This post contains a set of Frequency Asked Questions (FAQs) for the aluminum electrolytic capacitor. A representative surface mount example is shown in Figure 1.
Figure 1: Representative image of surface mount aluminum electrolytic capacitors.
What are the most important capacitor parameters?
For general purpose applications we can assume the capacitor is non-polarized. We then select for DC working voltage, 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.
Why did my capacitor explode? The dangers of reverse polarity and extended storage.
Electrolytic capacitors are polarity dependent. Polarity is a core function of the capacitor’s electrochemistry and their ability to form a high capacitance insulating oxide layer on one and only terminal.
Reversing polarity on an electrolytic capacitor will result in the destruction of that capacitor with potential damage to surrounding circuitry. In extreme cases, the capacitor’s electrolyte may boil. The resulting pressure may be released via the capacitor’s vent or cause the capacitor to explode. The associated traces may also vaporize. The result is a costly repair or equipment that is no longer worthy of repair—mistakes can turn equipment into scrap.
Careful installation is required to prevent capacitor damage. Consult the datasheet to determine the capacitor’s polarity.
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The negative lead of the capacitor is often marked.
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Radial capacitors as shown in Figure 2, may have different lead lengths, often with a shorter negative lead.
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For axial capacitor, the can is often the negative terminal.
Figure 2: Radial capacitor showing negative marking and short negative lead.
Tech Tip: Be careful when replacing capacitor. There have been reports of Printed Circuit Boards (PCBs) with improper silkscreen markings. We recommend you take pictures before removing the old capacitors. Orient the replacement the same way as the original.
Storage related failures
Regarding capacitor overheating and explosion, it is possible that the capacitor has been in storage for too long causing a degradation in the insulating oxide layer. This article explores the problem with a case study focused on the link capacitors as found in a Variable Frequency Drive (VFD)
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:/
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DC voltage rating
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Capacitance
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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:
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Through-hole: https://youtu.be/vLV6Lz030mA
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Surface mount: https://youtu.be/Zw_BzA0hWyQ
What is the shelf life of an aluminum electrolytic capacitor, and how can it be extended?
There are two parts to this question:
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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.
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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
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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.
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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.
Related information
Please follow these links to related and useful information:
About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.