Aluminium Electrolytic Capacitors: Construction and Applications

Simply put, electrolytic capacitors are polarized capacitors, whose anode voltage is higher, or more positive, than cathode. In aluminium electrolytic capacitors, the electrodes are made of aluminium. Positive electrode (anode) is made of pure etched aluminium foil. A thin oxide layer is formed on this foil by electrochemical process. This oxide layer acts as the dielectric. It is thinness of this layer, coupled with its little higher dielectric constant of 8-10, that gives the characteristic large capacitance values of electrolytic capacitors.

A non-solid electrolyte (in contact with this oxide layer) acts as cathode. A second aluminium electrode (cathode current collector), an etched foil, acts as means to connect to outer terminals of capacitor.  A paper separator between the foils acts as electrolyte absorbent as partition between foils. Let us go into more details of construction materials.

Aluminium electrolytic capacitors are made from as low as 0.1 μF to as high as 10,000 μF, and voltages from 4 V to 630 V for diverse applications. Foils, their processing, and other materials all vary vastly dep ending on application and environment.

Loss factor of electrolytic capacitors is much higher compared with electrostatic capacitors, and can vary from 1.5% to as high as 20% for different types of capacitors. In DC capacitors, the loss factor is acceptable, since the capacitor does not carry any current (except leakage current) when a steady state DC voltage is applied.

Let us first consider the main raw materials particular to electrolytic capacitors.

Aluminium Anode Foil

Anode is high purity (99.99%) aluminium foil, which is etched using electrochemical etching to create valleys and crests to increase the surface area significantly. The foil is then passed through a bath containing oxidizing chemical (electrolyte) under controlled speed, under regulated voltage and temperature conditions and a current is passed through the foil in successive baths to create an oxide layer (forming) on etched surface. Thickness of this oxide layer could be as small as 1.1 tp 1.5 nm/Volt of capacitor rating.

The oxide layer literally acts like a rectifier, can accept current in one direction only, while it is a direct short in opposite direction of current. Hence polarity is very important while connecting an electrolytic capacitor in circuit.

The oxide layer can then withstand this final forming voltage. This voltage decides the suitability of capacitor for a given design voltage. Degree of etching decides increase in surface area of foil, and forming voltage decides working voltage. Note that this oxide layer can be damaged by a reverse voltage between electrolyte and foil. This makes the aluminium electrolytic capacitor polar. Thickness of anode etched and formed foil varies from 50 to 100 μ, depending on application. As the oxide layer increases for higher voltages, the etched surface gets more oxidized, reducing available surface area for capacitor. Lower the voltage, higher is the surface area. Therefore, higher voltages, and higher surge / impulse currents for same voltage demand thicker films to be able to carry the currents.

Formation voltage is decided by capacitor rated voltage. Roughness of film (and depth or profile of etching) depends on capacitor duty and increase in area (gain) desired. Capacitance being proportional to foil surface, goal to is get maximum possible dielectric surface (oxide layer) after etching and forming process. Effective surface area after etching available as a ratio of plain foil surface is called ‘gain’. Gain of low voltage foils can go as high as 100, and for high voltage foils, it could be 20-25.

Cathode Foil

A second aluminium foil, used for cathode, is also etched to increase its area substantially. This foil, minimum 99% purity, serves as cathode foil. Its area has to be high, as it comes in contact with electrolyte in capacitor, and forms a very thin oxide layer. This layer is extremely thin, and etching is aimed at getting maximum capacitance density. The foil is 15 to 50 μ thick, and is etched lightly to get maximum surface area. When making a capacitor, cathode foil also develops a very thin oxide layer unavoidably, and a corresponding low voltage capacitance is created at the surface. Gain for cathode foil is much higher than that for anode foils.

This way, the capacitance at anode and cathode are in series with each other. Cathode capacitance being very large in comparison, a very small voltage appears across it.  This voltage is negligible if cathode capacitance is much larger than ten times that of anode. The entire capacitor voltage then appears across anode and capacitor value also is almost same as that formed at anode.

Tab foil

Aluminium foil tabs are inserted and fixed to both anode and cathode foils to make external contacts. Tab foil is typically 90 to 200 μ thick, and is fixed to anode and cathode foils by cold welding process or by punching and pressing to form cohesive joint. In low voltage DC electrolytic capacitors, cathode tab may simply be place in touch with cathode foil during winding of capacitor element.

Anode and cathode foils, as also tab foils are specialized processes, and are manufactured by few large companies in the world.

Separator

The two foils are separated by a permeable paper, usually a low density Kraft or manila paper, with a density of up to 0.6 g/cc, and thickness varying from 30μ to 75μ. It has high absorbency for electrolyte, which could be in fluid or paste form. This paper holds the absorbed electrolyte for capacitor functioning, and separates the two electrode foils.  While aluminium electrode foils have same width, paper separator extends beyond foil edges on both sides. Normally two layers of paper are used for high voltage capacitors, and paper type, thickness and other specs are chosen relative to the voltage and application of capacitor. The separator helps in re-forming of anode oxide layer during manufacture.

Electrolyte

Electrolyte acts to maintain the oxide layer and also reform it during manufacture. It also serves to repair the oxide layer (heal) in case of its degradation during service. An electrolyte is used during manufacture of cathode foil manufacturing process for forming of oxide layer. Thereafter another electrolyte is used for giving required properties to capacitor during manufacture. The electrolyte in a capacitor may be fluid, a gel or a paste form.

The electrolyte is a mixture of number of chemicals carefully chosen to suit operational requirement of capacitor to suit its voltage and application. It also acts as conduction medium of heat from element to container. Nature and composition of electrolyte, along with procedure used for manufacture of capacitor, plays a major role in loss factor of capacitor. Ethylene glycol, boric acid, liquid ammonia are some of the chemicals that go to make the electrolyte. Manufacturers usually have their own proprietary composition developed as per their experience. 

 Construction of aluminium electrolytic capacitor

Winding

Anode and cathode foils of desired widths may be cut, affixed with tabs and rolled into winding, with intermediate layers of paper for small capacitors or small quantities. The automatic windings have provision for feeding of tabs and their welding on the machine at predetermined positions.

Impregnation with electrolyte

Winding elements are impregnated with electrolyte using different processes, centrifugal impregnation (in a centrifuge) or vacuum impregnation.

Assembly into can

Winding is assembled in aluminium can and terminal disc. Disc is a laminated phenolic (Bakelite) based rubber one, having a gas release vent through phenolic thickness. Tabs are fixed to external terminals by aluminium rivets. The edge of can is then curled to seal the capacitor can. Alternately, gas release vent is provided at the bottom of can by having a dent structure, which makes wall thickness weak, and can puncture in case of bulging of can due to excessive heat formation.

Re-forming

The terminals of capacitors are connected to a variable voltage source, and voltage increased slowly, while controlling charging current. This recovers any weakness or discontinuity introduced in anode oxide layer during manufacturing process. Once the full rated voltage is reached, the re-forming is complete.

Outer Insulation sleeve

The outer can of capacitor holds the electrolyte, and hence is directly in contact with cathode. Therefore, it is covered with a heat-shrinkable insulation sleeve. The sleeve acts as insulation, and all markings are printed on this sleeve.

In many capacitors, an outer can of aluminium is used, isolated from inner capacitor. A cable is brought out for external connection. A disc on this can may also carry the markings. Such construction is common in motor start capacitors. Sometimes the outer can is made of moulded phenolic thermoplastic, which reduces the size of capacitor.

Markings of Electrolytic capacitors

Markings on aluminium must account for identification of terminal polarity. Body of DC capacitors is generally marked with minus sign strips on side to indicate the negative terminal side. Often terminal tag on disc is marked red to indicate positive polarity. In case of wire leads, or radial capacitors, a longer wire lead indicates positive pole, and negative polarity wire is short. Axial cans have arrow lines one side, pointing towards negative lead.

A.C. Electrolytic Capacitor

Sometimes, an electrolytic capacitor is designed and used for AC applications. One common example is motor start electrolytic capacitor. By basics, electrolytic capacitors are meant only for DC, being polar in nature. A special construction is used to enable electrolytic capacitors to work in DC, with their limitations.

Two DC capacitors are designed back-to-back in one winding. Two anodes form the outer terminals. Cathode is common to both these capacitor sections, making effectively two DC capacitors connected in series, with both anodes as external terminals. When AC voltage is applied, one section acts as capacitor, while the other section is a short-circuit to current in that direction. Reverse happens in other half cycle, when the second capacitor is active, while the first one is short circuit.

Two possibilities arise for construction. The electrolyte being the de facto cathode, there is no need for a separate cathode foil (current collector). Both capacitor sections use the common electrolyte as cathode.

In a second construction, a common cathode foil is used as in normal winding, then cathode foil is cut, and again inserted in winding after a gap. The second capacitor is thus formed in the winding, with same cathode foil and second section of anode foil.

This construction has the advantage of better heat dissipation because of the cathode foil, and also because of positions of the two anodes. There is additional electrolyte also available for heat dissipation. Costs are however higher because of additional materials involved.

Loss factor of electrolytic capacitors being very high compared to electrostatic capacitors, heat generation is significant, and capacitor cannot remain in circuit for long. It has to be disconnected from supply source before it gets too hot to avoid failure through thermal runaway and burning of capacitor. AC motor start capacitors are designed and specified to remain ON for just 3 seconds, and allowed 177 seconds OFF (cooling period) before it can be connected again to supply.

Aluminium Electrolytic Capacitor Applications

Typical applications of Aluminium Electrolytic Capacitors include the following.

• Input – output coupling / decoupling
• DC Link capacitor in AC / AC converters for variable Frequency Drives
• Motor start capacitors
• Energy Storage
• DC Filter / ripple filters
• Noise Filters
• Power supplies
• Computer motherboard

The list is not exhaustive, and there are several more applications of these capacitors.