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Design >> Analog Design >> basics Question
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Message started by raja.cedt on Mar 16^th, 2012, 2:39pm

Title: basics Question
Post by raja.cedt on Mar 16^th, 2012, 2:39pm

hello,
this is some purely theory based and graduate level question. If there any LC filter and if i apply some dc voltage at the input, the peak-voltage at the cap is 2 times the input voltage. I have derived and got ans, but is there any way understand intuitively without doing math.

thanks,
raj.

Title: Re: basics Question
Post by RobG on Mar 17^th, 2012, 9:15am

Do you mean that you are applying a stepped input?

Title: Re: basics Question
Post by raja.cedt on Mar 17^th, 2012, 11:09am

yes....

Title: Re: basics Question
Post by RobG on Mar 17^th, 2012, 5:13pm

I'm rusty, but I get the answer to be a sine wave with amplitude Vstep/sqrt(LC). Where do you come up with the number 2?

Title: Re: basics Question
Post by HdrChopper on Mar 17^th, 2012, 8:34pm

A step input will create an oscillation whose amplitude will depend on the values of L and C around whatever DC level the voltage step has. thus does not have to be 2x

Title: Re: basics Question
Post by raja.cedt on Mar 18^th, 2012, 1:51am

hello all,
i donno how you guys solved, but i have used normal circuit theory and i got it is v(1-cos(w*t)), so it's peak is 2*v, in fact this is quite common result in transmission lines also.

Thanks,
Raj.

Title: Re: basics Question
Post by RobG on Mar 18^th, 2012, 9:20am

raja.cedt wrote on Mar 18^th, 2012, 1:51am:

hello all,
i donno how you guys solved, but i have used normal circuit theory and i got it is v(1-cos(w*t)), so it's peak is 2*v, in fact this is quite common result in transmission lines also.

Oh hell, I did the impulse response, not step response. I now get the same answer as you. I guess chopper and I may have to migrate to management soon before we rust completely.

I guess you could say that all of the energy will slosh back and forth between L and C so the minimum energy in each will be zero at some point in time. So that explains the bottom peak being equal to zero. Furthermore, we could say the midpoint (average value) must be v since the average value of the input must equal the average value of the output given the inductive (DC) coupling (and also the steady state solution with a series resistance that goes to zero will be v). Finally, we know the answer is a sinusoid that must start at zero... It is easy to say such things once you know the answer ;)

Title: Re: basics Question
Post by HdrChopper on Mar 19^th, 2012, 7:53am

RobG wrote on Mar 18^th, 2012, 9:20am:

[quote author=raja.cedt
Oh hell, I did the impulse response, not step response. I now get the same answer as you. I guess chopper and I may have to migrate to management soon before we rust completely.

I'm working on that already :P

My mistake was that I always thought of a RLC response, not a purely LC one (do not know why since the question was clear from very beginning! So Raja you were correct!

Title: Re: basics Question
Post by Vladislav D on Mar 19^th, 2012, 1:14pm

For the lazy persons (like me) http://www.lccircuits.com/

Title: Re: basics Question
Post by raja.cedt on Mar 19^th, 2012, 3:06pm

hey Vladislav D,
cool man...interesting website. Keep posting this type of websites.

Thanks,
Raj.