# complex RC circuit

Thread Starter

#### google.com

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.

Thanks in advance for any advice

#### 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.

Thanks in advance for any advice
[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.

Thread Starter

#### google.com

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?

Thread Starter

#### google.com

Joined Mar 27, 2005
4
In case anyone in the future has a similar problem, once you get Rth & Eth you can now find voltage across the cap vc(t)
E[1-e^(-t/tau)]
& the current through it ic(t)
E/R[e^(-t/tau)]
where tau = Rth*C

Then you can use those values along with nodal or loop equations to solve for current etc. through the circuit.

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