# Basic Second Order Transient Circuit (RLC)

#### wiz0r

Joined May 2, 2008
64
Hello,

I'm having problems with this circuit. I'm trying to solve this problem, but I'm stuck right now. Okay, well, I'm trying to figure out if this is a Parallel RLC or a Series RLC (we have not cover series + parallel RLC circuits on my class). Seems to me that it is a series+parallel RLC, tho.

The circuit is on the picture. First, I turn off the sources, and try to find the equivalent resistence. What I do is:

R12 = (R1 * R2) / (R1 + R2) = 3/2

R123 = R12 + R3 = 11/2

Now, how can I add R4 to R123? It doesnt seem to be in series, or parallel. Am I doing something wrong? Please help, I know this is a stupid/easy question, but bleh, I don't know what I'm doing wrong. Unless, this is a series + parallel RLC circuit?

Sadly, I can't continue doing the exercise, 'cause of this little problem. Can someone help me?

Any help will be greatly appreciated.

~wiz0r

#### mik3

Joined Feb 4, 2008
4,843
Hello,

I'm having problems with this circuit. I'm trying to solve this problem, but I'm stuck right now. Okay, well, I'm trying to figure out if this is a Parallel RLC or a Series RLC (we have not cover series + parallel RLC circuits on my class). Seems to me that it is a series+parallel RLC, tho.

The circuit is on the picture. First, I turn off the sources, and try to find the equivalent resistence. What I do is:

R12 = (R1 * R2) / (R1 + R2) = 3/2

R123 = R12 + R3 = 11/2

Now, how can I add R4 to R123? It doesnt seem to be in series, or parallel. Am I doing something wrong? Please help, I know this is a stupid/easy question, but bleh, I don't know what I'm doing wrong. Unless, this is a series + parallel RLC circuit?

Sadly, I can't continue doing the exercise, 'cause of this little problem. Can someone help me?

Any help will be greatly appreciated.

~wiz0r

if you remove the voltage source by a short and the current source by an open circuit, thus remove them, then R4 is in series with R123.

#### wiz0r

Joined May 2, 2008
64
thanks alot, mike.

Now I know that R4 is in series with R123. But, I supposed I did something wrong before that.

the α(series) = Req / (2*L)

which according to our professor, α(series) should be 0.5.

according to my calculations;

Req = R123 + R4 = 15/2

therefore;

α(series) = (7.5) / (2*2) = 1.875

Obviously, 0.5 != 1.875

So, can anyone please tell me what am I doing wrong, or where is my mistake? anyone?

again, any help will be greatly appreciated.

EDIT: Well, I supposed I did a little mistake when I drew the circuit. The closed switch should look like .---->. instead of having that 45 angle. I don't know if it matters or not, so yea..

#### wiz0r

Joined May 2, 2008
64
mm, well, i figured out what was wrong.

since the switch is closed, i need it to open it to find the equivalents R, L, and C to check if its an series RLC or parallel RLC.

i opened the switch, and took out the sources, everything cancels out, and i'll have only the rightmost loop with the 2 ohm resistance, 2H inductance, and 1F capacitance.

anyway, could somone tell me if im right or not?