What is Electrolytic Capacitor : Construction, Symbols & Advantages

An electrolytic capacitor is popularly known as a polarized capacitor, wherein the anode has more positive voltage than the cathode. They are used in filtering applications, low-pass filters, audio amplifier circuits, and many more. Metals like aluminum, tantalum, niobium, manganese, etc. form an oxide layer in the electrochemical process, which blocks the electric current flowing in one direction but permits the flow of current in the opposite direction. This phenomenon was first observed by Johann Heinrich Buff (1805–1878), the German physicist and Chemist in 1857. The French researcher and founder Eugene Ducretet in 1875 was the first person to implement this idea and invented the term “valve metal” for these metals. The actual development of electrolytic capacitors with wound foils is separated by paper began by A. Eckel of Hydra-Werke (Germany) in 1927 combined with Samuel Ruben’s idea of stacked construction.

What is an Electrolytic Capacitor?

An electrolytic capacitor definition is, it is a polarized capacitor whose anode has a higher or more positive voltage than the cathode. As the name suggests it is a polarized capacitor and an electrolytic capacitor function is, it uses an electrolyte to operate with a higher or more positive voltage on the anode than the cathode. Therefore, the anode terminal is denoted with a positive sign, while the cathode with a negative sign. Applying a reverse polarity voltage of 1 to 1.5 volts may destroy the capacitor and dielectric and the result is hazardous, leading to an explosion or fire.

An electrolytic capacitor uses an electrolyte, in the form of solid, liquid or gel – serves as cathode or negative plate to achieve much higher capacitance per unit volume. On the other hand, a positive plate or anode made of metal acts as an insulating oxide layer formed through anodization. This allows an oxide layer to work as the dielectric of the capacitor.

Construction

The electrolytic capacitor construction involves two thin layers of aluminum foil – the plain foil and the etched foil. These two foils are separated by an electrolyte. To set up the polarity of two foils, they are anodized by chemically growing a thin layer of aluminum oxide to form the anode and distinguishes itself from the cathode. In the process of electrolytic capacitor construction, the cathode and the anodized anode are formed, which is separated by an electrolyte (paper soaked with electrolyte).

During standard operation, the anode is held at positive concerning the cathode, hence cathode is indicated with negative (-) sign on the capacitor’s body. As aluminum is a polarized device, applying a reverse voltage on these terminals would produce the insulation in the capacitor, damaging the capacitor.

The unique property of an aluminum capacitor is the self-healing process of a damaged capacitor. During the reverse voltage, the oxide layer gets removed from the foil, yet allowing the current to pass from one foil to another.

Electrolytic Capacitor Symbol

The electrolytic capacitor symbol is shown in the figure below. The capacitor symbols are of two types. The second symbol (b) represents the polarized capacitor, which can be an electrolytic or tantalum capacitor. The curved plate on the symbol implies that the capacitor is polarized and is the cathode, which is held at a lower voltage than the anode. The first symbol (a) in the figure below represents the non-polarised capacitor.

Polarity

Knowing the polarity of any device is important for building any electronic circuits. Connecting in the otherwise around can destroy the capacitor. Though some capacitors are not polarized, like ceramic capacitors (1 µF or less), they can be connected in either way.

In certain cases, the capacitor’s positive lead would be longer than the negative lead. Sometimes, the capacitor terminals are trimmed, wherein the user must be careful in connecting the capacitor.

The tantalum and aluminum capacitors consist of a polarity marked with the plus (+) sign indicating anode side.

Non-solid electrolyte type of electrolytic capacitor consists of a polarity marked with the minus (-) sign indicating the cathode side.

The solid electrolyte type of electrolytic capacitors consists of a polarity marked with the plus sign indicating anode side, but absent for cylindrical led and SMD polymer capacitors.

Electrolytic Capacitor Values

Depending on the anode and electrolyte structure, the electrolytic capacitance values tend to get influenced. With non-solid electrolyte, electrolytic capacitors display a wider deviation for frequency and temperature ranges than the solid electrolytes.

The electrolytic capacitor’s basic unit is expressed as microfarad (μF). In the datasheets prepared by manufacturers, the capacitance value is mentioned as rated capacitance (CR) or nominal capacitance (CN). These are the values for which the capacitance is being designed.

The electrolytic capacitors are the large, cylindrical structure, which is polarised and has higher capacitance.

Electrolytic Capacitor Values and units are legibly printed on the body of capacitors. Beginning from left to right, 1µF, 10µF, 100µF, 1000µF.

Electrolytic Capacitor Types

Based on the type of material and electrolyte used, the electrolytic capacitors are classified into the following types.

Aluminum Electrolytic Capacitor

Aluminum electrolytic capacitors are polarized capacitors, in which the anode (+) terminal is formed of aluminum foil along with an etched surface. The anodization process produces a thin insulating layer of oxide, which acts as a dielectric. The cathode is formed through second aluminum foil when a non-solid electrolyte masks the rough surface area of the oxide layer.

Non-Electrolytic Capacitor

The non-electrolytic capacitors are those capacitors that comprise of an “insulating material” as a dielectric in non-electrolytic form. Such type of capacitors are non-polarized and has many uses.

Tantalum Electrolytic Capacitor

Tantalum electrolytic capacitor offers lower leakage current and ESR. It uses tantalum metal that works as an anode, enclosed by a layer of oxide to work as a dielectric, and further wrapped with a conductive cathode. These capacitors are innately polarized devices and are highly stable. It operates efficiently with exceptional frequency when connected correctly.

Niobium Oxide- Electrolytic Capacitor

The construction of Niobium oxide-electrolytic capacitors is similar to tantalum capacitors. It used niobium oxide instead of tantalum metal to act as the anode. Niobium oxide is available abundantly and offers extremely stable characteristics than the tantalum capacitor.

Uses/Applications

A wide range of electrolytic capacitor applications are as follows

• Used in filtering applications to reduce the ripple in the power supplies
• Used as a low pass filter to smoothen the input and output signals
• Used in audio amplification circuits as filters to reduce hum

Advantages and Disadvantages

The advantages of the electrolytic capacitor are

• Used to achieve high capacitance value
• Used in low-frequency applications
• Tantalum capacitors are preferred over other types because of the high stability disadvantages of the electrolytic capacitor are as follows:
• Must be attentive to ensure that the capacitors are corrected with right terminals
• Reverse voltage may damage the capacitor
• Easily gets affected due to temperature change
• Capacitor when used with a combination of non-electrolyte increases the capacitor size

FAQ’s

1. Where electrolytic capacitors are used?

They are used in filtering applications, audio amplification circuits, and in low pass filters

2. How do you identify an electrolytic capacitor?

Electrolytic capacitors are usually marked with a stripe, which indicates the negative lead. The positive lead is usually longer than the negative lead.

3. Do capacitors have oil in them?

Yes. Oil-filled capacitors are available and they are generally of high power and high voltage.

4. Are electrolytic capacitor AC or DC?

Electrolytic capacitors are generally used in circuits with DC power supply. AC voltages may damage the capacitor.

5. What is the average lifespan of a capacitor?

The average lifespan of a capacitor is expected to be 15years. The lifespan can be reduced if the ripple current is excessive and heats the capacitor.

In this article, the reader gets to know the insights of the electrolytic capacitor. We discussed the definition, construction, polarity and marking, applications and advantages and disadvantages. Further, the reader can know the types of electrolytic capacitors.