An electrolytic capacitor is a type of capacitor that uses an electrolyte to produce a higher capacitance than other types of capacitors. The electrolyte is a fluid or gel with a large number of ions. The high capacitive reactance of electrolytic capacitors has advantages and disadvantages.
They are characterized by high leakage current, equivalent series resistance, and limited service life. Electrolytic capacitors can be wet electrolytes or unidirectional polymer capacitors. They are usually made of aluminum or tantalum, but other materials can also be used. Most electrolytic capacitors have polarization, which means that the voltage at the positive electrode must be greater than the voltage at the negative electrode.
The figure above is the internal structure diagram of a commonly used electrolytic capacitor. Most electrolytic capacitors (also known as e-caps) are polarized capacitors that are mainly formed of two thin layers of metal foil and paper spacer filled with electrolyte. A dielectric oxide layer acts as a dielectric medium - electrical insulation amid anode and cathode foil.
The supercapacitor is a unique electrolytic capacitor, usually called a double-layer capacitor. Their power range can range from hundreds of farads to thousands of farads.
The diverse qualities of oxide structures have an impact on the electrolytic capacitors' following characteristics. Electrolytic capacitors with stable long-life properties, capacitors with low leakage current values, and e-caps with rated voltages up to roughly 100 volts all employ amorphous oxide anode foils. Capacitors with greater voltages, such as photoflash capacitors, frequently use crystalline oxide anode foils.
There are two symbols for capacitors typically employed to represent capacitors in the field of electronics. One symbol is used for capacitors that are polarized, and another symbol is used for capacitors that are not polarized.
In the diagram above the symbol with one curved plate is a Polarized Capacitor. The curved portion represents a cathode ( negative) of the capacitor, while the opposite plate represents one of the electrodes ( positive). Sometimes a plus sign is also added to the positive side. The most common symbol for an electrolytic capacitor (polarized capacitor). The left diagram is by IEC standard (Europe) while the right one is by ANSI standard (the US).
An electrolytic capacitor can adapt to higher frequency circuits than a usual ceramic capacitor can do because of its high capacitance values. Besides, they do a better job than super-capacitors in handling ripple current. And they are built to low volume which saves lots of space on a board, saving direct cost.
Non-solid electrolyte electrolytic capacitors have a polarity marking on the cathode (minus) side and a shorter lead.
Except for cylindrical led (single-ended) and SMD polymer capacitors, electrolytic capacitors with solid electrolytes have a polarity marking on the anode (plus) side.
This is not the most common marking on electrolytic capacitors, but it is more common on axial capacitors like this one. The arrows contain plus signs. As a result, they point to the positive terminal. On the right is the negative side. Capacitors with both leads exiting the same side of the can typically have a stripe with "minus" signs or dashes inside it, near one of the legs, indicating the negative lead. This lead will be slightly shorter on new capacitors as well.
As one of the most important passive components, the electrolytic capacitor is a big family and has developed for nearly a century as a classic passive electronic component. Among the wide spectrum of e-cap, there are 3 kinds of them widely used in industry practice. They vary from each other in size, capacitance, and applications.
Aluminum, Tantalum, and Niobium Electrolytic Capacitor
This kind of electrolytic capacitor has passed the longest period of design and manufacture, from the “Wet” type to the “Dry” type. They are named from the aluminum can which rolls up the anode and cathode and can be divided into the solid type and the non-solid type.
Nowadays, aluminum electrolytic capacitors are largely used in camera flashes, automotive airbags, power-factor correction, etc.
Tantalum capacitors are much more long-price than aluminum electrolytic capacitors. They produce low leakage with a high capacity and are often used with ceramic or film capacitors in power supply decoupling targets. Tantalum electrolytic capacitors feature a self-healing mechanism that functions to reduce MnO2 electrolytes into insulating Mn2O3. In addition, they behave superiorly stable in harsh environments (especially high-temperature ambient).
The niobium capacitor is relatively a young competitor in the market for the tantalum capacitor. They have a thicker dielectric layer and a lower breakdown voltage than tantalum capacitors. Besides the self-healing mechanism of tantalum capacitors inherent, niobium capacitors have one more feature: a self-arresting mechanism. The mechanism operates to protect the chip from short circuits fault on account of local breakdown. What's more, they excel in resisting vibrations and shocks.
Niobium electrolytic capacitors favor their use in consumer, industrial, automotive, aircraft fields, and so on.
Noted: Although not all SMD capacitors employ electrolytic construction, some do. Electrolytic capacitors are commonly referred to as through-hole capacitors due to their greater capacitance levels and voltage compatibility. Even though your board space is restricted and small SMD capacitors appear to be required, you may be able to find an electrolytic capacitor that will suffice.
In the case of SMD (surface area mounted) electrolytic capacitors, there are 2 basic marking types. The very first one specifies the value in microfarads and the operating voltage. For instance, using this technique, a 4.7 μF capacitor with an operating voltage of 25 volts would bear the marking "4.7 25V. In the other marking system, a letter is followed by 3 numbers. The letter represents the voltage score according to the table listed below. The first two numbers represent the worth in picofarads, while the third number is the absolute zero to be contributed to the first 2. For example, a 4.7 μF capacitor with a voltage score of 25 volts would bear the marking E476. This equates to 47000000 pF = 47000 nF = 47 μF.
An Exploding Capacitor
Although some bipolar e-caps can withstand a certain reverse voltage for a while, the constructing characteristics determine that electrolytic capacitors should be run from a higher voltage on the positive end and a lower one on the negative end.
Furthermore, general polarized electrolytic capacitors are designated to work only in a DC circuit, not AC circuits. Errors or damages may take place if the reverse voltage is connected to an e-cap.
Opens and shorts are the two most common capacitor failure types for paper and plastic film capacitors. Intermittent openings, shorts, and high resistance shorts all fall within this category. Capacitors may also fail to owe to capacitance drift, thermal instability, high dissipation factor, or low insulation resistance, addition to these failures.
The capacitor plague was the cause of several consumer electronics failures in the mid-to-late-2000s. The capacitor plague was caused by faulty electrolytic capacitors manufactured between 1999 and 2007, with the first problems appearing around 2002. The signs are simple to spot: if a gadget from this era isn't working, check the power source. You have capacitor plague if there is goo surrounding the caps or the tops are broken open.
A failure rate and useful life must be considered when building a device with aluminum electrolytic capacitors for its dependability. The bathtub curve is a good approximation of the failure rate of aluminum electrolytic capacitors.
In general, their chemical failure rate (life consumption) increases as temperature increases. The changing pattern is known as the 10-degree-rule. That is, every 10% of uplift by temperature brings about double the length of an electrolytic capacitor.
The graph below demonstrates the typical pattern of an electrolytic capacitor lifetime.
Failure rates are frequently expressed as percentages per 1000 hours (10-5/hour). Units of Failure In Time (FIT) (10-9/hour) are used for greater reliability devices with a lower failure rate.
Besides, due to their structure, electrolytic capacitors are very subject to degradation in regards to long-term storage to PCB. Electrolyte evaporation and dielectric dissolution are two main degradation modes of electrolytic capacitors.
Electrolytic capacitors' lifespan calculation can be demonstrated through the Arrhenius equation (figure 4) when designing circuits. Although it should be noted that the life calculation formulas vary from manufacturer to manufacturer, they are all similar.
L: Estimated life (Hr)
L0: Life at rated temperature (Hr)
Tmax: Maximum Rated Temperature (°C)
Ta: Ambient Temperature (°C)
You can refer to the datasheets of specific capacitor products for those parameters listed above.
A simple way to go is to check and replace the electrolytic capacitors every several years to prevent them from hindering the operation of the circuits or causing further damage.
In simplest terms, cooling the capacitor makes it work to a longer lifespan and perform better power carry capacity, too. It's a double benefit method. As time passes, the industrial design requirement of more condensed packages outpaced the heat-sink consideration for capacitors’ building path, leading to a shorter capacitor lifetime.
So what do we do? At the current time, an aluminum chassis or panel attached to the capacitor part could help remove the heat. What's more, an external heat dissipater could take away more heat (see the graph below) and the electrical parameters of the capacitor are not compromised.
As the two most frequently implemented capacitors, film capacitors and electrolytic capacitors differ from each other in many ways.
Electrolytic capacitors utilize a dielectric material that cannot offer perfect insulation to separate the conductive plates, resulting in large ESL (Equivalent Series Inductance) and ESR (Equivalent Series Resistance) leakage. Thus, the heat dissipation factor of electrolytic capacitors is more serious than the non-inductive winding film capacitors when experiencing a large ripple current. That's why low ESR is important in capacitor design.
As for this capacitance tolerance, electrolytic capacitors are generally valued at about 20% while film capacitors often rate at 10% or 5%, which is the most common value of capacitors.
Usually, the size of film capacitors is larger than that of the electrolytic capacitor. And it is one of the main factors that contribute to the low impedance of electrolytic capacitors.
An electrolytic capacitor has two leads of uneven length within which the shorter one is negative and the longer one is positive. Film capacitors are non-polar so they have two leads of equal length.
The capacity value of the electrolytic capacitor can be made very large. The relative capacitance of the film capacitor is relatively small.
The film capacitors are relatively safe and easy to use compared with electrolytic capacitors under the same storage conditions. The environment required by the electrolytic capacitor in the process of use is more stringent.
Because of the characteristics of electrolytic capacitors, they are usually engraved with grooves at the bottom of the body to prevent explosions from bringing secondary damage to other components on PCB boards.
Among all kinds of film capacitors, DC-link capacitors are assumed to be an appropriate replacement for electrolytic capacitors. DC-link capacitors are made with polypropylene (pp) films which is suitable for high voltage power electronics.
Electrolytic capacitors have lower stability compared to ceramic caps and their capacity could alter under high voltage and temperature changes.
Ceramic capacitors can sustain a high frequency, high voltage condition with their capacity remaining stable.
Though they may occur in the same application as a personal computer mainboard, electrolytic capacitors do a great job in low-frequency decoupling/filtering and thus can be used as low-frequency filters.
On the other hand, ceramic capacitors are a better choice when dealing with high-frequency noise.
As for the appearance, an electrolytic capacitor looks like a cola can with versatile sizes.
Ceramic capacitors are disc-shaped components and take the size of about a coin or even smaller.
As has been mentioned, an electrolytic cap is generally polarized (but still there are some special ones designed for AC circuit use).
Ceramic capacitors are typically non-polarized and don’t require a forward-biased voltage.
Several factors you need to think twice about when choosing an electrolytic capacitor include appropriate voltage ratings, ESR, ripple current, heat dissipation, and surge. Also, we are glad to hear your thinking about the topic. If it helps you, please share it with your friends.
(by Easybom)
Marshall is a knowledgeable power electronics engineer with a focus on renewable energy systems. He has a strong background in circuit analysis and simulation and has worked on a variety of projects ranging from small-scale solar installations to large-scale wind farms.
Laura · October 14,2021
there are three methods for telling the positive and negtive of a electrolytic capacitor.
Basic Electronics Technology/TutorialsAccess：5057
Laura · October 12,2021
When the capacitor is connected to a DC voltage source, the capacitor will be charged
Basic Electronics Technology/TutorialsAccess：874
Laura · October 12,2021
Depending on the capacity of the electrolytic capacitor, the R×10, R×100, and R×1K gears of the multimeter are usually used for testing and judgment
Basic Electronics Technology/TutorialsAccess：1319
Laura · October 10,2021
Use the resistance file of a multimeter to check the quality of the electrolytic capacitor.
Basic Electronics Technology/TutorialsAccess：2843
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