# RLC, RL, RC and LC circuits and ODE

1. Sep 25, 2015

### Bruno Tolentino

Look this schematic picture:

S means source and can be a current source or a voltage source. C_1, C_2 and C_3 are linear components, can be resistor, capacitor and inductor.

In the everycircuit, I tried set up all possible combination and the everycircuit denied some combinations, but, those denied combinations are displayed how possibles in the wiki page about RLC, RC, RL and LC circuits. So, my questions is: what combinations are really impossibles and what are possibles?

And about the possibles, how are the EDO of this system? In other words, I want to write the voltage in function of the time and the current in function of the time for each possible case. I don't know do it.

2. Sep 25, 2015

### Staff: Mentor

It would probably help people to answer you if you give specific examples of these "denied" combinations.

3. Sep 25, 2015

### Svein

I cannot see any impossibilities. I mean, some of them could cause sparks and heat, but give a box with resistors, capacitors and inductors I can connect them any way I want.

4. Sep 26, 2015

### Bruno Tolentino

All circuits below don't works, especially of two of the right...

Why?

5. Sep 26, 2015

### vanhees71

It's very hard to read your circuits, but the two right ones seem to connect an ideal coil directly to a voltage source, which means you produce a short circuit, which I'd try to avoid ;-).

6. Sep 26, 2015

### Bruno Tolentino

Só, what are the convenient combinations and inconvenient (using the parameters that I provided in first post, above) and why?

7. Sep 27, 2015

### Daz

I don’t know what software you are using but most of the circuit simulators around today are descendents of SPICE (LTSpice, PSPice, etc.) and they all work the same way.

When you start a simulation in SPICE, it first calculates the DC operating point by running a nodal analysis using Kirchoff’s law. To do this, it replaces all the capacitors with open-circuits and the inductors with short-circuits, and then solves the nodal equations for the DC operating point. (At DC an open-circuit is a perfect approximation to a capacitor and a short is a perfect approximation to an inductor.)

If SPICE cannot get the DC operating point it bails out and refuses to go any further.

The circuits that your simulator is rejecting all have either:-

1) A capacitor in series with the voltage source. (The three on the left.) No DC current, therefore not possible to analyse in SPICE, or
2) An inductor directly across the voltage source. (The two on the right.) DC current infinite, therefore not possible to analyse in SPICE.

I’m guessing that your simulator works the same way.