Selection Guide for Aluminum Electrolytic Capacitors

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What is an aluminum electrolytic capacitor?

Capacitors are passive electronic devices that store electric charge. Polarized aluminum electrolytic capacitors are typically constructed using two rolled layers of aluminum foil separated by a paper insulator saturated with a wet electrolyte. This capacitor’s dielectric consists of this insulator in conjunction with a thin oxide layer formed on the positive foil anode. Electrolytic capacitors have a relatively high capacitance-voltage (CV) rating per unit volume at a low cost. Applications include power supply energy storage as well as signal bypass, coupling, and decoupling.

Image showing the variety of aluminum electrolytic capacitors offered by DigiKey
Image showing the variety of aluminum electrolytic capacitors offered by DigiKey2068×1185 261 KB

An engineer’s perspective

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The aluminum electrolytic capacitor is a common component found in nearly every electronic device. You will certainly encounter these devices in school. They are a miracle of modern technology, packing a large amount of charge into a compact, low-cost package. Unfortunately, they do have a limited operational life as described in the Lifetime @ Temp specification.

Must-know facts about polarized electrolytic capacitors

There are a few must-know facts about aluminum electrolytic capacitors:

  • Safety: The aluminum electrolytic capacitor will hold a charge for a long time. This presents a safety hazard as unpowered equipment can still deliver an electric shock. Be sure to discharge the capacitor according to the manufacturer’s suggested procedures. Also, be sure to incorporate bleeder resistors, especially for high voltage applications. One of the worst electrical shocks I received was from a photoflash capacitor bank.

  • Polarity: The aluminum oxide dielectric is polarity dependent. Incorrect installation will destroy the capacitor. This could range from boiling of the electrolyte with steam exiting the capacitor’s vent to a full explosion where the “guts” of the capacitor are blown out of the case with pieces of aluminum foil strewn over the enclosure. We should all wear safety goggles as a preventative safety measure when working on electronics. As a rule, the negative terminal of the capacitor is marked. Be sure to consult the datasheet as there may be exceptions.

  • Degradation: The aluminum oxide dielectric degrades with time to the point where the structure is degraded in terms of capacitance and the ability to withstand voltage. Full application of voltage to a degraded capacitor can lead to its destruction. Slow application of current can often restore the capacitor to full health. For example, large Variable Frequency Drives (VFD) have a reforming procedure to restore the capacitor. Search the web to find examples of VFD capacitor restoration.

  • Too much of a good thing: Capacitors have a finite life. People will often restore vintage audio and guitar equipment by “recapping” the device. This will often restore the equipment to full operation. However, take it slow as it is very easy to make a mistake. Do not replace all capacitors at once, instead replace a few at a time and then test the results. This will limit your troubleshooting to the small areas you just repaired as opposed to the entire device.

Looking forward to continuing the conversation on this forum.

Best wishes,

Aaron Dahlen
LCDR USCG (Ret.), MSEE
DigiKey Applications Engineer

Frequently asked questions

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What are the most important capacitor parameters?

That’s not an easy question to ask as the answer is dependent on your application. For general purpose application we can assume the capacitor is non-polarized. We then select for DC working voltage and capacitance, and construction (radial vs axial vs surface mount). As we work up the ladder, the lifetime at temperature specification becomes important. At the top of the design hierarchy, we focus on ESR, impedance, and other specification that are applicable to your project’s specific needs.

How do I select a replacement capacitor for vintage audio amplifier and guitar amplifier?

The good news is that modern capacitors are physically smaller than their vintage counterparts. This simplifies the restoration of vintage equipment such as audio amplifiers, receivers, and guitar amplifier.

You will need to focus on several parameters including:

  • DC voltage rating
  • Capacitance
  • Physical size including height, diameter, and most importantly, the distance between the capacitor’s terminals. In other, words, make sure your replacement capacitor will fit on the PCB and not be so tall that the cover cannot be installed.

These videos will help you with the selection process:

What is the shelf life of an aluminum electrolytic capacitor, and how can it be extended?

There are two parts to this question:

  • natural degradation of the oxide layer: The aluminum electrolytic capacitor’s operation is dependent on the formation of a thin oxide layer. This layer is formed/reformed when the DC voltage is applied. The layer degrades when the capacitor is in storage for a long period of time.

  • solderability: Oxidation of the capacitor’s terminals can lead to manufacturing defects. Consequently, shelf life is a function of the conditions under which the part is stored. As a conservative measure, many suppliers recommend a 2-year shelf life - consult the manufacturer’s data sheet. You may also be interested in this article.

How long will the capacitors in my equipment last?

There can be no definitive answer to this question as life of a capacitor is determined by operating conditions as well as temperature. As a general rule capacitors follow the rule to 10 where each 10° C reduction in temperature doubles the capacitor’s life.

How do I reform an aluminum electrolytic capacitor?

An aluminum electrolytic capacitor is reformed by slow application of a controlled voltage. The best results are obtained when the process is conducted over several hours. Consult the manufacturer’s literature for recommendations.

A simple procedure is to use a high value series resistor with a RC time constant between 15 and 60 minutes. The process could also be done manually using a variable DC power supply. Occasionally you will see people use a series connected light bulb and a variac to slowly increase the voltage.

What are the disadvantages of aluminum electrolytic capacitors?

This question is best answered by comparing the aluminum electrolytic capacitor to other technologies such as metal film, mica, and paper-n-oil capacitors. By comparison, the electrolytic capacitor has a relative short life, temperature sensitivity, and loose tolerance. However, despite these shortcomings, the aluminum electrolytic capacitor is inexpensive. Consequently, it is common component found in nearly every electronic device made in the last century.

What is the difference between a polarized and a non-polarized electrolytic capacitor?

The vast majority of aluminum electrolytic capacitors are polarized - they are immediately damaged if installed backwards. This is reflected in the capacitor’s singular DC working voltage specification. The bipolar capacitors such as the specialized motor starting capacitors include an AC working voltage. Typical single phase starting capacitors include 125, 165, 250, and 330 VAC ratings.

What is aluminum electrolytic capacitor leakage?

Leakage has two meaning for an electrolytic capacitor

  • physical: The electrolytic capacitor is often constructed with a wet electrolyte. A damaged or defective capacitor may leak electrolyte. Older failing capacitor will often bulge and vent electrolyte. Capacitors in this condition should be replaced immediately to prevent damage to the equipment.

  • electrical: Electrical leakage occurs when DC is applied. It’s as if a high value resistor were installed in parallel with the capacitor’s terminals. The leakage parameter is typically specified in terms of the capacitance and the working DC voltage with an associated time constraint. For example, leakage < 0.01CV after 2 minutes. Consult the datasheet for additional information.

Introduction to DigiKey’s Product Selection Guide

This page is one of many in DigiKey’s Product Selection Guide (PSG). This particular page is focused on aluminum electrolytic capacitors. It provides a description of the individual parameters used to characterize the part. This information will allow you to better understand and navigate DigiKey’s parametric search engine. Click here for a case study showing how to use the search tools.

How are aluminum electrolytic capacitors categorized?

The DigiKey search tools include the following specification for aluminum electrolytic capacitors.

  • Capacitance: A measurement of the device’s ability to store an electrical charge. It is measured in Farads (F) and uses the metric system for values.

  • Tolerance: Characterizes the extent of permitted variability of overall device value among samples of a given part number, resulting from variations in manufacturing.

  • Voltage - Rated: Typically this represents the maximum voltage rating to which a device will meet its given specifications. Exceeding this voltage rating may damage the device and other system components.

  • ESR (Equivalent Series Resistance): All capacitors have an internal resistance. This is similar to the Thevenin source with an ideal capacitor in series with a small output resistor. This ESR is an important consideration for high current circuits that require a low source impedance such as when the capacitor is used as a power supply energy reservoir. A low ESR implies that the capacitor can deliver high current without a drop in voltage.

  • Lifetime @ Temp: Characterizes a device’s longevity in service at the indicated temperature. Refer to the manufacturer’s documentation for supporting information.

  • Operating Temperature: Recommended operating temperature, typically given in a range or as a maximum. Exceeding these temperatures may affect performance or damage the device and other system components. Many capacitors follow the “rule of 10” where service life is doubled for every 10° C reduction in temperature.

  • Polarization: Indicates whether or not a capacitor is designed to tolerate a reversal of applied voltage during normal operation.

  • Ratings: Minimum performance standards of the device, which are typically set by a third party.

  • Applications: Identifies the type of equipment, environment, or function for which the device is intended for use.

  • Ripple Current @ Low Frequency: The maximum allowable current flow through a device at the indicated test frequency. Temperature qualifications apply, see the datasheet for details.

  • Ripple Current @ High Frequency: The maximum allowable current flow through a device at the indicated test frequency. Temperature qualifications apply, see the datasheet for details.

  • Impedance: Impedance is related to ESR, but accounts for the frequency-dependent response of the capacitor. Impedance is usually specified at a given frequency such as 100 kHz.

  • Lead Spacing: Indicates the center-to-center spacing between adjacent leads.

  • Size / Dimension: The physical dimensions of the main body of the device. Typically listed as diameter x height.

  • Height - Seated (Max): Indicates how far the device will extend above the surface to which it has been attached.

  • Surface Mount Land Size: The length and width the space of the device’s main footprint take up on the board.

  • Mounting Type: Indicates how the device is attached.

  • Package / Case: Indicates the type of protective case used on an electronic component to allow easy handling, installation, and protection. This selection was determined to be the closest industry standard applicable to the supplier’s device packaging. Typically, it is best to check the actual dimensions from the datasheet rather than depend on this terminology when designing your circuit.

Examples of aluminum electrolytic capacitors

Image of the Panasonic EEE-HC1C101XP
Image of the Panasonic EEE-HC1C101XP640×640 52 KB


  • The Nichicon brand LNK2H103MSEJ is chassis mount 100,000 uF 500 VDC aluminum electrolytic capacitor shown with mounting bracket and terminal bolts. Datasheet

Image of the Nichicon LNK2H103MSEJ
Image of the Nichicon LNK2H103MSEJ640×640 38.8 KB


Image of the Cornell Dubilier 228TTA050M
Image of the Cornell Dubilier 228TTA050M640×640 24.7 KB


Schematic symbol

The schematic symbol for the polarized and non-polarized aluminum electrolytic capacitor are shown below.

Polarized

Schematic image of a polarized electrolytic capacitor

Non-Polarized

Non-polarized image 2

Schematic symbols courtesy of Scheme-it.

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Follow up

If you have any general questions on the specifications for selecting aluminum electrolytic capacitors, please reply using the button below. If you have questions on a specific capacitor, please post your questions in the forum’s capacitor category.

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