# DC blocking versus AC coupling

Discussion in 'General Electronics Chat' started by TheBigPablo, Nov 22, 2008.

1. ### TheBigPablo Thread Starter Member

Nov 22, 2008
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I am havign a hard time understanding the concept of dc blocking and AC coupling. If there is a
dc blocking cap between amp stages. The DC current can not get through the insulating material
and as such blocks it. How then does the AC signal get coupled throgh this same capacitor?

2. ### vvkannan Active Member

Aug 9, 2008
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Hi pablo
For a capacitor the reactance is inversely proportional to the frequency(Xc=1/2 pi f c).For DC ,frequency is zero and hence it offers infinite
impedence which is not the case for AC signal

3. ### TheBigPablo Thread Starter Member

Nov 22, 2008
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The dc is blocked by the insulating material between the plates of the capacitor ( which is a open in respect. ) The AC somehow over comes this open if you will and is allowed to pass though the insulating material. How is AC allowed through. Please explain

Apr 5, 2008
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5. ### TheBigPablo Thread Starter Member

Nov 22, 2008
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I would have to assume it is my fault, either i am not specifying the issue correctly or no one can really answer the question at hand. Does anyone have a written answer for the question, this is just a simple question that came to mind but can not get a answer from any book or forum yet. I am looking for someone to EXPLAIN how a AC signal gets through a blocking capacitors insulating material while the DC signal is not allowed to. Although this seems to be a simple question not one has answered it yet. Please any percise answer would be appreciated.

6. ### Distort10n Active Member

Dec 25, 2006
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It sounds like that you are looking for an answer based on physics. There are several forum members who have a strong physics background, but most are EE's so you are going to get an EE answer.

Vvkannan has already answered you perfectly. The impedance of the capacitor is inversely proportional to the frequency across the terminals: 1/(2∏fC). The lower the frequency is then the higher the impedance offered by the capacitor.

You can also derive the impedance mathematically, Wikipedia has a nice page on capacitive reactance.

Now the more I think about your question, it sounds like you are interested in understanding the properties of the dialectric vs. frequency; i.e., how the dialectric is working at the molecular level to prevent DC but pass AC.

I do not know the answer to this question so you made me grab my physics book on an early Saturday morning, but alas, no mention of this phenomenon.

7. ### TheBigPablo Thread Starter Member

Nov 22, 2008
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Fianlly a partial answer. Yes i am looking for the exact thing i asked for, How does ac get through the insulator but dc is blocked. By stating that capacitive reactiance is inv proport does not answr the question is just states the formula listed in every text book and basic electronics course. I am sure i will find the answer after hours of research just figured there were engineers in here that could answer the question. Its really not physics anymore that it is electronics. If we look at it in terms of Xc-1/2pi f c. The cap , the pi and constants are not changing. So it can only be dependant on the freq changing that alows the ac sig to get through the insulating material. if freq is low or zero like dc then it should not pass. What does the freq do to the insulating material???? anything, nothing,...

8. ### beenthere Retired Moderator

Apr 20, 2004
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The signal does not cross through the capacitor's dielectric. That is not the way it works.

Changing the charge on one plate causes a corresponding quantity of charge to be attracted to or driven off the other plate, depending on the polarity of the applied voltage. The capacity of the capacitor measures the quantity of charge it can handle. That limits how much current flows from/into it resulting from that change in charge on the other plate.

Since the capacitor is limited in how much current it can supply to an attached circuit (thinking of the input of an AC coupled amp), then we can see that, as the frequency increases and the time for sourcing/sinking current decreases, the capacitor looks less and less resistive.

9. ### Distort10n Active Member

Dec 25, 2006
429
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Man, did I overthink the question. This is true, the current does not pass through the dialetric material; rather, it charges and discharges on the plates.

I do believe the dialetric material does align itself with the polarity of the signal however.

10. ### mik3 Senior Member

Feb 4, 2008
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In simple words, frequency does nothing to the material.
DC is blocked by a capacitor because DC pushes electrons always in the same direction. Thus the electrons move from plate 1 of the capacitor to plate 2 of the capacitor. Thus plate 1 becomes positively charged and plate 2 becomes negatively charged, so a voltage difference (say Vi) opposite to that of the battery starts to be created between the plates. This goes on until Vi equals in magnitude the battery's voltage (I say magnitude because its of opposite polarity) where the charge flow (current) stops and thus DC is blocked.
On the other hand, in AC, when its polarity its positive it moves electrons from plate 1 to plate 2. When its polarity changes it moves electrons from plate 2 to plate 1. Thus in AC electrons move from one plate to the other and it seems that AC current passes through the capacitor. In reality AC current does not pass through the capacitor but it just move back and forth form one plate to the other. Just for information this fantastic current through the capacitor was named "Displacement current" by Maxwell.

Did you get an idea or did I confuse you more?
Just let your imagination to work. Imagine yourself as an electron surfing through the wires from plate 1 to the battery and finally to plate 2 and then the reverse way.!!!

11. ### SgtWookie Expert

Jul 17, 2007
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A capacitor blocks DC current, but allows the effects of AC to pass through it.

Imagine a hollow sphere with a flexible rubber diapragm stretched tightly through the middle of it, and fittings on either side. If gas or fluid pressure were applied to one side of the diapragm, the effects of that pressure would be felt on the other side of the diapragm (and the exit fitting), however the gas or fluid would not transfer to the other side of the diapragm - unless the force applied to the diapragm exceeded its tensile strength.

Same thing with capacitors; if you exceed their rated voltage, the voltage (which is pressure) will punch holes through the dielectric.

12. ### TheBigPablo Thread Starter Member

Nov 22, 2008
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Mik3 Thank You. The explanation moved me in the right direction. Sometimes the simple things catch me. In the case of the DC blocking cap, the cap charges to the VS and then when it equals VS there is no more current so this effectively is an open to the DC? They say The DC blocking cap isolates so if amp 1 on the left and amp 2 on the right with DC block cap between it isolates the 5vdc on left from the 9 vdc on the right so they dont interfere with each others operation. If this is the case does the cap measure the diff between these voltages or does it charge to the greater.
Now i need to read more to fix the assumtion i was under and correct it. So many thanks!!!!!!

13. ### mik3 Senior Member

Feb 4, 2008
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Yes you are right, the voltage across the capacitor will be 9-5=4

14. ### DickCappels Moderator

Aug 21, 2008
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When an electronics student who worked part time in our lab askted this question, it took us nearly an hour to get to the answer that satisfied him: There is electrostatic attraction between the atoms in the plates of the capacitor.

The charges in the plates can "feel" each other through space, or in capacitors, through the dielectric, much as you can feel the hairs on your arm attracted to the face of a television set when the power is first switched on or off.

When the polarity of the voltage on one plate of the capacitor changes, the other plate can feel the change.

Hope this helps a little more.

Last edited: Nov 24, 2008
15. ### markm Member

Nov 11, 2008
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So many ways to describe the same thing, and sometimes you've just got to keep trying until you find the description that clicks in your mind...

A charge on one side induces an equal and opposite charge on the other side. At a high-school physics level, I'd put it like this: pushing electrons into one plate of the capacitor repels electrons from the other, so for an instant it does look like current passed through the capacitor. In a DC circuit, when you turn the source on you get a pulse of electrons entering the capacitor on one side and leaving it on the other, but those electrons find their way around the rest of the circuit, and then the current stops. With AC, the charge is pushed into the capacitor and pulled back out on every cycle, with a corresponding charge movement on the other side - and it repeats on every cycle, rather than stopping.

Of course, at a college physics level, you should work out the math - and with some guidance from the teacher, eventually you may come to the equations EE's use for practical calculations. (One genius came to that solution on his own, one time a century ago...)

Aug 9, 2008
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17. ### vvkannan Active Member

Aug 9, 2008
138
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I have been taught capacitors are charge storage devices and is this really a misconception?
What would happen if a capacitor is connected to a supply with one side connected(say negative side) to ground.will electrons move from the plate (connected to the positive side) to the positive terminal.will this not induce positive charge on the second plate (connected to ground)?.The author of that page says no electron will get injected (into the plate connected to positive terminal)if this is the case.So there would no potential across the capacitor.am i right?.if the battery potential is not dropped across the capacitor then how would the net emf in the loop be 0(KVL).
Thank you

Last edited: Nov 27, 2008
18. ### mik3 Senior Member

Feb 4, 2008
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Electrons are moving from the plate connected to the positive battery terminal to the plate connected to the negative battery terminal. Thus, a voltage is developed across the capacitor.

19. ### hobbyist AAC Fanatic!

Aug 10, 2008
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may I use an analogy that may clear up how you could view this.

Think of a cappacitor as a thin sheet of paper standing on edge (paper dividing wall) now picture a simple bar magnet laying down on the right side of the paper with it's North pole facing the paper dividing wall. Now your going to demonstrate direct current first. with your left hand hold another bar magnet with the North pole facing the wall. Now slowly move this magnet towards the paper wall divider and when you touch this paper wall STOP moving the magnet. What happens to the magnet on the other side of the paper wall. It repels away from the wall. where is the magnet in your hand located, it is touching the paper wall. what happened on the other side of this wall. You had an initial movement of a magnet away from the wall. What action is happening NOW. Nothing because you have repelled the magnet away from the other side.
Well how can we have contunue movement of magnets with this wall in between. (remember we are not allowed to go past the wall) Hmmm. how about if we take our hand off of the magnet we used to move on the left side, leaving it stationary at the paper wall and lets take the magnet on the right side and move it back against the paper wall, whoaaa, the other magnet on the left side of the paper wall moved away from the wall, remember the rule states we can only touch one magnet at a time, so if we take turns moving each magnet back to the paper wall we can cause the other magnet to automatically move away from the wall so we can ALTERNATE the pushing of ONE magnet at a time back to this wall and if we do it fast enough we can effect movement of magnets on both sides of the wall as if it were oscillating back and forth, through this wall.
See if that helps any....

It seems like you got yourself into a quandry with understanding capacitor functions in a circuit, that your getting way confused with trying to figure it out on a physics chemistry level.
I know what it's like, because when I first got into this hobby, I wanted to learn how to design transistor circuits, that I got so bogged down in thinking I needed to know how all this electrons and holes and charges and bla bla bla... worked, that I had a major mental block, and the Lord spoke to my heart and said "STOP it, your trying to figure out how transistors are made, when all you have to know is how to make transistors work, your learning to design circuits not transistors", well thank God He set me straight and the mental block was gone and I've been learning more and more about transistor circuit design, and that was back in the 80's.

Unless your interested in physics when it comes to electronics, then just look at electronic circuit components using analogies, they may not be acurate to a physics major but they sure help us understand the working Concept of a component.

Just using your question above, if you took a capacitor which has electrons on a plate and connect it to the positive sypply of a battery then in THEORY (bogged down) we figure it has to attract the electrons from the plate to the positive battery terminal. (in theory YES probably so) but in ACTUALALITY NO, so how do you get out of this (bogged down condition) take the actual results and come up with your own analogy to help you accept the real result. for instance when it comes to this particular application, picture the capacitor plate with electrons locked on the plate and the only way that lock can be released is if the other plate of the capacitor was energized with a voltage oposite of the voltage on this plate. So no matter How strong the positive battery terminal is these electrons are locked in position they can't move until the other plate is energized with voltage of opposite polarity.

Last edited: Nov 26, 2008
20. ### leftyretro Active Member

Nov 25, 2008
394
5
Well stated and quite practical advice. It works the same in most engineering disciplines. A design engineer doesn't need to know everthing about how his materials and components are made and constructed, just their specifications and how he can best utilize them.