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Letely I've been seeing circuits with a lot of misunderstandings of the capacitor and what it's function is. I decided to explain it in a most understandable way so that anyone should be able to grasp it.
First a little about the phenomena capacitance. It occurs on two different plates that are isolated with a non conductive material like eletrolyte,ceramic,air,paper. Electrons of opposite charge begin to build up on the plates but having no way to reach each other due to the isolation (you can see the polarity and how it shifts in the 1mF capacitor). The magnitude of capacitance is measured in Farads (F). A typical capacitor would be in uF (uF is mostly used in rectifiers and audio decoupling). That's 0,000001F. There are even lower values. 1pF would be 0,000000000001F. (radio frequency). Capacitors work by passing AC (green) and blocking DC through it (yellow) That makes it a decoupling capacitor. Other features include smoothing a rectified voltage (red). When used in conjunction with an inductor (which is the exact opposite of a capacitor,since it passes DC and blocks AC) they may be used as an oscillator tank circuit (blue). In resonance the voltage in the LC circuit is maximum and is several times larger than the original amplitude. There are many types of capacitors. Big round ones are called electrolytic since the insulation material is an electrolyte. They have polarity and should not be wired Incorrectly. They are usually from 1uF to 10000uF. These are used where the frequency involved is from 20Hz-20Khz whether to rectify it or decouple it. Smaller capacitors like the ceramic ones or the ones with the air isolation have a value of around 1pF to 1uF. They are mostly used in higher frequencies up to the radio frequencies. You can view capacitors as a sort of a battery,but with one major difference. Batteries hold their charge for quite long and have a certain current limit that restrains them from discharging quickly. Capacitors discharge very quickly in a sudden burst of energy which is solely dependant on their capacitance value.A little about why capacitors pass AC while blocking DC. Capacitors have an almost infinite resistance to DC because once the electrons on the two plates have built up to their maximum nothing else happens. DC doesn't change polarities like AC to discharge the built up electrons. The capacitor just remains still with no current passing through it. AC on the other hand constantly shifts between positive and negative every time charging and discharging the capacitor allowing it to pass the AC and restricting DC. The current draw is quite dependant the value of the capacitor or the frequency. The higher the value (Farads) the more current through the capacitor. For example for 20Hz and 8ohms the corresponding capacitor value would be above 470uF. For 1Khz at 8ohms it'll be just a few uF. Even 20uF will do. Same goes for the resistance. At 20Hz 10k the cap should be ~1uF. At 1Khz 10k it should be around 15nF. See where I'm going with that.
A TABLE OF TYPICAL VALUES AND DESIGNATIONS:
1mF=0,001F=1.10^-3F (electrolytic)
1uF=0,000001F=1.10^-6F (electrolytic)
1nF=0,000000001F=1.10^-9F (ceramic)
1pF=0,000000000001F=1.10^-12F (Air)
1fF=0,000000000000001F=1.10^-15F (not applicable in almost any circuits. Used to represent parasitic capacitance in Radio Frequency applications)
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