Electrolytic Capacitors

Electrolytic capacitors are important components in power supplies and many solid state circuits. A circuit designer should be aware of the strengths and weaknesses found in these components.

This web page heavily borrows from the good information in this Nichicon PDF article. Some sections have been copied verbatim, while others have been abbreviated for the hobbyist.

Principles of Aluminum Electrolytic Capacitors

An aluminum electrolytic capacitor consists of cathode aluminum foil, capacitor paper (electrolytic paper), electrolyte, and an aluminum oxide film, which acts as the dielectric, formed on the anode foil surface.

Electrolytic Basics Smooth aluminum foil offers less surface area and thus less capacitance. To create larger capacitors in smaller size, oxidized aluminum films are used since the oxidized surface greatly increases the capacitor plate surface area.

Cross Sections As previously mentioned, an aluminum electrolytic capacitor is constructed by using two strips of aluminum foil (anode and cathode) with paper interleaved. This foil and paper are then wound into an element and impregnated with electrolyte. The construction of an aluminum electrolytic capacitor is illustrated at right:

Construction Since the oxide film has rectifying properties, a capacitor has polarity. If both the anode and cathode foils have an oxide film, the capacitors would be bipolar (nonpola) type capacitor.

Packaging These technical notes refer to “non-solid” aluminum electrolytic construction in which the electrolytic paper is impregnated with liquid electrolyte. There is another type of aluminum electrolytic capacitor, which is the “solid” that uses solid electrolyte.

The images at left show a low (upper) and high (lower) voltage cross section of the oxidized aluminum foil.

Anode foil and a cathode foil facing each other are interleaved with electrolytic paper and wound into a cylindrical shape. This is called a “capacitor element.” At this stage, it has configuration of a capacitor but the unit has low capacitance.

When this capacitor element is impregnated with liquid electrolyte, the anode foil and cathode foil are electrically connected. With the aluminum oxide layer formed on the anode foil acting as the sole dielectric, a capacitor with a high value of capacitance is now attainable. That is to say that the electrolyte is now functioning as a cathode.

Equivalent Series Resistance

The equivalent circuit of an aluminum electrolytic capacitor is shown at left. The equivalent series resistance is also known as “ESR”.

A reactance value due to the equivalent series inductance “L” is extremely small at low frequencies (50Hz~1kHz) and can be regarded as zero. See PDF referenced at top, for formulas.

The graph of ESR response to frequency is shown at right.

Leakage Current

The causes of leakage current in aluminum electrolytic capacitors are listed below :

  1. Distorted polarization of dielectric (aluminum oxide layer)
  2. Resolution and formation of dielectric
  3. Moisture absorption by dielectric
  4. Breakdown of dielectric due to the existence of chlorine or iron particles.

The leakage current value can be decreased by proper selection of materials and production methods; however, cannot be totally eliminated.

Leakage current is also dependent upon time, applied voltage and temperature.

The specified leakage current value is measured after the rated voltage of the capacitor is applied at room temperature for a specified time period. When selecting a capacitor for a particular application, characteristics such as temperature dependency, aging stability and etc. must be taken into account.

Temperature Characteristics

Aluminum electrolytic capacitors have liquid electrolyte. This electrolyte has properties (conductivity, viscosity, etc.) that have rather conspicuous temperature characteristics.

Electrical conductivity increases as the temperature increases and reduces as the temperature decreases. Therefore, the electrical characteristics of aluminum electrolytics are affected by temperature more than other types of capacitors. The following section explains the relationship between temperature and capacitance, tangent delta, ESR, impedance and leakage current.

Capacitance

Capacitance vs Temperature The capacitance of aluminum electrolytic capacitors increases as the temperature increases and decreases as the temperature decreases. The relationship between temperature and capacitance is shown in Fig. 1-9.

Tanδ, Equivalent Series Resistance (ESR), Impedance

ESR, Impedance vs Frequency The Tanδ, equivalent series resistance (ESR) and impedance changes with temperature and frequency. An example of the general characteristics is shown in Fig. 1-11 (see PDF for Tanδ Fig 1-10).

Impedance Ratio

The ratio between the impedance at 20oC and the impedance at various temperatures is called the impedance ratio. Impedance ratio becomes smaller as smaller change of ESR and capacitance with temperature. The quality of performance at low temperatures is particularly expressed with the impedance ratio at 120Hz.

Leakage Current

The leakage current increases as the temperature increases and decreases as the temperature decreases. Fig. 1-12 shows the relationship between temperature and leakage current.

Application Guidelines

  • Operating temperature and applied ripple current should be within the specification.
  • The capacitor shall not be used in an ambient temperature which exceeds the operating temperature specified in the specification.
  • Do not apply excessive current which exceeds the allowable ripple current.
  • Appropriate capacitors which comply with the life requirement of the products should be selected when designing the circuit.
  • Aluminum electrolytic capacitors are polarized. Make sure that no reverse voltage or AC voltage is applied to the capacitors. Use bi-polar capacitors for a circuit that can possibly see reversed polarity. Note that even bi-polar capacitors can not be used for AC voltage applications.
  • For a circuit that repeats rapid charging/discharging of electricity, an appropriate capacitor that is capable of enduring such a condition must be used. Welding machines and photo flash are a few examples of products that contain such a circuit. In addition, rapid charging/discharging may be repeated in control circuits for servomotors, In which the circuit voltage fluctuates substantially.
  • Make sure that no excess voltage (that is, higher than the rated voltage) is applied to the capacitor.
  • Please pay attention so that the peak voltage, which is DC voltage overlapped by ripple current, will not exceed the rated voltage.
  • In the case where more than 2 aluminum electrolytic capacitors are used in series, please make sure that applied voltage will be lower than rated voltage and the voltage be will applied to each capacitor equally using a balancing resistor in parallel with the capacitors.
  • Aluminum electrolytic capacitors must be electrically isolated.
  • The aluminum case and the cathode foil are connected by the unstable resistance of a naturally formed oxide layer inside the aluminum case and the electrolyte.
  • Outer sleeve of the capacitor is not guaranteed as an electrical insulator. Do not use a standard sleeve on a capacitor in applications that require the electrical insulation.
  • Capacitors may fail if they are used under the following conditions:
  • Environmental (climatic) conditions
    • Being exposed to water, high temperature & high humidity atmosphere, or condensation of moisture.
    • Being exposed to oil or an atmosphere that is filled with particles of oil.
    • Being exposed to salty water or an atmosphere that is filled with particles of salt.
    • In an atmosphere filled whith toxic gasses (such as hydrogen sulfide, sulfurous acid, nitrous acid, chlorine, bromine, methyl bromide, ammonia, etc.)
    • Being exposed to direct sunlight, ozone, ultraviolet ray, or radiation
    • Being exposed to acidic or alkaline solutions
  • Under severe conditions where vibration and/or mechanical shock exceed the applicable ranges of the specifications.
  • When designing a P.C. board, please pay attention to the following:
    • Have the hole spacing on the P.C. board match the lead spacing of the capacitor.
    • There should not be any circuit pattern or circuit wire above the capacitor pressure relief vent.
    • Allow adequate clearance above the pressure relief vent (see PDF).
    • In case the vent side is placed toward P.C. board (such as end seal vented parts), make a corresponding hole on the P.C. board to release the gas when vent is operated. The hole should be made to match the capacitor vent position.
    • Screw terminal capacitors must be installed with their end seal side facing up.
    • When you install a screw terminal capacitor in a horizontal position, the positive terminal must be in the upper position.
  • The main chemical solution of the electrolyte and the separator paper used in the capacitors are combustible. The electrolyte is conductive. When it comes in contact with the P.C. board, there is a possibility of pattern corrosion or short circuit between the circuit pattern which could result in smoking or catching fire. Do not locate any circuit pattern beneath the capacitor end seal.
  • Do not design a circuit board so that heat generating components are placed near an aluminum electrolytic capacitor or reverse side of P.C. board (under the capacitor).
  • When you install more than 2 capacitors in parallel, consider the balance of current flowing through the capacitors. Especially, When a solid conductive poIymer aluminum electroIytic capacitor and a standard aIuminum electroIytic capacitor are conected in parallel.
  • If more than 2 aluminum electrolytic capacitors are used in series, make sure the applied voltage will be lower than the rated voltage and that voltage will be applied to each capacitor equally using a balancing resistor in parallel with each capacitor.

Safety

Do not bring your face near the capacitor when the pressure relief vent operates. The gasses emitted from that are over 100C.

  • If the gas gets into your eyes, please flush your eyes immediately in pure water.
  • If you breathe the gas, immediately wash out your mouth and throat with water.
  • Do not ingest electrolyte. If your skin is exposed to electrolyte, please wash it away using soap and water.

The internal pressure of the capacitor will rise due to gas generation caused by heat generation, evaporation of electrolyte or electrolytic dissociation if the following is applied : extreme voltage, reverse voltage, AC current or extreme ripple. With this in mind, the pressure relief vent is provided to release internal pressure. There are two types of pressure relief vents classified by their location on the capacitor :

  1. end seal
  2. aluminum case

Storage

lt is recommended to keep capacitors between the ambient temperatures of 5C.

Misc

See the Nichicon PDF article for more technical information about lifetime, balancing resistors etc.

electrolytic_capacitors.txt · Last modified: 2011/08/02 21:24 by ve3wwg
 
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