An Inside Look At Capacitors by Steven Hamilton, P.E., CFEI

There are many types of capacitors as well as many applications for them.  One common type is the Aluminum Electrolytic (AE) capacitor.  This type of capacitor is commonly employed in power supply circuits.  The unique construction of aluminum electrolytic capacitors makes them very efficient, offering large capacitance in a small space.  They do however have a finite lifespan which is often ignored.

Figure 1. Typical AE Capacitor

Figure 1. Typical AE Capacitor

Construction:

All capacitors are the same in the respect that they are two conductors separated by a non-conductor.  The size, shape, separation distance and material of the conductor all play a significant role in defining the performance of the capacitor.  The other important feature is the dielectric, which is the non-conductor that separates the conductors.

An aluminum electrolytic capacitor is made of thin sheets of aluminum foil that are rolled into a cylinder.  Before being assembled, the anode layer of the capacitor undergoes a chemical process to form an oxide layer on it.  The cathode layer remains free of oxide. These sheet electrodes are separated by a layer of paper or film.  The rolled assembly is then placed into an aluminum cylinder which is then filled with an electrolyte solution and sealed.

The difference in the anode and cathode (the oxide layer on the anode) makes these capacitors polarity sensitive.  The anode must always be at a higher voltage in order to protect the dielectric oxide layer.

Applications:

Figure 2. Basic Construction of AE Capacitor

Figure 2. Basic Construction of AE Capacitor

Aluminum electrolytic capacitors are frequently seen in computers, audio amplifiers, CFL and LED lighting and in variable speed motor drives.  The capacitors in these applications are used to filter out spikes in voltage or to store energy.

Life span:

Aluminum electrolytic capacitors have a limited life span.  They are designed to be used at a rated voltage and operated below a temperature maximum.  Under those conditions the manufacturer assigns a typical life in hours; 1,000 to 10,000 hours is the standard range of lifespans.  When you consider there are 8,760 hours in a year it gives perspective on the life expectancy of a heavily utilized capacitor.

The life span of a capacitor is often increased through design by ensuring that it runs at temperatures well below the maximum rating.  Common temperature ratings of these capacitors are 85°C (185°F) and 105°C (221°F).  If a capacitor is run at a cooler temperature than rated it will last longer—operated at higher temperatures and it will fail sooner.  This temperature rating applies to the core temperature of the capacitor, which can often be higher than the ambient environment.

As an aluminum electrolytic capacitor ages the electrolyte dries out.  The electrolyte serves to transfer heat and to provide a means to trap hydrogen gas released as a byproduct of the decomposition of the oxide layer on the anode.  Eventually the capacitor’s ability to perform is greatly compromised.

Failures:

Aluminum Electrolytic capacitors can fail quietly or dramatically.  Aluminum electrolytic capacitors that are used within their rating will eventually lose their ability to function, typically becoming an open circuit – which is a safe failure mode.  An aged capacitor may exhibit bulging of the top; this condition is caused by pressure building up inside the container.  Bulging can also be a result of power surges, polarity reversal or high AC ripple currents.  Though capacitors are designed to vent internal pressure these vents occasionally become blocked or are overcome by rapid pressure buildup.

Aluminum electrolytic capacitors that are exposed to abuse such as high temperatures, power surges, polarity reversal or high AC ripple currents can fail violently.  They can explode or catch on fire in extreme cases.

Responsible equipment design should include several measures for products containing aluminum electrolytic capacitors.  If the expected equipment life exceeds the capacitor design life the consumer should be warned and advised how to monitor the health of the capacitors.  Access to the capacitors should be readily accessible to allow easy, frequent inspection.  The equipment should also be designed to maintain low ambient temperatures where the capacitors are mounted to maximize their life expectancy.  Aluminum electrolytic capacitors are a popular design due to their low cost and high performance; however they do have a finite life.  They have become ubiquitous in many common devices in use in homes and in industry.  As these capacitors age they will degrade and typically become an open circuit, sometimes bulging.  If exposed to extreme conditions they can fail much quicker and result in explosive ruptures or fires.

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