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Old 21st September 2011, 11:55 PM   #1
illinox is offline illinox  Canada
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Default concerned about caps

there are about 17,000 threads on this site that contain the word Capacitor. I'm in school for electronics, and my instructor just can't explain it to me in a way I understand: Really simple; how does a cap work? Or is there a thread I should see?

Thank you very much
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Old 22nd September 2011, 12:12 AM   #2
evanc is offline evanc  United States
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I like the water analogy. A cap is a bucket, it can only hold so much water and then it needs to be dumped out....
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Old 22nd September 2011, 12:22 AM   #3
illinox is offline illinox  Canada
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thats good

but where i'm having trouble, is i have two instructors teling me two different things.

One says DC current flow is always (-) to (+) and caps charge until theyre full, then squeeze out and repeat.

The other says direction doesn't matter, but caps charge on the positive plate for the first half-cycle and then charge on the negative plate for the second half-cycle but doesn't say anything about the charge conducting into the cct.
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Old 22nd September 2011, 12:32 AM   #4
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I hope you aren't paying too much for that edumacatin'.
Here: Capacitor - Wikipedia, the free encyclopedia

Mike
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Old 22nd September 2011, 12:39 AM   #5
illinox is offline illinox  Canada
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nah only $5000

WIKIPEDIA was too technical. I feel like a dummy... First year made sense and now third year is so... weird.
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Old 22nd September 2011, 01:06 AM   #6
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Quote:
Originally Posted by illinox View Post
One says DC current flow is always (-) to (+) and caps charge until theyre full, then squeeze out and repeat..
Obviously your instructor has gotten through life without being handed a charged capacitor (or more commonly, having a charged cap tossed to him)....

I'd be askin for a $5k refund right quick.......
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Old 22nd September 2011, 01:37 AM   #7
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If you want to understand caps, play with a couple of magnets turned like pole to like pole. Electrostatically, thats kinda what electrons "feel" between capacitor plates.
Using a water analogy, filter caps are like a pressure surge tank with an air bladder or spring piston. Used for AC feedthrough coupling they are like a pipe with an elastic diaphram that blocks through flow of constant pressure (DC) while coupling sudden pressure changes (AC) from one side to the other.
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Old 22nd September 2011, 02:26 AM   #8
Enzo is offline Enzo  United States
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I like water analogies too.


Imagine a pipe with water flowing through it. That is a wire. Now lets make a wide place in the pipe, just to give us some room. Like the way a subway tunnel opens up to make room for a station, then closes back down past the station. Once that wide spot fills with water, it goes back to being a pipe.

Now right in the middle of our wide spot, put a rubber sheet across the chamber, blocking the flow. A diaphragm, so to speak. This is our capacitor. Water can no longer flow directly through the pipe. We can pump water into one end, and the diaphragm will stretch, allowing water to accumulate in the chamber. And the rubber diaphragm will hold the pressure against the water. If we then open the pipe on the supply side, the diaphragm will forrce the water back out the pipe. SO we charged out cap, and then discharged it.

And now here is a value of a cap. It blocks the flow of DC - the direct one way flow of the water - but if we have a system full of water, putting a varying pressure on one side will flex the diphragm one way of the other, which in turn will move the water on the far side. SO the motion itself will be transfered from one side to the other - flow right through in a sense - but the water itself cannot pass through.

Forgive another analogy, the mail. If I write a letter, mail it to you, and you open it, the actualletter I wrote turns up in your hands. On the other hand email, if I write a letter here, it comes out in your computer where you can read it, but the actual physical messsage did not travel. The signal - the motion of the water in our water analogy - passes through, but not the flow of the water. Just as the post above me stated.

The cap allows the changes to pass through, but not the steady state.

I think you are confusing two messages from the two instructors. Like if one person tells you to walk to school, and another person tells you to carry your lunch, you should not confuse the message to mean you should walk on your lunch.

A cap will charge up to a DC potential (potential is another word for voltage) and hold it. An ideal cap would hold it forever. No cap is 100% efficient, so eventually leakages in the cap would allow it to discharge itself. But a good cap can hold a lot of charge a long time. It won;t "squeeze out" the charge unless it has the circuit around it to allow that. Like a battery. Current doesn;t flow from a battery unless a circuit is connected to it to provide a path for that current.

In fact, I sometimes think of a cap as a crummy battery. A battery you can charge up, but it won;t hold much. You have to recharge it 60 times a second.

In the real world, caps are often used to "filter" power supplies. In those cases, the pulsing DC voltage of the circuit will alternately fill the cap, and then between pulses, the cap will discharge into the circuit. This is because the circuit has a steady demand for voltage. I suspect that might be what instructor A was talking about.


And the caps charge up by accumulating electrons on one plate. I think what instructor B was talking about was how caps charge. I don;t like how you stated it, and if that was a quote, the instructor should be smacked. But as a varying signal is applied to a cap, current tries to flow one way or the other, and so the electrons on the plate will pick one side or the other depending upn polarity of flow.

Two parallel conductive plates form a cap. I recall a tesla coil project from 50 years ago where we needed a very high voltage capacitor for the circuit. It was made by taking a pane of glass, and glueing a layer of foil to either side of it. SO a basic cap has no polarity other than what you charge it to. However some types of cap do have polarity, meaning you can only apply voltage to them one way or they will be damaged. Caps like electrolytics and tantalums are this way. The reason is that these types rely on a chemical formulation to work. They are made that way because they can cram a much larger capacitance into a space than other techniologies like film or ceramic caps. But you have to watch polarity. FIlm caps work either direction. SO instructor B was not talking about a polarized cap.

Instructor B's cap sounded like an electrolytic in a filter cap application. On the other hand, he may also have been talking about the flow of current - and there are two camps on that, the "conventional current" camp, and the "electron flow" camp. The two look at currents in exact opposite directions. Until you understand electricity, this can be confusing. I just imagine I am riding a bus. I can look out the back or out the front, the two views have opposite directions of motion, but either way we are still moving 35 miles an hour. If we hit a tree, the important thing is a 35mph collision, not whether the tree ran out and hit us or if we ran into the tree. SO back to focus... the current always flows from + to - could be the guy's take on this issue, and then when it huts a cap, well, charging occurs.

Pardon my ramble, but does that help? In all my years of training and technical education, capacitance is the least well explained phenomenon. I have yet to hear a clear and concise definition. Resistance they can define, but when they go to define capacitance it always comes out, "Capacitance: you take to conductors and place them..." That is not a definition, that is a description of the phenomenon.

I am not poo-pooing physics, but thinking in practical terms for technicians.
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Old 22nd September 2011, 06:50 AM   #9
benb is offline benb  United States
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Quote:
Originally Posted by illinox View Post
thats good

but where i'm having trouble, is i have two instructors teling me two different things.

One says DC current flow is always (-) to (+) and caps charge until theyre full, then squeeze out and repeat.

The other says direction doesn't matter, but caps charge on the positive plate for the first half-cycle and then charge on the negative plate for the second half-cycle but doesn't say anything about the charge conducting into the cct.
These instructors could be talking about two different things, capacitors in different circuits. I'm also wondering about current flow - to +. Does he mean electron current? That was taught to electronics technicians (at least in the USA) many decades ago, but I think it's been dropped, and everyone now uses conventional current flow (current through a passive device such as a resistor is said to travel into it through the more positive terminal, and out of it through the more negative terminal - the opposite of electron current).
Quote:
Originally Posted by illinox View Post
nah only $5000

WIKIPEDIA was too technical. I feel like a dummy... First year made sense and now third year is so... weird.
Maybe try any year/edition of ARRL's Radio Amateur Handbook. It has a highly condensed (no pun intended) electronics course in the first 100 or so pages. There's about a page or so on capacitors near the beginning.
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Old 22nd September 2011, 11:16 AM   #10
DF96 is online now DF96  England
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Do your instructors understand caps?
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