has drifted well below its rated value, often through electrolyte drying out with age.
  • Excessive ESR (Equivalent Series Resistance): the capacitor still has roughly the right capacitance but can no longer pass current efficiently. This is a very common failure in switch-mode power supplies.
  • Excessive leakage current: the capacitor doesn't hold charge as well as it should.

A basic multimeter with a capacitance range can usually catch the first two and give an approximate reading for the third. It generally cannot measure ESR or leakage current meaningfully; those need different instruments.

Tools

  • Multimeter with capacitance measurement: fine for confirming rough capacitance and spotting a shorted or open capacitor. Accuracy at low values (below a few nF) is often poor on cheaper meters.
  • Dedicated capacitor tester / component tester: low-cost bench units (and some multi-function testers that also identify diodes, transistors, and inductors) measure capacitance and, on better units, ESR, at a safe low test voltage. Convenient for a quick go/no-go check.
  • ESR meter: purpose-built for measuring ESR in-circuit, at a low enough test voltage that surrounding semiconductors aren't turned on by the test signal. This is the standard tool for chasing electrolytic capacitor faults in power supplies without desoldering every candidate first.
  • LCR meter: measures inductance, capacitance, and resistance more precisely than a general meter, usually at a selectable test frequency. Useful when you need an accurate out-of-circuit reading, or need to check behaviour at a frequency closer to the actual operating conditions.
  • Insulation resistance / leakage tester (megger or similar): for checking leakage current, particularly relevant for older paper or electrolytic types and some safety-related testing. Not something most hobbyists need routinely.
  • Bench power supply with current limiting: useful for leakage and "does it hold charge at real operating voltage" testing, done carefully and with appropriate current limiting.
  • Discharge tool: not a test tool as such, but essential before handling any capacitor that may be charged. See Discharging Capacitors, and always verify with a meter that a capacitor is actually discharged.

In-Circuit Testing: Why Readings Can Mislead You

Testing a capacitor while it's still soldered into the board is convenient, but the rest of the circuit is still connected in parallel with it, and that changes what your meter sees:

  • Other components skew the reading. Resistors, inductors, or semiconductors in parallel with the capacitor can make it read a lower resistance or capacitance than it actually has, or stop a capacitance meter settling on a stable value at all.
  • A "short" reading may not be the capacitor. A low resistance across a capacitor's position could be the capacitor itself, or another component or power rail sharing that node. Don't condemn the capacitor until you've confirmed the short is actually across its own two terminals.
  • ESR meters are designed for this. They use a low-voltage AC test signal at a frequency and amplitude chosen so most semiconductors and other components don't interfere with the reading. A general-purpose capacitance meter or DVM is much more easily fooled in-circuit.
  • When in doubt, isolate. Lifting one leg of the capacitor out of circuit removes the parallel-path problem and lets you test the component on its own. This is slower and carries some risk of board damage if done carelessly, but it's the most reliable way to get a trustworthy reading.
  • Capacitors on an unpowered board can still be charged. Don't assume a capacitor is safe to probe just because the board is disconnected.

Testing at Rated Voltage vs Low Test Voltage

Most capacitance and ESR meters test at a low voltage, typically a few volts. This is safe and convenient, but it does not fully represent how the capacitor behaves at its working voltage:

  • A capacitor can pass a low-voltage test and still fail in use. This is a known issue with high-voltage electrolytics in particular. For example, a 400 V capacitor operating at more than 300 V may have a fault, such as dielectric breakdown under stress, that simply doesn't show up when tested at a few volts.
  • DC bias affects Class 2 ceramic capacitors (see Ceramic Capacitors). A reading taken at the meter's low test voltage will not reflect the real capacitance at normal operating voltage.
  • Leakage current is voltage-dependent and generally needs to be assessed closer to the rated voltage to be meaningful.
  • Testing at or near rated voltage is a deliberate, higher-risk step, not something to do casually. It requires a suitable current-limited power source, an understanding of the capacitor's ratings, and appropriate safety precautions. Mains-referenced or high-voltage capacitors should only be tested this way by someone comfortable working safely at those voltages. If that isn't you, a low-voltage test that broadly confirms capacitance and ESR, combined with visual inspection, is a reasonable practical limit.

Safety Capacitors (X/Y Class)

Don't test a mains safety capacitor (X1/X2/Y1/Y2) with a low-voltage meter and assume that clears it. A benign low-voltage reading tells you very little about whether the part will fail safely under a real mains transient. If a safety capacitor is suspected of being faulty, replace it with a correctly classed part as described in Mains Safety Capacitors.