in diode equivalent circuit , why do we represent capacitor in parallel rather in series with resistor ? and what is the importance of dynamic ac resistance ..when it is taken into consideration?

 

plzz anyone explain that ?

 

It would be clearer if you posted the schematic in terms of ideal components.

 

Diode pass DC current, so capacitor can not be put in series, becaues capacitor block DC current.

 

A diode has a capacitance which you will notice when it is backward biased and the diode has also a dynamic impedance. You are asking why, please search for basic facts of a diode. It rather complicated to explain in a few words.

http://en.wikipedia.org/wiki/Diode

Curious, why are you wondering about this?

 

I was curious why all these english as a second language posters are asking questions about such simple theory stuff. A quick look at their public profile reveals them to be 'students'. If that is the case, should we really be answering their homework questions for them? Doesn't that defeat the purpose of a homework assignment and cripple their ability to think for themselves, or to build knowledge that they will REMEMBER at some point down the road? I'm not suggesting they shouldn't ask any questions, but perhaps answers for simple first/second year theory should not be given as freely or quickly without more information as to the need/inability to find the information themselves.

 

I'm not suggesting they shouldn't ask any questions, but perhaps answers for simple first/second year theory should not be given as freely or quickly without more information as to the need/inability to find the information themselves.

Good point and well said. If these are supposed to be students the only way to learn is to figure tha majority out with your own research and only ask about the minor areas they just can't quite click on.

A lot of this is the fault of the teacher as well. Anyone ever take an advanced math class and just put down the final answer instead of illustrating how you worked through the problem?

Nothing, absolutely nothing, beats hands on experience for learning.

Teaching also involves structure, it's important that the student understand the basics that lead up to the questions they're asked to answer.

In this case I believe the question was somewhat legitimate but gives away a lot as to tthe teacher's faults.

It's my guess the question was why a capacitor appears in a spice model of a simple diode. Well, as mentioned above diodes do exhibit capacitance - matter of fact capacitance exists between any two conducting surfaces that have anything but a dead short between them.

 

i am sorry for what i said.
thanks for your guidance friends.

 

I don't think they are feeling superior, but even a little research and reading will answer your questions. At least take the time to draw schematics, it is a universal requirement. It takes most of us more time to answer questions than it does you to ask them. Do the research, draw an example of what you're asking about, these take as much time as we take to look up the references you could have found to begin with.

A diode that is biased off resembles a capacitor most of all. There is a nonconducting dielectric between two conducting areas caused by the reverse bias. There are some diodes, called varicaps, that enhance this and are used in various oscillators for both tuning and FM modulation.

Looking back over the replies only one post referred to non-english speaking students, most were wondering why you didn't already know this. We assume you have a text book, and this site is home to a good one.

At the level of school you are at you should know how to look up answers. The way you phrase a question makes it appear you have not tried.

 

I think a lot of us are a bit on edge what with the start of the school year, there has been a sudden inrush of people asking very basic questions as if their textbooks haven't arrived yet. (Or they underestimated cost of same and partied away too much of their starting money beforehand as I did one semester)

Also, as you've also noticed, there are a few that you can't even understand - then they seem to get mad when you can't give them an immediate answer, design, bill of materials and where to buy all the parts.