I want to ask that how to get the time constant for simple RCL circuit in order to get the natural response later, Here is the sample circuit. Assume that S is close at t=0 and the circuit is open for a long time
simulate this circuit – Schematic created using CircuitLab
when S close, in order to calculate the time constant \$\tau\$ in this case, I omit the source \$V_S\$ but then I don't know how the circuit which will be used to calculate \$\tau\$ look like. I thought \$V_S\$ will act like sort circuit so it'll look like this
but my lecturer said it actually looked like this, which doesn't have \$R_1\$
So what is the rule when omitting the source in case of: voltage source and voltage source. What if the switch is in series but not parallel?
Best Answer
This is why you're confused.
In this circuit, we don't have any control over \$V_S\$ - it is a constant, unchanging, DC voltage. However, we do have control over the switch in the circuit. When, we close the switch, the circuit splits into two separate pieces.
The left side of the circuit becomes:
simulate this circuit – Schematic created using CircuitLab
Since this circuit has nothing to do with the inductor, we can ignore it.
The right side of the circuit is what your teacher drew, so we can simply analyze that on its own. Note that \$V_S\$ hasn't changed - instead, we've totally taken it out of the circuit.