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Electrolytic capacitor

An electrolytic capacitor is a type of capacitor that uses an ionic conducting liquid as one of its plates. Typically with a larger capacitance per unit volume than other types, they are valuable in relatively high-current and low-frequency electrical circuits. This is especially the case in power-supply filters, where they store charge needed to moderate output voltage and current fluctuations, in rectifier output. They are also widely used as coupling capacitors in circuits where AC should be conducted but DC should not.

Electrolytic capacitors can have a very high capacitance, allowing filters made with them to have very low corner frequencies.

History

There is no clear inventor of the electrolytic capacitor. It is one of the many technologies that spent many years as a laboratory curiosity, the classic "solution looking for a problem".

The principle of the electrolytic capacitor was discovered in 1886 by Charles Pollak, as part of his research into anodizing of aluminum and other metals. Pollack discovered that due to the thinness of the aluminum oxide layer produced, there was a very high capacitance between the aluminium and the electrolyte solution. A major problem was that most electrolytes tend to dissolve the oxide layer again when the power is removed, but he eventually found that sodium perborate (borax) would allow the layer to be formed and not attack it afterwards. He was granted a patent for the borax-solution aluminium electrolytic capacitor in 1897.

The first application of the technology was in making motor start capacitors for single-phase alternating current motors. Although most electrolytic capacitors are polarized, that is, they can only be operated with DC, by separately anodizing aluminum plates and then interleaving them in a borax bath, it is possible to make a capacitor that can be used for AC systems. 19th and early 20th century electrolytic capacitors bore little resemblance to modern types, being constructed more along the lines of a car battery. The borax electrolyte solution had to be periodically topped up with distilled water, again reminiscent of a lead acid battery

The first major application of DC versions of this type of capacitor was in large telephone exchanges, to "quieten" relay hash on the 48 volt DC power supplies.

The development of AC-operated domestic radio receivers in the late 1920s required the production of fairly large capacitance (for the time) high voltage capacitors, typically at least 4 microfarads and rated at around 500 volts DC. Waxed paper and oiled silk capacitors were available but devices with that order of capacitance and voltage rating were bulky and prohibitively expensive. The first attempt at a modern electrolytic capacitor was patented by Julius Lilienfeld in 1926. Lilienfeld's design was constructed rather along the lines of a silver mica capacitor, but with electrolyte-soaked paper sheets in place of the mica. It proved impractical to adequately seal the devices, and in the hot conditions inside typical AC operated radio receivers they quickly dried out and failed.

Retired US Navy engineer Ralph D Mershon is credited with developing the first commercially available "radio" electrolytic capacitor that was used in any quantity, (although other researchers produced broadly similar devices). The "Mershon Condenser" as it was known, was constructed similarly to a conventional paper capacitor, with two long strips of aluminum foil interwound with strips of insulating paper, but with the paper saturated with electrolyte solution instead of wax. Rather than trying to hermetically seal the devices, Mershon's solution was to simply fit the capacitor into an oversize aluminum or copper can, half-filled with extra electrolyte. (These are referred to by vintage radio enthusiasts as "wet electrolytics", and ones with liquid still inside are prized collectors items).

Although "Mershons" were an immediate success, (and the name "Mershon Condenser" was for a short time synonymous with quality radio receivers in the late 1920s), due to a number of manufacturing difficulties their service life turned out to be quite short and Mershon's company went bankrupt in the early 1930s.

It was not until World War II when sufficient resources were finally applied to finding the causes of electrolytic capacitor unreliability, that they became the reliable components they are today.

Construction

Aluminum electrolytic capacitors are constructed from two conducting aluminum foils, one of which is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil act as cathode. 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.

Polarity

In aluminum electrolytic capacitors, the layer of insulating aluminum oxide 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 volume. 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.

Unlike most capacitors, electrolytic capacitors have a voltage polarity requirement. The correct polarity is indicated on the packaging by a stripe with minus signs and possibly arrowheads, denoting the adjacent terminal that should have lower electrical potential (i.e. negative terminal). Also the negative terminal lead of radial electrolytic capacitors are shorter. This is necessary because a reverse-bias voltage above 1 to 1.5 V will destroy the center layer of dielectric material via electrochemical reduction (see redox reactions). Without the dielectric material the capacitor will short circuit, and if the short circuit current is excessive, then the electrolyte will heat up and either leak or cause the capacitor to explode.

Special capacitors designed for AC operation are available, usually referred to as "non-polar" or "NP" types. In these, full-thickness oxide layers are formed on both the aluminium 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.

Modern capacitors have a safety valve, 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. Electrolytics can withstand a reverse bias for a short period of time, but they 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.

Capacitor Polarized
Capacitor
Variable
Capacitor

These are the different schematic symbols for electrolytic capacitors. Some schematic diagrams do not print the "+" adjacent to the symbol. Electrolytic capacitors are marked to show the polarity of the leads.

Electrolyte

The electrolyte is usually boric acid or sodium borate in aqueous solution together with various sugars or ethylene glycol which are added to retard evaporation. Getting a suitable balance between chemical stability and low internal electrical resistance is very tricky 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.

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 sulfuric acid, however most of these are no longer in service due to corrosion.

Electrical behavior of electrolytics

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

where Rleakage is the leakage resistance, RESR is the equivalent series resistance, LESL the equivalent series inductance (L being the conventional symbol for inductance).

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

It should be pointed out that this is only a simple model and does not include dielectric absorption (soakage) and other non-ideal effects associated with real electrolytic capacitors.

Since the electrolytes evaporate, design life is most often rated in hours at a set temperature. 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.

Capacitance

The capacitance value of any capacitor is a measure of the amount of electric charge stored per unit of potential difference between the plates. The basic unit of capacitance is a farad, however this unit has been too large for general use until the invention of the Double-layer capacitor, so microfarad, nanofarad and picofarad are more commonly used. These are usually abbreviated to μF or uF, nF and pF.

Many conditions determine a capacitor's value, such as the thickness of the dielectric and the plate area. In the manufacturing process, electrolytic capacitors are made to conform to a set of preferred numbers. By multiplying these base numbers by a power of ten, any practical capacitor value can be achieved, which is suitable for most applications.

A standardized set of capacitor base numbers was devised so that the value of any modern electrolytic capacitor could be derived from multiplying one of the modern conventional base numbers 1.0, 1.5, 2.2, 3.3, 4.7 or 6.8 by a power of ten. Therefore, it is common to find capacitors with values of 10, 15, 22, 33, 47, 68, 100, 220, and so on. Using this method, values ranging from 0.1 to 4700 are common in most applications. Values are generally in microfarads (µF).

Many electrolytic capacitors have a tolerance range of 20 %, meaning that the manufacturer is stating that the actual value of the capacitor lies within 20 % of its labeled value. Selection of the preferred series ensures that any capacitor can be sold as a standard value, within the tolerance. Also many electrolytic caps have asymmetric tolerances, typically -20% but with much larger positive tolerance. This eliminates any need to test and grade individual caps.

Variants

Unlike capacitors that use a bulk dielectric made from an intrinsically insulating material, the dielectric in electrolytic capacitors depends on the formation and maintenance of a microscopic metal oxide layer. Compared to bulk dielectric capacitors, this very thin dielectric allows for much more capacitance in the same unit volume, but maintaining the integrity of the dielectric usually requires the steady application of the correct polarity of direct current else the oxide layer will break down and rupture, causing the capacitor to fail. In addition, electrolytic capacitors generally use an internal wet chemistry and they will eventually fail if the water within the capacitor evaporates.

Electrolytic capacitance values are not as tightly-specified as with bulk dielectric capacitors. Especially with aluminum electrolytics, it is quite common to see an electrolytic capacitor specified as having a "guaranteed minimum value" and no upper bound on its value. For most purposes (such as power supply filtering and signal coupling), this type of specification is acceptable.

As with bulk dielectric capacitors, electrolytic capacitors come in several varieties:

  • Aluminum electrolytic capacitor: compact but lossy, these are available in the range of <1 µF to 1 F with working voltages up to several hundred volts DC. The dielectric is a thin layer of aluminum oxide. They contain corrosive liquid and can burst if the device is connected backwards. The oxide insulating layer will tend to deteriorate in the absence of a sufficient rejuvenating voltage, and eventually the capacitor will fail if voltage is not applied. Bipolar electrolytics (also called Non-Polarised or NP capacitors) contain two capacitors connected in series opposition and are used when the DC bias voltage must occasionally reverse. Bad frequency and temperature characteristics make them unsuited for high-frequency applications. Typical ESL values are a few nH.
  • Tantalum: compact, low-voltage devices up to several hundred µF, these have a lower energy density and are more accurate than aluminum electrolytics. Tantalum capacitors are also polarized because of their dissimilar electrodes. The cathode electrode is formed of sintered tantalum grains, with the dielectric electrochemically formed as a thin layer of oxide. The thin layer of oxide and high surface area of the porous sintered material gives this type a very high capacitance per unit volume. The cathode electrode is formed either of a liquid electrolyte connecting the outer can or of a chemically deposited semi-conductive layer of manganese dioxide, which is then connected to an external wire lead. A development of this type replaces the manganese dioxide with a conductive plastic polymer (polypyrrole) that reduces internal resistance and eliminates a self-ignition failure.

Compared to aluminum electrolytics, tantalum capacitors have very stable capacitance, little DC leakage, and very low impedance at high frequencies. However, unlike aluminum electrolytics, they are intolerant of voltage spikes and are destroyed (often exploding violently) if connected in the circuit backwards or exposed to spikes above their voltage rating.

Tantalum capacitors are more expensive than aluminum-based capacitors and generally only usable at low voltage, but because of their higher capacitance per unit volume and lower impedance at high frequencies, they are popular in miniature applications such as cellular telephones.

See also

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