sfogg Posted February 26, 2004 Share Posted February 26, 2004 Craig, "No he doesn't like questions answered with questions !" Yeah, it is a real PITA. I had a teacher that used this method many moons ago. It drove all of us completely nuts as he would never answer any question... he justed helped us to find the answers out for ourselves. That in turned helped us to become better thinkers and very good problem solvers on our *own.* Much later on in life I realized how valuable this skill he taught us was and he was by far the finest instructor I even had. So humor me... why does a capacitor block constant DC when it has charged up? If you want a hint just ask. Again there is no shame in attempting to learn something new. Shawn Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 Shawn, Very good response !! A capacitor acts like a electrical storage device for direct current once it is charged it blocks all DC current from flowing thru it to the next stage. It allows AC through it because it alternates and never completely charges the Capacitor to its limit before it switches this is why if your not getting good low end response after the coupling cap you increase its size so as to not allow the AC to charge it before switching or changing its polarity. Do I get at least a C Craig Quote Link to comment Share on other sites More sharing options...
Ray Garrison Posted February 26, 2004 Share Posted February 26, 2004 Can anyone join in? Because once the capacitor is fully charged, it has the same voltage as the constant DC source, thus there is no potential to drive current flow. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 Sure Ray !! Quote Link to comment Share on other sites More sharing options...
sfogg Posted February 26, 2004 Share Posted February 26, 2004 Craig, For now lets just talk about constant DC (or consider it 0hz AC). You are correct that when the DC charges the capacitor DC stops flowing through it.... but *why* does it block it? Get out a non-leaking cap, your multi-meter and a DC power source. Measure voltage coming out of your DC source. Measure the voltage coming out of the uncharged cap. Connect the cap to the voltage source and let it charge up. Disconnect the cap from the power source and quickly measure its voltage. What do you see? Shawn Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 Shawn, I know exactly what your getting at it will leak until its charged and while its descharging been there done that. I have a very good capacitor analyzer that does this for me no multimeter required. But what point are you trying to make as it relates to any type of DC getting through the cap ? Why must we do this in puzzle fashion ? I really have way to much real work to do for this if you know what I mean Craig Quote Link to comment Share on other sites More sharing options...
sfogg Posted February 26, 2004 Share Posted February 26, 2004 Craig, "I know exactly what your getting at it will leak until its charged and while its descharging been there done that." No, you don't know exactly what I'm getting at. I'm not asking what occurs while it is leaking, I'm asking what occurs when it ISN'T leaking (passing DC) and why that is so. " But what point are you trying to make as it relates to any type of DC getting through the cap ?" We are getting there. "Why must we do this in puzzle fashion ?" For the reason I already gave. You haven't been able to get the whole picture, so instead we are doing it in blocks building up to the whole picture/ Shawn Quote Link to comment Share on other sites More sharing options...
Audio Flynn Posted February 26, 2004 Share Posted February 26, 2004 I would have puked if you guys woulda started singing Kumbaya or something. It seeems like a festive perky little thread. Only missing the jazz puppy. ...CONTINUE Quote Link to comment Share on other sites More sharing options...
sfogg Posted February 26, 2004 Share Posted February 26, 2004 Ray, "Because once the capacitor is fully charged, it has the same voltage as the constant DC source, thus there is no potential to drive current flow." Define 'fully' charged....... Shawn Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 Shawn, I am not a child or your student ! I appreciate your efforts but you just don't get me or I you. We might just as well give this up now I'm done playing to busy for this ! Thanks Craig Quote Link to comment Share on other sites More sharing options...
sfogg Posted February 26, 2004 Share Posted February 26, 2004 Craig, " or your student !" Then why did you ask for an answer if not to learn? Shawn Quote Link to comment Share on other sites More sharing options...
mike stehr Posted February 26, 2004 Share Posted February 26, 2004 "And it is so exactly appropriate for a few of Mobile's groupies." I happen to like Holsten. You calling me a ballwasher, parrot? Quote Link to comment Share on other sites More sharing options...
3dzapper Posted February 26, 2004 Share Posted February 26, 2004 A perfect capacitor is fully charged when the negative plate collects enough electrons to equal the depletion of electrons on the positive plate. The number of electrons required varies by the amount of capacitance of the unit. A perfect capacitor blocks DC voltage because of it's dielectric. Which, unlike a battery, is an insulator seperating the plates and preventing current flow. A perfect capacitor would pass DC waveforms perfectly. However since we are in an imperfect world there is some resistance in capacitors. This RC internal circuit has a charging time constant which rounds off the leading edge of the waveform. That is why selecting capacitors with a low ESR is important. Rick Quote Link to comment Share on other sites More sharing options...
sfogg Posted February 26, 2004 Share Posted February 26, 2004 Ray, By defining fully charged I meant do you mean the capacitor is fully charged such that it can hold no higher voltage or do you mean that the capacitor is fully charged relative to the constant DC voltage source? Shawn Quote Link to comment Share on other sites More sharing options...
3dzapper Posted February 26, 2004 Share Posted February 26, 2004 Not as fancy as Mark's but I do not claim to be a Paint expert! Rick Quote Link to comment Share on other sites More sharing options...
3dzapper Posted February 26, 2004 Share Posted February 26, 2004 Thanks Ryan. "A perfect capacitor blocks DC voltage because of it's dielectric. Which, unlike a battery, is an insulator seperating the plates and preventing current flow." By that I meant current flow within the capacitor. In a tube circuit the negative plate recieves it's electrons from the grid and grid bias resistor until it reaches equalibrium and bias stabilizes. Then as a signal is impressed across the capacitor it either recruits more electrons or sheds them as the voltage varies. Rick Quote Link to comment Share on other sites More sharing options...
sfogg Posted February 26, 2004 Share Posted February 26, 2004 Rick, " Then as a signal is impressed across the capacitor it either recruits more electrons or sheds them as the voltage varies." Now we are cooking with fire and will start wrapping up Craig's question. So... the constant DC (say B+ or whatever) is applied to a coupling capacitor. The capacitor 'fully' charges against this constant DC so that the charge the capacitor is holding is equal to the constant DC that is being applied to the capacitor (perfect world) so that there is no voltage potential difference between the two and the end result is there is no DC current flow past the capacitor. Now we impress a square wave signal (which has DC components) onto the capacitor along with the constant DC voltage. What happens now? Shawn Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 Now were getting to my real question ! So who can answer it ? Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 I don't see that happening ! If it did it would disrupt the bias of the output tube wouldn't it ? Quote Link to comment Share on other sites More sharing options...
sfogg Posted February 26, 2004 Share Posted February 26, 2004 Craig, "Now were getting to my real question ! So who can answer it ?" Without understanding what makes a capacitor block DC you can't understand the answer to your question above. Ryan, "The difference in DC potential between the constant source (B+) and the superimposed DC signal would have an effect on the charging/discharging state of the RC circuit, so current would again flow in the circuit while the capacitor was charging and discharging, and this would cause the capacitor to "pass" DC leakage?" There we have it. The square wave signal changes the potential between the source (B+ and signal) and the capacitor. The two are no longer balanced either making the capacitor 'overcharged' or 'undercharged' relative to the source. In either direction it is unbalanced so that DC passes through the capacitor again. So then a bigger question becomes how is this different then how 'DC' in a square wave passes through a capacitor that doesn't have a constant DC source applied to it? Or is there no difference? Shawn Quote Link to comment Share on other sites More sharing options...
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