# complex RC circuit

Joined Mar 27, 2005
4
Hi. I've been given a "complex" RC circuit that's been giving me trouble for days. Source is a square wave (-10,10V) but Unit Step is fine for now. As shown in the attachment, the circuit has just a single capacitor so I can reduce the circuit down to a Thevenin Equivalent to get the time constant, total current etc. But I'm looking for the current i(t) through the top resistor, R5, not through the capacitor. This is where I run into trouble..

Since I've found the total current, is setting up a current divider at t=0 and t="infinity" the right approach here? I've done this and my Final values agree but my Initial values (t=0) do not -- I believe this is because at t=0 I treat the capacitor as just a closed circuit ... but this isn't right is it? The current from the cap. is going to flow opposite the current elsewhere.... which to me makes things very complicated.

#### Brandon

Joined Dec 14, 2004
306
Originally posted by google.com@Mar 27 2005, 04:35 PM
Hi. I've been given a "complex" RC circuit that's been giving me trouble for days. Source is a square wave (-10,10V) but Unit Step is fine for now. As shown in the attachment, the circuit has just a single capacitor so I can reduce the circuit down to a Thevenin Equivalent to get the time constant, total current etc. But I'm looking for the current i(t) through the top resistor, R5, not through the capacitor. This is where I run into trouble..

Since I've found the total current, is setting up a current divider at t=0 and t="infinity" the right approach here? I've done this and my Final values agree but my Initial values (t=0) do not -- I believe this is because at t=0 I treat the capacitor as just a closed circuit ... but this isn't right is it? The current from the cap. is going to flow opposite the current elsewhere.... which to me makes things very complicated.

[post=6456]Quoted post[/post]​
Remember, in steady state a cap is an OPEN circuit and a inductor is a SHORT circuit.

This may be the cause of your problem.

Joined Mar 27, 2005
4
Thanks for the quick response. For the steady state, I do treat it like an open circuit but for the initial (t=0), can I treat it closed?