aluminum electrolytic capacitors

Aluminum electrolytics have their problems however; noise, high leakage, high temperature drift, high dielectric absorption and high inductance. Additionally, low temperature is a problem for most aluminum capacitors. For most types, capacitance falls off rapidly below room temperature while dissipation factor can be ten times higher at -25C than at 25C. Most limitations can be traced to the electrolyte. At high temperature, the water can be lost to evaporation, and the capacitor (especially the small sizes) may leak outright. At low temperatures, the conductance of the salts declines, raising the ESR, and the increase in the electrolyte?s surface tension can cause reduced contact with the dielectric. The conductance of electrolytes generally has a very high temperature coefficient, +2%/C is typical, depending on size.

Since the electrolytes evaporate, is most often rated in at a set . For example, typically as 2000 hours at 105 degrees Celsius (which is the highest working temperature). Design life doubles for each 10 degrees lower, reaching 15 years at 45 degrees. However a great number of capacitors much older than this are still in service. Most Electrolytic capacitors are rated for 85 degrees Celsius maximum.

It should be pointed out that this is only a simple model and does not include associated with real electrolytic capacitors.

R_ESR must be as small as possible since it determines the when the capacitor is used to smooth voltage. Loss power scales with the ripple flowing through and linearly with R_ESR. Low ESR capacitors are imperative for high efficiencies in power supplies.

where R_leakage is the leakage resistance, R_ESR is the , L_ESL the (L being the conventional symbol for inductance).

A common modeling circuit for an electrolytic capacitor has the following :

There are a number of non-aqueous electrolytes, which use only small amount of water. The electrolytes are generally composed of a , a salt of weak acid, and a , and optional and other additives. The electrolyte is usually soaked into an electrode separator. The weak acids are usually organic acid or . The salts employed are often or metal salts of organic acids  or weak inorganic acidsSolvent-based electrolytes may be based on organic hydroxyl alkylor (diethylene glycol, , etc.).

There are three major types of water-based electrolytes for aluminium electrolytic capacitors: standard water-based (with 40-70% water), and those containing or (both with less than 25% water). The water content helps lowering the , but can make the capacitor prone to generating gas, especially if the electrolyte formulation is faulty; this is a leading cause of , to which the high water content electrolytes are more susceptible. The lower voltage ratings (thinner oxide layer) and lower operating voltage (slower regeneration of oxide layer) are further aggravating factors.

Electrolytes may be toxic or corrosive. Working with the electrolyte requires safe working practice and appropriate protective equipment such as gloves and safety glasses. Some very old tantalum electrolytics, often called "Wet-slug", contain corrosive ; however, most of these are no longer in service due to .

The electrolyte is usually or in aqueous solution together with various sugars or which are added to retard evaporation. Getting a suitable balance between chemical stability and low internal electrical resistance is not a simple matter, and in fact, the exact composition of high-performance electrolyte is a closely guarded trade secret. It took many years of painstaking research before reliable devices were developed. The electrolyte has to have high , high , low , with addition of facilitators.

The above are the most common schematic symbols for electrolytic capacitors. Some do not print the "+" adjacent to the symbol.

Modern capacitors have a , typically either a scored section of the can, or a specially designed end seal to vent the hot gas/liquid, but ruptures can still be dramatic. An electrolytic can withstand a reverse bias for a short period, but will conduct significant current and not act as a very good capacitor. Most will survive with no reverse DC bias or with only AC voltage, but circuits should be designed so that there is not a constant reverse bias for any significant amount of time. A constant forward bias is preferable, and will increase the life of the capacitor.

Special capacitors designed for AC operation are available,^{[ ]} usually referred to as "non-polarized" or "NP" types. In these, full-thickness oxide layers are formed on both the aluminum foil strips prior to assembly. On the alternate halves of the AC cycles, one or the other of the foil strips acts as a blocking diode, preventing reverse current from damaging the electrolyte of the other one. Essentially, a 10 microfarad AC capacitor behaves like two 20 microfarad DC capacitors in inverse series. These should not be used in sensitive circuits without a constant bias, since the diode action can cause distortion. They are primarily used in circuits where the bias changes occasionally.

To minimize the likelihood of a polarized electrolytic being incorrectly inserted into a circuit, polarity is indicated on the capacitor's exterior by a stripe with and possibly arrowheads adjacent to the negative lead or terminal. Also, the negative terminal lead of a radial electrolytic is shorter than the positive lead. On a , it is customary to indicate the correct orientation by using a square through-hole pad for the positive lead and a round pad for the negative.

Most electrolytic capacitors are polarized and may catastrophically fail if voltage is incorrectly applied. This is because a reverse-bias voltage above 1 to 1.5 V will destroy the center layer of dielectric material via electrochemical reduction (see reactions). Following the loss of the dielectric material, the capacitor will , and with sufficient short circuit current, the electrolyte will rapidly heat up and either leak or cause the capacitor to burst.This because if the aluminium foil with a layer of aluminium oxide on it is made +ve the oxide ion will get oxidised and will convert into oxygen gas generating a high pressure and hence may burst up the capacitor. This is same as the electrochemical principle in an electrolytes with 2 electrodes.

In aluminum electrolytic capacitors, the layer of insulating on the surface of the aluminum plate acts as the dielectric, and it is the thinness of this layer that allows for a relatively high capacitance in a small . The aluminum oxide layer can withstand an electric field strength of the order of 10⁹ volts per meter. The combination of high capacitance and high voltage result in high energy density.

Aluminum electrolytic capacitors are constructed from two conducting foils, one of which is coated with an insulating layer, and a paper spacer soaked in . The foil insulated by the oxide layer is the while the electrolyte and the second foil act as . This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminium casing. The two most popular geometries are axial leads coming from the center of each circular face of the cylinder, or two radial leads or lugs on one of the circular faces. Both of these are shown in the picture.

Construction

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