Electrolytic Capacitor – Definition, Symbol and Characteristics

Electrolytic Capacitor

A capacitor that uses an electrolyte to obtain high capacitance is an electrolytic capacitor. An electrolyte is a fluid or gel with a very high concentration of ions. So, before learning about electrolytic capacitors, it is important to learn about the terms electrolyte and capacitor.

What is a Capacitor?

A capacitor is similar to a battery, but the two work differently from one another. A battery is an electronic device that converts chemical energy into electrical energy. A capacitor stores electrostatic energy in an electric field.

A capacitor is a two-terminal electrical device. It can store energy in the form of an electric charge. It comprises two electrical conductors separated by a distance, and the space between them is filled by a vacuum or an insulating material called a dielectric.

Capacitance is the ability of the capacitor to store charges. So, a capacitor stores energy by holding pairs of opposite charges at a distance. The simplest design of capacitors comprises two metal plates with a gap between them.

What is an Electrolytic Capacitor?

A capacitor with an electrolyte between its two charged ends is called an electrolytic capacitor. It is a general term covering three different capacitors:

• Aluminum electrolytic capacitor
• Tantalum electrolytic capacitor
• Niobium electrolytic capacitor.

The electrolytes of these capacitors contain aluminum or tantalum and other metals. The ability of large capacitance makes electrolytic capacitors useful for sending low-frequency signals. These capacitors are broadly used for noise filtering. They also find applications in decoupling power supplies. However, along with the benefit of large capacitance come some drawbacks, such as leakage currents, a limited lifetime, and equivalent series resistance.

Did you know: Some special electrolytic capacitors have capacitances up to thousands of farads. These are called supercapacitors or double-layer electrolytic capacitors.

Varieties of Electrolytic Capacitors

Each of the three electrolytic capacitors uses non-solid and solid manganese dioxide. It can also contain solid polymer electrolytes.

Depending on the anode metal and the electrolyte used, the varieties of electrolytic capacitors are as follows:

Aluminum Electrolytic Capacitors	Non-Solid Electrolyte Organic /inorganic solvents	Solid Electrolyte MnO2
Tantalum Electrolytic Capacitors	Non-Solid Electrolyte Sulphuric Acid Wet slug	Solid Electrolyte MnO2
Niobium Oxide Electrolytic Capacitors	–	Solid Electrolyte MnO2

Construction of Electrolytic Capacitors

Aluminum electrolytic capacitors comprise the following:

• two aluminum foils
• paper spacer soaked in an electrolyte

One aluminum foil stays covered with an oxide layer; this foil acts as the anode. The uncoated aluminum foil acts as a cathode.

In normal operation, the anode is at a positive voltage in contrast to the cathode. Thus, the cathode is often marked with a minus sign on the body of the capacitor.

The electrolyte-soaked paper (the anode) and cathode are piled up. The pile is then rolled and placed into a cylindrical enclosure. They are connected to the circuit via pins.

The two common geometries are:

• Axial
• Radial

Axial capacitors consist of one pin on each end of the cylinder. Contrastingly, radial geometry capacitors have both pins on the same cylinder end.

Did you know: Why are some capacitors so big? A capacitor with large capacitance is bulky and larger in size. The breakdown voltage of a dielectric layer is proportional to the layer thickness. So, making thicker layers creates capacitors with larger voltage ratings. The capacitance of an electrolytic capacitor depends on several factors, and plate area and electrolyte thickness are among those factors too.

Electrolytic Capacitor Symbol

It comprises an aluminum or tantalum plate with an oxide dielectric layer. A liquid electrolyte is the other electrode. These polarized capacitors provide high capacitance. However, they have low tolerance and high explosion risk. The following figure shows the symbol of electrolytic capacitor:

Symbol Electrolytic Capacitor

Electrolytic Capacitor Polarity

Electrolytic capacitors are polarized. This polarization is due to their asymmetrical construction. They need to be operated with a higher voltage wherein there is more positive charge on the anode than on the cathode. Often It has polarity marks on the housing.

Nearly every electrolytic capacitor is polarized, i.e., the voltage of the anode is always higher than the cathode.

Characteristics of Electrolytic Capacitors

The electrical characteristics of electrolytic capacitors are majorly influenced by the electrolyte and the anode used. The primary characteristics are as follows:

1. Capacitance and Tolerance:

The electrolyte and anode of the electrolytic capacitor affect the value of capacitance. Capacitance largely depends on frequency and temperature. Moreover, capacitors with non-solid electrolytes exhibit a wider sense of frequency and temperature than capacitors with solid electrolytes.

Capacitance is measured in microfarad. The capacitance value specified by manufacturers is called nominal or rated capacitance. The percentage deviation from the rated value is called capacity tolerance.

2. Capacitance Drift:

The capacitance has a large tolerance of 20%. However, it drifts from nominal value with the passage of time. So, an aluminum electrolytic capacitor with a nominal capacitance of 47µF will have a capacitance between 37.6µF to 56.4µF.

Although tantalum capacitors also have higher tolerances, a low maximum operating voltage. Therefore, they can’t be used as a perfect replacement for aluminum capacitors.

3. Shelf life of electrolytic capacitors:

It is important to note that capacitors made using old technologies had a shorter shelf life of only up to a few months.

Do you know why?

Because when they were left unused, the oxide layer deteriorated. However, it is possible to rebuild the deteriorated layer in a process called capacitor reforming. This process is done by connecting the electrolytic capacitor to a voltage source via a resistor. The voltage is slowly increased until the oxide layer is completely rebuilt.

Modern electrolytic capacitors come with longer shelf lives of two years or more. However, if you leave these capacitors unpolarized for longer periods, they will also have to be reformed before use.

Applications of Electrolytic Capacitors

Some applications of electrolytic capacitors are as follows:

• They are used in filtering devices to reduce voltage fluctuations.
• They are suitable for output and input smoothing to filter when there is a weak DC signal with the AC component.
• They are widely used for noise filtering.
• These are used for decoupling power supplies.
• They are also utilized for coupling signals between amplifier stages.
• They help store energy in flashlamps.

How would you read the capacitance value?

Usually, in the case of a through-hole capacitor, its capacitance value and maximum rated voltage are given on the body.

You will come across a printed text like “4.7μF 25V“ on it. This text implies that an electrolytic capacitor has 4.7μF as its nominal capacitance value. 25V means that its maximum voltage rating is 25 volts, and this value should never be exceeded.

In the case of surface-mounted electrolytic capacitors, you will find two basic markings. The first one is for the capacitance value in microfarads, while the second is the operating voltage.

So, these capacitors will bear a marking as follows:

4.7 25V

It means that the capacitor has 4.7 μF capacitance and 25 Volts operating voltage.

In another marking system, you will see a letter followed by three numbers. This letter stands for voltage rating. The meaning of different letters is shared in the table below. Amongst the three numbers, the first two represent the value in picofarads, and the third number tells how many zeros you must add to the first two. The following example will help you understand better.

If you come across an electrolytic capacitor bearing the marking– E476, it means E stands for 25 volts while 476 means six zeros are to be added to 47 to get the capacitance in picoFarads. Therefore, the capacitor reading will be 47000000 pF = 47000 nF = 47 μF.

The letters on the capacitors represent the following maximum voltage values:

Letter	Voltage
e	2.5
G	4
J	6.3
A	10
C	16
D	20
E	25
V	35
H	50

How is an Electrolytic Capacitor Different from a Ceramic Capacitor?

The following table enumerates the key differences between an electrolytic capacitor and a non-electrolytic or ceramic capacitor:

Characteristics	Electrolytic Capacitor	Ceramic Capacitor
Polarization	It is a type of polarized capacitor.	It is not a polarized capacitor.
Anode terminal	Its anode terminal is greater than the cathode.	Both terminals are equal.
Range	0.1 μF to 4700 μF	10 pF to 0.1 μF
Temperature Stability	Poor	Good
Tolerance	High	Low
Life	Shorter	Longer

Conclusion

An electrolytic capacitor helps attain high capacitance with the introduction of an electrolyte. Its anode terminal is always higher than the cathode. These capacitors find applications in various fields, but the most common use is to reduce voltage fluctuations in filtering devices.

Frequently Asked Questions

Q1. What is capacitance?

Capacitance describes the ratio of variation in the electric charge (Q) of an electrical system to the corresponding electrical potential variation.

Q2. What is a capacitor?

Capacitors are an electrical component that stores energy as electric charges. It consists of dual conductors separated by a marked distance. The space between the conductors is filled with a dielectric or vacuum.

Q3. What is the unit of capacitance?

The SI unit of capacitance is Farad (abbreviated as F). It can also be measured in microFarads. The unit is named after the scientist Michael Faraday.