Like so many important scientific discoveries, the discovery of electrolytic capacitance was quite fortuitous. It happened while Charles Pollak was studying the process of anodizing aluminum and other metals in 1886. he discovered that the high capacitance that developed between the thin oxide forming on the aluminum and the electrolyte solution was a serious problem because it led to the dissolution of the oxide coating after the voltage was removed. After many long hours of experimenting with different electrolyte solution he found the solution to the problem, a Sodium Borate electrolyte that allowed the thin oxide coating to form and then not attack the coating after the voltage was removed. Pollak patented the Borax-solution electrolytic capacitor in 1897.
Almost all electrolytic capacitors made today are polarized capacitors and can only be used in Direct Current (DC) circuits, the first electrolytic capacitor were non-polarized units and could be used in Alternating Current (AC) circuits. In fact the first practical use of electrolytic capacitors were as starting capacitors for single phase AC motors. Motor starting capacitors are still made in the same way by interleaving two aluminum plates with a Borax-solution impregnated dielectric material. A dielectric material is an electrical insulating material that the two plates of the capacitor from shorting out. The first major use of the DC, polarized versions of the electrolytic capacitor was in telephone exchanges to filter the harsh noise generated by the electromechanical relays used in their switching circuits. Those early electrolytic capacitor looked more like car batteries than they resembled today's electrolytic capacitors.
It was the invention of the AC powered radio in the 1920s that led to the development of the precursor to the modern electrolytic capacitors by Julius Lilenfeld in 1926. Even these electrolytic capacitors of the 1930s bear little resemblance to the modern electrolytic capacitors.
There were still plenty of radios from the 1930s and 1940s around when I started tinkering with them in the mid-1950s.
Construction of a modern electrolytic capacitor.
Modern capacitor are constructed of two aluminum plates separated by a paper dielectric as shown in this photo of a disassembled capacitor.
One of the two aluminum foil plate shown at the top of the photo has a thin insulating oxide coating formed on it. The oxide coating is the actual dielectric used in these capacitors. The plate with the insulating oxide coating is the capacitor's Anode or Positive (+) plate. The second aluminum foil plate along with the Borax impregnated paper separator become the capacitor's Cathode or Negative (-) plate.
The two most popular modern electrolytic capacitor designs are the radial lead and the axial lead designs.
The Axial-Lead capacitor shown at the top in the above photo has one lead extending from each end of the case. The Radial-Lead capacitor has both leads exiting the same end of the case as shown at the bottom of the photo.
All DC electrolytic capacitors are polarized and the technician make certain that he/she install these capacitor's observing the correct polarity. Installing a polarized capacitor backwards in the circuit where a positive voltage is applied to the cathode and a negative voltage is applied to the anode will destroy one of these capacitors in a matter of seconds. Any reverse-polarized voltage greater than 1.0 to 1.5 Volts will destroy the oxide insulating layer through an electrochemical reduction process, which causes the capacitor's plates to short circuit. Back in the day, many capacitors literally exploded when their plates short circuited. Modern capacitors have a safety-valve built into their cases that prevent them from exploding.
As shown above, the capacitor's safety-valve opens much as the safety-valve on a tank water-heater opens to release the pressure before the tank explodes.
Unless you are dealing with a pictorial diagram where parts actually appear as they actually look, they will be represented by the following symbols.
Those representing polarized electrolytic capacitors appear in the center column. The symbol in the first column represents any non-polarized capacitor. The Variable Capacitor in the right hand column will be covered in a future article.
There are many different electrolytes used today but their chemical compositions is beyond the scope of this articles. Besides the complex chemical formulas involved in understanding the differences between them, you do not need to understand the differences between them to use them.
Capacitance and Tolerances.
The capacitance of a capacitor is determined by many factors, but the two major factors are the area of the plates and the thickness of the capacitor's dielectric material. I have included this information for those of you who have inquiring minds and just need to know how capacitors are designed. The capacitance of any capacitor can be computed using the following formula.
The following is a table listing the relative permittivities (also known as the "dielectric constant") of various common substances:
OK, there you have, play around with it when you are bored.
This is also information that I have included as an extra bonus much like they do in those side bars in the “For Dummies” books. You say that it is information that is interesting and nice to know but not necessary for this article. Still, as you advance in your studies, you will eventually encounter what are known as equivalent circuits. Equivalent circuits are composed of virtual components, component that have no existence in reality. For example the electrical equivalent circuit for an electrolytic capacitor is shown below.
represents the leakage resistance between the capacitors plates, which will determine the capacitor's leakage current. The
represents the equivalent series resistance inserted in the circuit by the capacitor. The
represents equivalent series inductance added to the circuit by the capacitor.
Do not worry about this now, I just wanted to give you an idea of what an equivalent circuit was.
Most electrolytic capacitors have a 20 percent tolerance, which they can test good even if their test value is 20 percent higher or lower than the value stamped on their case.
I will cover several methods that can be used to test capacitors in another article.