Pspice help RLC parrallel circuit.

[xXOfNiRXx]

Hello everybody. I have an over-damped RLC parallel circuit. Using techniques taught, I solved for v(t) = 70e^(-10,000t)+20e^(-40,000t) V for t> zero. This indicates that my initial voltage is 90v. Using MATLAB (Part of the assignment) I verified this. I also found that Current as a function of time in my inductor is I(t)= -28e^(-10,000t)-2e^(-40,000t) mA for t>0. Which indicates that at t=0 current is -30mA; through the inductor. Which again, agrees with both my Matlab graph and the answer in the book.

C1 has a capacitance of 10nF and an initial current of -15mA (solved for current, agrees with book)
R1 has a resistance value of 2000ohms
L1's value is 250mH with an initial current of -30mA. (Given in problem)

However, upon running Pspice with this code:

C1 1 0 10e-9F IC = -15e-3A
R1 1 0 2.0e3ohm
L1 1 0 250e-3H IC = -30e-3A
.tran 5e-6 1e-3 UIC
.plot tran v(1)
.plot tran I(L1)
.end

My voltage graph doesn't agree with Matlab or my voltage function. According to Pspice, my initial Voltage at node 1 is only 60 not 90. I'm confused because my current plot for L1 is right on with an initial of -30mA, as it should.

Does anybody see anything wrong with my code? I've tried sing .IC =V(1)=90, but that didn't do anything either. Anyhelp would be greatly appreciated...it's driving me crazy. Thanks in advance.

 

[phyzguy]

I think the problem is that pspice does not recognize the current through the capacitor as a valid initial condition. You can specify a capacitor node voltage as an initial condition, but not a current. So it ignores the 15 mA flowing through the capacitor and assumes that the 30mA flowing through the inductor is flowing back through the resistor, leading to the 60V initial node voltage. I couldn't find a way to get it to do what you want.

 

[xXOfNiRXx]

Thanks for your reply, appreciate the help. It's an interesting problem. What you said makes sense though. Setting IC = 90V for the capacitor does nothing. Wow, I'm at a loss; I have literally tried everything that I can think of.

 

[xXOfNiRXx]

So, I finally got it to work. However, I changed the code around many-a-time until it finally worked. Curiously enough, the same code doesn't give me the same answer every time! Weird. Here is he code that code:

C1 1 0 10e-9F IC = 90V
R1 1 0 2.0e3ohm
L1 1 0 250e-3H IC= -30e-3A
.tran 5e-6 5.5e-4
.plot tran V(1)
.plot tran I(L1)
.end