# Simple question in L.P.F

Discussion in 'Homework Help' started by omerysmi, Oct 10, 2014.

1. ### omerysmi Thread Starter Member

Oct 10, 2014
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In this L.P.F the input is step signal, I need to find an experssion of Vo(0+) and of Vo(∞).

Vo(0+) = (E*R2)/(R1+R2)
Vo(∞) = E

Am I right?

Feb 17, 2009
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3. ### omerysmi Thread Starter Member

Oct 10, 2014
46
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So in fact the transient is not blocked in this circuit? (Unlike standart L.P.H circuit of capatiance in series to resistor that the transient is blocked)

As we can see in the drawing, there is a transient of E*R2/R1+R2

Last edited: Oct 10, 2014
4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Yes, this circuit will behavior exactly llike you show.

5. ### JoeJester AAC Fanatic!

Apr 26, 2005
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This simulation had the step function occurring at t=1 mS.

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6. ### WBahn Moderator

Mar 31, 2012
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The thing to remember is that a capacitor only forces the voltage across the capacitor to be continuous (while in inductor only forces the current through the inductor to be continuous). The voltages and currents anywhere else are free to change instantly.

7. ### MrAl Distinguished Member

Jun 17, 2014
3,599
754
Hello,

The simple answer here is that the capacitor is a short circuit at t=0 and an open circuit at t=infinity. So either short the cap or open the cap to get the two required results. If you need the actual transition period too then you need to calculate the exponential, but only if you need that also.

In the case where the capacitor has a non zero initial voltage, then you have to replace the capacitor with a battery equal to the initial voltage for the t=0 solution, and again an open circuit for the t=infinity solution.

So all these solutions come from a little topological transformation that just involves the capacitor, which always transforms the circuit into a simple voltage divider or even less, and a little more math for the exponential part but again only if you need that part too.