Why are electrolytic caps polarity sensitive? What is physically going on inside of them that makes them sensitive to polarity and subject to rupturing if they are wired in reverse?

Randy

Electrolytic caps have polarity because of the way they are formed. Someone else on here will have to describe to you exactly what happens when they are made, as I forgot the exact details. I have them in a book, but I have to go right now and don't have time to read it. Anyway, an ordinary condenser (capacitor) simply has two plates (electrodes, elements, whatever) which are separated by some di-electric material, otherwise known as an insulator (air, insulative material, etc.). None of the ordinary insulators can be reduced to such a thickness that would allow for compact space and still hold back a lot of voltage. An ordinary condenser (paper, plastic, mica, or air insulated) of say 30 MFD would be huge.

An electrolytic is made by wrapping two aluminum foils around with a paper insulator in between. This is similar to an ordinary condenser. Some older "wet" electrolytics were made by placing an aluminum plate of sorts (often a helix wound perforated plate) inside of an aluminum can. The more modern "dry" electrolytics have the paper moistened with an electrolytic fluid. The old wet electrolytics are filled with this fluid. The unit is then charged with direct current. An oxide of sorts forms on one of the two aluminum electrodes. This oxide provides a sort of insulator. Otherwise the fluid itself has no insulating properties what-so-ever. At this point my knowledge falls off. Though I have several books on this subject and have read the subject many times (it really is quite interesting), I forget all the time. I forgot which electrode gets coated with the oxide substance. I think, as well, that the electrolytic fluide aids in increasing the capacitance of the condenser, as it increases electrode surface area. This is obvious, because when an electrolytic dries up, it loses capacitance.

If you wire up an electrolytic backwards, you will remove the oxide from the electrode it originally adhered to. The oxide only stops current in one direction (kind of like a diode) The oxide may then form on the other electrode. In the mean time there will be no insulator and current will flow rapidly through the condenser. Damage will occur, especially with the current available in a radio. The heat created by current rushing through the fluid will build up steam and may explode the condenser.

I am not actually sure if an electrolytic of the commercial type can be reformed backwards after it has been formed one way. Perhaps it can. I know that home made ones can be reformed with an opposite polarity. Home made electrolytics often use more crude materials, though--not thin foil.

Thomas

I didn’t reply to this post earlier Thomas, because I didn’t want to do a bunch of explaining. Now that you have done most of the writing, I’ll give Randy a more simple explanation, heh, heh.

Roughly speaking, an electrolytic is about the same thing as a wet cell battery. If you connect the battery up backwards in your car while the alternator is running, sparks are likely to fly. Electrolytic condensers are charged like a battery too, and do almost the same thing if you hook one up backwards while the power supply is on. As a matter of fact, early experimenters knew that wet cell batteries could be adapted for use in a filter circuit, and amateur operators made banks of them at home for use in low power transmitters. It just took several years to develop one that was practical.

Very true. I was thinking about that point, but was in a hurry to get to school. They are quite a bit like a battery, though the chemical change that takes place within them is semi-permanent and is not really affected by whether the condenser is charged up or not. In my Popular Science Cyclopedia of Things To Make, they show how to do electroplating using various sources of direct current. One method is to use a bank of light bulbs as a variable resistor and then make an electrolytic rectifier. Both a half wave and full wave design are given. Then a transformer design is given. Also is a design using a radio filament rheostat to take the place of the light bulbs. Further designs make use of two Ford model T generators. One is modified so that it will be a motor capable of running at 110 volts AC, and the other supplies the direct current when driven by the motor, which is then modified by a bank of light bulbs for various current requirements. I tried to make a motor with my 1951 Chevrolet generator, but it is not capable of this due to several differences when compared to a model T generator, but I will not get into this. Incidently, my Chevrolet now uses a 12 volt alternator regulated at 6 volts, which is far superior to the original generator, which is why I have a generator at hand.

I have another set of books (about 8 volumes) from 1917 called Library of Practical Electricity. One of them goes into great detail about electrolytic lightning arrestors used at transmission plants and such. Really interesting to read. It goes on and on about incredible choke and spark gap designs which are designed to handle different types of lightning (high or lower frequency), as well as handling the various types of electricity in use (high or low voltage, AC or DC), and methods of blowing out arcs at lightning arrestors, should they form.

Anyway, the electrolytic thing is very fascinating. Amazing that such a simple device can actually be rather fascinating. The electrolytic rectifier, by the way, makes use of the electrolytic's polarity preference. It should be noted, however, that making use of regular electrolytic condensers as rectifiers will quickly destroy them just like putting them in backwards.

Thomas

Thanks guys. I think I'm getting the idea. I will have to see if I can find some more reference material on the subject though. Some of the history you refer to does sound like it would be interesting.

In a nutshell though, it sounds like they use "practical" physics to achieve the larger capacitance ratings. And that smaller capacitances are achieved in a way that is more cost effective for those values.

Randy

Here is a good article that answers the question and pretty much anything else you would want to know about electrolytics.

[url]http://electrochem.cwru.edu/ed/encycl/art-c04-electr-cap.htm[/url]

:Why are electrolytic caps polarity sensitive? What is physically going on inside of them that makes them sensitive to polarity and subject to rupturing if they are wired in reverse?
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:Randy
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WOW! I think I'm going to read the rest later! All that information about the oxide tubes and stuff is amazing! They're talking about maximizing capacitance and such by determining the diameter and thickness of the oxide tube structure, which is incredibly small. I didn't realize it was that complicated, but I guess that's why to-day's electronics are a bit smaller and more efficient than those of yesterday.

It is interesting to note that they say that the electrolytic condenser was invented in the early 1920s. This may be true in the radio department, but they did have similar devices that worked on the same principal much earlier. They weren't used for radio, but were those lightning arrestor devices I was talking about before. I'll have to see if I can scan the chapter about this. Unfortunately I don't have my own web site, but if anyone wants a copy, I can e-mail it to them.

Thomas