Electrolytic Capacitor - Polarity

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. This oxide has a dielectric constant of 10, which is several times higher than most common polymer insulators. It can withstand an electric field strength of the order of 25 megavolts per meter which is an acceptable fraction of that of common polymers. This combination of high capacitance and reasonably high voltage result in high energy density.

Most electrolytic capacitors are polarized and require one of the electrodes to be positive relative to the other; they may catastrophically fail if voltage is reversed. 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 redox reactions). Following the loss of the dielectric material, the capacitor will short circuit, and with sufficient short circuit current, the electrolyte will rapidly heat up and either leak or cause the capacitor to burst, often in a spectacularly dramatic fashion.

To minimize the likelihood of a polarized electrolytic being incorrectly inserted into a circuit, polarity is very clearly indicated on the case. A bar across the side of the capacitor is usually used to indicate the negative terminal. Also, the negative terminal lead of a radial electrolytic is shorter than the positive lead and may be otherwise distinguishable. On a printed circuit board 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.

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 of the foil strips acts as a blocking diode, preventing reverse current from damaging the electrolyte of the other one.

Modern capacitors have a safety valve which is 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.

The above are the most common schematic symbols for electrolytic capacitors. Some schematic diagrams do not print the "+" adjacent to the symbol. Older circuit diagrams show electrolytic capacitors as a small positive plate surrounded below and on the sides by a larger dish-shaped negative electrode, usually without "+" marking.