What is an Electrolytic Capacitor?

Electrolytic Capacitor

– How Electrolytic Capacitors are made

The diagram depicts an aluminum 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 – an electrical insulation amid anode and cathode foil.

– Electrolytic Capacitors Symbol

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).

– Why choose electrolytic capacitors

An electrolytic capacitor can adapt to higher frequency circuits than a usual ceramic capacitor can do because of their 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.

Types & Applications

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.

– Aluminum 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 capacitors are largely used in camera flashes, automotive airbags, power-factor correction, etc. 

– Tantalum Electrolytic Capacitor

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). 

– Niobium Electrolytic Capacitor

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 as tantalum capacitors inherent, niobium capacitors have one more feature: 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.

Cautions 

– Do not Reverse Bias an Electrolytic 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 circuits, not AC circuits.

Errors or damages may take place if the reverse voltage is connected to an e-cap.

·Explosion due to short circuits between two terminals

·Blast from the pressure generated by H2 in the oxide layer 

·Overheat resulting from smaller resistivity and damage the overall capacitor

– Electrolytic Capacitor may Wear Out

·How long do electrolytic capacitors last? 

In general, their chemical reaction 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.

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 manufacture, they are all similar.

L: Estimated life (Hr)

L0: Life at rated temperature (Hr)

Tmax: Rated Temperature (°C)

Ta: Ambient Temperature (°C)

You can refer to the datasheets of specific capacitor products for those parameters listed above.

·Constant Test is Needed

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.

Difference between Film & Electrolytic Capacitors

As the two most frequently implemented capacitors, film capacitors and electrolytic capacitors differ from each other in many ways. 

– Structure & Performance

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 of electrolytic capacitors is more serious than the non-inductive winding film capacitors when experiencing a large ripple current. 

– Capacitance Tolerance

As for this capacitance tolerance, electrolytic capacitors are generally valued at about 20% while film capacitors often rate 10% or 5%, which is the most common value of capacitors.

– Size

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.

– Lead Length:

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.

– Capacity Value

The capacity value of the electrolytic capacitor can be made very large. The relative capacitance of the film capacitor is relatively small. 

– Safety

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.

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