Why can't we use non-electrolytic capacitor ?

Properly derated, good quality electrolytic capacitors, (temperature, temperature rise, voltage) can last a lot longer than 5 years. Poor quality caps that are running at several of their maximum ratings can have a very short life. One of the wear out mechanisms is loss of electrolyte through the seal material. I do not know what you are referring to by, "Due to crystaline in the material".

You misread the situation. People don't use electrolytics becasue of some special characteristic, they use them because that's the only feasible way to get larger capacitances. Find a non-electrolytic with a value of 1uF or more, and it gets rather large. But switch to electrolytic and they become much smaller. It's the construction of the electrolytic that allows the higher capacitance within a reasonable package.

So you will find that virtually all the capacitors over 1uF or so are electrolytic. The exceptions are when there is a very specific need for something else, such as specific value (electrolytics don't come in tight tolerances). There is little other reason to justify the size and cost of non-electrolytics in those larger values.

And once you have that path, then people simply live with any drying out of electrolytics.

Michael

Hi thank all for the response, I read from some book ,is just like lead acid battery if not use for a long period the plate in the battery will be polarize.

polarize.

It is like a battery only because it involves chemistry. In electrolytic capacitors, one of the plates is metal (aluminum for most, tantalum for some) and one plate is the conductive electrolytic solution surrounding that metal. The insulation that separates them is a thin layer of oxide that is made of the metal and oxygen released by driving current through the electrolyte in one direction during the manufacturing process. If the unit sits around for years, there is some degradation of the oxide, reducing its insulating properties. There is also a possibility that the electrolyte will escape and there will be no conductive plate opposite the metal. Both these aging problems are much improved since I started electronics, 40 years ago. But long periods of hot storage or operation will eventually wear out any electrolytic capacitor. It is only a question of how hot and how long. That is why capacitors are life expectancy rated based on several conditions (temperature, voltage, ripple current). Sitting on the shelf, they last much longer, generally.

polarize.

Thanks John for your explaination. I have another question. A ceramic disc capacitor is cheaper than any of the polyester film capacitor, If both having the same value why can't i use the ceramic disc instead of polyester film capacitor if the cicuit is user in a 20w fluorescent light dc to dc driver by car battery, is this some thing to do with working frequency of the capacitor ? Best Regards same value o

It may be possible to use the cheaper capacitor. No capacitor is perfect, and you have to decide if the imperfections of a particular type will be a problem in a given application. Ceramic capacitors made to be small and cheap have very strange dielectrics that change capacitance with temperature and also with applied voltage. They also act as transducers that produce voltage when force is applied to them and change dimensions when voltage is applied to them. They also tend to get warmer than film capacitors when charged and discharged rapidly.

Here is a web site that explores many of the details of the various kinds of capacitors.

polarize.

Hi John Thank for your reply, it take sometime to understand the capacitor topic Best Regards

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