From: EmScott (msb54doc@email.msn.com) Date: 10/1/2001 4:10 AM Subject: silly capacitor question ...Even though I've made three homebrews, I've got a question that I've been embarassed to ask: WHY do capacitors block DC ? I try to make a mental analogy to some sort of "storage tank" in a "plumbing system"-based mental model of a circuit, and it doesn't help me get to any understanding. Go ahead, have at it !! From: Ray Ivers (RAGEray@aol.com) Date: 10/1/2001 12:27 PM Subject: Re: silly capacitor question Em, Capacitors block DC because of their construction; basically an insulator (dielectric) sandwiched between two conductors. No electrons flow through a properly-functioning capacitor at all. Here's another question - one that took me a long time to feel comfortable answering: How do capacitors pass AC? :) The 'storage tank' model you mentioned isn't too far off IMO. If you really want to toast your noodle, take a schematic and white-out all the coupling caps, and then figure out the signal AC current-flow paths, both charge and discharge. I soon came to think of capacitors as circuit obstructions, which is basically what they are; signal AC doesn't flow through them, it's got to flow around them. Ray Ivers From: kiwizen Date: 10/1/2001 1:11 PM Subject: Re: silly capacitor question I like to see electricity as a vacuum that the earths static charge is always trying to fill.As all current actually travels from earth this is a helpful analogy.So when a positive charge enters a cap it fills up its "field",lets call it A, for the length of the rising then falling wave and simultaneously in the other field on the other side of the fence,from whatever grounds are available,the earth is seeking equalibrium and fills the field,B, with an opposite charge.Full cap,A and B equalised,but then an AC current keeps coming into the first field A with a negative charge so the opposite field B starts lets go its negative charge and starts letting the positve charge come accross to try and get that equalibrium back.Therefore,after initial charging,a coupling cap will pass an AC charge and a cap to ground will act as a reservoir for positive charges.Am I right? The one I'm still figuring out but its coming on fast is how the size of the cap in farads is proportional to the size of the wave it will pass.As a coupling cap thats an easy thing to grasp but figuring their usage in cathode bias is still a bit of a mystery but I'm starting to get the resonant frequency stuff so it shouldn't be long.Good luck.P.S.I'm the same in that I usually figure things out after I've done them.Even if it involves high voltages!! From: meh (mark@mhuss.com) Date: 10/1/2001 12:44 PM Subject: Re: silly capacitor question Given: Electricity is the flow of electrons from one atom to the next. Remember that a cap is basically two plates (big pieces of aluminum foil, actually) that do not touch - therefore no electrons can actually cross from one plate to the next (keeping it simple). If you put a DC voltage across a cap, a bunch of electrons will be crammed onto one of the two plates, but none can leave, so (like a water tank, for a given pressure) no more will go in after a certain point. These electrons have to come from somewhere. Effectively they are pulled off the other plate. Imagine a closed tank with a rubber membrane dividing it in half, full of water. This tank is then connected from one side to the other by a large pipe, with a pump pumping as hard as it can from one to the other. The pressure in one side of the tank will go up (as the rubber membrane stretches) and the other down, until the pump can't force any more in. You could use the water *pressure* to do work, but could not use the water *flow* to do work once the tank was charged. This is the DC metaphor. If you had a bi-directional pump, it could pressurize one side, then the other, back to the first, etc. so water would be almost always flowing. Even though no water would ever make a complete circuit because of the membrane, you could still use the water *flow* back and forth to do work (and the pressure too, at the right moments). This is the AC metaphor. To mangle this AC metaphor even further, you could have two separate circuits connected by one of these divided tanks and a pump in one circuit pressurizing and emptying one side of the tank. Water would flow in and out of the other circuit due to the membrane/pressure changes, even though not one drop of water is shared by the two sides. HTH, --mark From: kiwizen Date: 10/1/2001 1:30 PM Subject: Re: silly capacitor question What I'd like to add is that the earths static charge which is referenced at zero volts will actually see negative and positive voltages as a vacuum and try to equalize them all back to the zero volts reference.Initially the positive as power,which it is full of,helped in understanding electricity but after a while it really starts to get confusing unless a realisation is made that current actually comes from earth .Nuff said!Though I wonder what happens around the magnetic ley lines. Anybody know?Would the biasing on an amp set up at the equator need to be rebiased at the poles :-) From: Dan Z (zebelman@hotmail.com) Date: 10/1/2001 5:56 PM Subject: Re: silly capacitor question Mark Isn't it the dielectric in the cap that prevents electron flow, not the fact that the plates don't touch? The plate and cathode in a tube don't touch, but electrons flow there, correct? Taking your water pump analogy, caps transfer AC only by the input/output of the two "pump" sides happening at the signal frequency? Is this basically the inverse of the way a tube transfers pulsating DC to convey the AC signal information? This is a great thread. I'm starting to understand what's happening in tube amps, but I still need help on the "why". Thanks, Dan Z From: ChrisM (cjmclean@alum.mit.edu) Date: 10/1/2001 7:28 PM Subject: Re: silly capacitor question The way to really think about caps, inductors, and resistors are as "impedance" sources. A resistance is a constant impedance, a constant blocking of electron flow, like a screen. A capacitor and inductor are impedances which change with frequency. A capacitor behaves like an impedance of 1/(CS), where C is the cap value, and S is the frequency. So at DC, the frequency is 0, so the impedance is infinite. This would be equivalent to an open circuit. As the frequency rises, the impedance drops, until the capacitor eventually looks like a short circuit at high frequencies. Inductors behave like an impedance of LS, with the inductor value as L and S as the frequency. So at DC, an inductor is a short circuit, and the impedance goes up with frequency. At very high frequencies, it behaves like an open circuit. Just think about caps and inductors as variable resistors, and you'll be fine. It makes circuit analysis much quicker if you do that instead of worrying about the formulas too much. -ChrisM From: kiwizen Date: 10/1/2001 10:30 PM Subject: Re: silly capacitor question ,...With silly analogy answers, Imagine,if you will,and being wer'e all male it shouldn't be too hard,that the dialectic in a cap is like the man in the middle of a fight who tries to keep the two antagonists from trading punches(electrons)but as the heat rises and the anger reaches peak the man in the middle ducks and the fight is on,the two are going at it until the currency of anger subsides. I think we all get lost when it it gets more complicated with the A-Z of analogies and metaphors. The dialectic is a semiconductor or possibly an insulator that under stress becomes a conductor and this stress is what makes them eventually breakdown.I have found that I read lots of tech lit which initially means nothing but I keep reading till its all meaningless nonsense then at weird moments later on,say just gazing at a cup or a pickup,whatever,it all suddenly dawns on me whats going on.So just keep spading into that uncomprehensible techno babble and before you know it you'll be quotinng capacitive reactance formulas at parties.Welcome to geek land,enjoy your complimentary multimeter.