ELECTROLYTIC CAPACITOR

                Electrolytic capacitor development

     The electrolytic capacitor has been in use for many years. Its history can be traced back to the very early days or radio around the time when the first broadcasts of entertainment were being made. At the time, valve wireless sets were very expensive, and they had to run from batteries. However with the development of the indirectly heated valve or vacuum tube it became possible to use AC mains power.

    While it was fine for the heaters to run from an AC supply, the anode supply needed to be rectified and smoothed to prevent mains hum appearing on the audio. In order to be able to use a capacitor that was not too large Julius Lilienfield who was heavily involved in developing wireless sets for domestic use was able to develop the electrolytic capacitor, allowing a component with sufficiently high capacitance but reasonable size to be used in the wireless sets of the day.

                       Electrolytic capacitor symbols

     The electrolytic capacitor is a form of polarised capacitor. The electrolytic circuit symbol indicates the polarity as it is essential to ensure that the capacitor is fitted into the circuit correctly and is not reverse biased.

Circuit symbols used for polarised capacitors like the electrolytic capacitor

There is a variety of schematic symbols used for electrolytic capacitors. The first one '1' is the version that tends to be used in European circuit diagrams, while '2' is used in many US schematics, and '3' may be seen on some older schematics. Some schematic diagrams do not print the "+" adjacent to the symbol where it is already obvious which plate is which.

                           Electrolytic capacitor technology

As the name indicates, the electrolytic capacitor uses an electrolyte (an ionic conducting liquid) as one of its plates to achieve a larger capacitance per unit volume than other types.

           The capacitors are able to increase the capacitance in a number of ways: increasing the dielectric constant; increasing the electrode surface area; and by decreasing the distance between the electrodes. Electrolytic capacitors use the high dielectric constant of the aluminium oxide layer on the plate of the capacitor which averages between 7 and 8. This is greater than other dielectrics such as mylar which has a dielectric constant of 3 and mica of around 6 - 8.

In addition to this, the effective surface area within the capacitors is increased by a factor of up to 120 by roughening the surface of the high-purity aluminium foil. This is one of the keys to producing very high levels of capacitance.

               Construction of electrolytic capacitors

     The plates of an electrolytic capacitor are constructed from conducting aluminium foil. As a result they can be made very thin and they are also flexible so that they can be packaged easily at the end of the production process.

     The two plates, or foils are slightly different. One is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte is placed between them. The foil insulated by the oxide layer is the anode while the liquid electrolyte. The thickness of the anode oxide thin film in an aluminium electrolytic capacitor is selected by the required working withstand voltage. The second foil acts as the cathode and although this does have a naturally occurring oxide layer, this is very much thinner.

Electrolytic capacitor structure

In order to package them the two aluminium foils with the electrolyte soaked paper are rolled together to form a cylinder, and they are placed into an aluminium can. In this way the electrolytic capacitor is compact while being robust as a result of the protection afforded by the can.

There are two geometries that are used for the connection leads or tags. One is to use axial leads, one coming from each circular face of the cylinder. The other alternative is to use two radial leads or tags, both of which come from the same face of the cylinder.

The lead styles give rise to the descriptions used for the overall capacitors. Descriptions of axial and radial will be seen in the component references.

For manufacture of the electrolytic capacitor it is necessary to use high purity foil for the anode. Typcailly this is between 50 and 100µm thick. The cathode still uses well refined aluminium but the requirements are not as stringent as those for the anode. The foil used is between about 20 and 50 µm thick.

To increase the surface area of both anode and cathode to increase the capacitance, the surface is roughened by etching. There are two methods that are used but both involve the use of hydrochloric acid.

Electrolytic capacitor properties

There are a number of parameters of importance beyond the basic capacitance and capacitive reactance when using electrolytic capacitors. When designing circuits using electrolytic capacitors it is necessary to take these additional parameters into consideration for some designs, and to be aware of them when using electrolytic capacitors.

1. ESR Equivalent series resistance:   Electrolytic capacitors are often used in circuits where current levels are relatively high. Also under some circumstances and current sourced from them needs to have a low source impedance, for example when the capacitor is being used in a power supply circuit as a reservoir capacitor. Under these conditions it is necessary to consult the manufacturers datasheets to discover whether the electrolytic capacitor chosen will meet the requirements for the circuit. If the ESR is high, then it will not be able to deliver the required amount of current in the circuit, without a voltage drop resulting from the ESR which will be seen as a source resistance.                                                                                                                 Frequency response:   One of the problems with electrolytic capacitors is that they have a limited frequency response. It is found that their ESR rises with frequency and this generally limits their use to frequencies below about 100 kHz. This is particularly true for large capacitors, and even the smaller electrolytic capacitors should not be relied upon at high frequencies. To gain exact details it is necessary to consult the manufacturers data for a given part.

    Leakage:   Although electrolytic capacitors have much higher levels of capacitance for a given volume than most other capacitor technologies, they can also have a higher level of leakage. This is not a problem for most applications, such as when they are used in power supplies. However under some circumstances they are not suitable. For example they should not be used around the input circuitry of an operational amplifier. Here even a small amount of leakage can cause problems because of the high input impedance levels of the op-amp. It is also worth noting that the levels of leakage are considerably higher in the reverse direction.

     Ripple current:   When using electrolytic capacitors in high current applications such as the reservoir capacitor of a power supply, it is necessary to consider the ripple current it is likely to experience. Capacitors have a maximum ripple current they can supply. Above this they can become too hot which will reduce their life. In extreme cases it can cause the capacitor to fail. Accordingly it is necessary to calculate the expected ripple current and check that it is within the manufacturers maximum ratings.

     Tolerance:   Electrolytic capacitors have a very wide tolerance. Often capacitors may be quoted as -20% and +80%. This is not normally a problem in applications such as decoupling or power supply smoothing, etc. However they should not be used in circuits where the exact value is of importance.

                                Polarisation

       Unlike many other types of capacitor, electrolytic capacitors are polarised and must be connected within a circuit so that they only see a voltage across them in a particular way. The capacitors themselves are marked so that polarity can easily be seen. In addition to this it is common for the can of the capacitor to be connected to the negative terminal.

       It is absolutely necessary to ensure that any electrolytic capacitors are connected within a circuit with the correct polarity. A reverse bias voltage will cause the centre oxide layer forming the dielectric to be destroyed as a result of electrochemical reduction. If this occurs a short circuit will appear and excessive current can cause the capacitor to become very hot. If this occurs the component may leak the electrolyte, but under some circumstances they can explode. As this is not uncommon, it is very wise to take precautions and ensure the capacitor is fitted correctly, especially in applications where high current capability exists.