What Is the Service Life and Reliability of Capacitors?

Capacitors are an essential element of many circuit designs, but they can also be an expensive part of a system. This is why, when buying components for an electronic device, it's important to know what the service life and reliability are of various capacitor types.

A capacitor is a device that stores electrical charge, consisting of conductors separated by an insulating material called the 'dielectric'. This can be air, paper, plastics, mica, or ceramics. The dielectric has a number of properties that are important in the design of a capacitor, and these properties influence how much capacitance the capacitor can achieve.

The most common type of capacitor is the aluminium electrolytic capacitor, which is constructed from two conducting aluminium foils, one of which has an insulating oxide layer, and a spacer soaked in conductive liquid electrolyte. This is rolled up, placed in a cylindrical casing, and fitted with two connection pins, or legs.

These caps are polarised, meaning that any DC voltages applied across them must be oriented in accordance with the positive or negative markings on their body. They're cheap to manufacture, and, unlike most other types of capacitors, are physically small for the amount of capacitance they provide.

Their service life is typically 15 years or more, and they can usually be reused for a long time after that. Often, they will have lost some of their rated capacitance due to the evaporation of the electrolyte, but they will still be usable.

In audio circuitry, the most common uses of capacitors are for three functions: coupling DC voltage from one circuit element's output to another; decoupling and storing power supply rails; and frequency-selective circuits such as filters and equalisers. In these applications, they act as a very effective 'blocker' of DC flow, reducing the effect of interference that can disrupt audio signal quality by introducing a large amount of noise to the system.

They also act as a 'filter' by reducing the effect of low-frequency signals and, sometimes, enhancing the response of higher frequencies to make these signals easier for the amplifier to reproduce. They are used in a variety of different circuits, and can be found in many types of devices, from tube radios to computers and audio gear.

The service life of a capacitor depends on several factors, including the material it's made from, its application, and the ambient temperature at which it is stored or operated. It's also influenced by local heating, reverse polarity DC, and current surges, and the application of ripple currents in a circuit.

Generally speaking, electrolytic capacitors are more susceptible to degradation than non-electrolytic ones, and their ESR (internal equivalent series resistance) tends to increase as they age. This is because the insulators on electrolytics are thinner and have larger surface areas than their non-electrolytic counterparts, which makes them more prone to leakage current.

However, a well-designed power supply can prolong the lifespan of electrolytic caps by preventing heat from building up on them. This is where it's a good idea to use a power supply that has built-in protection features to prevent thermal runaway, and to replace any electrolytics that have reached the end of their service life when it becomes necessary.