# DC Transients

#### jagjit Sehra

Joined Feb 29, 2008
25
Hi Folks,

Got a quick question regarding the second order differential equation for calcculating (modellling) the transient responce for an RLC series circuit.
I am aware that the there are actually two ressponses which must be added together. 1) The naturaal reesponse (Response when the supply voltage is zero however the capacitor will have an intial voltage or the inductor will have an intial current. The response will die away in one of three ways).
2) The forced response (Response when there is a supply voltage to the circuit).
To account for the case where there is an intial value and a supply voltage both cases must be accounted for. Voltage across the capacitor is the one of intrest therefore Vc=Vn+Vf. However I have some confusion about this aswell
if the there is no supply voltage connected to the circuit but the capacitor does have an intial voltage why does the forced part of the equation still have to be inccculded.

If the overdamped case is considered then Vc= Ae(m1t)+Be(m2t) where A and B are constants. I know how to calculate the values of A and B but really dont understand what they are. For example for a RC series circuit the model looks as follows: vr= Ae(m1t). I know that A is the supply voltage in my head that makes sense. But I cant understand what A and B are. I can get there values by knowing what the intial value of vc is and then differentiating that to obtain a simmulatanous equation. But they are just numbers to me. Can anyone please give me some advice or a nudge in the right direction. Thank you in advance for your help.

Jag.