# Transient analysis

Hey, there…
I’ve drawn simple circuit and I can’t figure out where is transient response… simple RC circuit should have time constant of RC… in my case= 1msec… but the result is steady state… any help would be appreciated…
thanks
this is the log:

``````Circuit: KiCad schematic
Doing analysis at TEMP = 27.000000 and TNOM = 27.000000
Warning: v1: no DC value, transient time 0 value used
Initial Transient Solution
--------------------------
Node                                   Voltage
----                                   -------
in                                           4
out                                          0
v1#branch                                    0
Reference value :  0.00000e+00
No. of Data Rows : 2508
``````

The simulation sequence is:
Calculate the (dc) operating point of the circuit.
Use these values as starting point for the transient simulation.
The op calculation is done by removing the capacitors (and short circuiting any existing inductor).

The circuit in the picture does show a sinusoidal input voltage. The output should not be steady state, but also a sine wave.

The printed results you have shown seem to stem from a 4 V const input. ngspice has calculated the op, (in: 4V, out 0V), as described above, as its starting node values. And then nothing changes anymore, everything remains constant. There is no jump in any voltage, no current flowing through C1.

If you want to see any action, you should use a ramped voltage, e.g. from 0 to 4 by a pulse source, with a short delay, before the ramp starts.

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by the steady state I meant that the charging duration of cap is somehow ignored. I want response to this input voltage not a for example ramp voltage… should I be able to see the transient response if I add a delay to this input voltage? in addition v1 has a dc value of 4v why the log says this: “v1: no DC value”. and the second part is the why I can’t see the transient response?
Thanks, holger…

ngspice manual 4.1.2 Sinusoidal
SIN (VO VA FREQ TD THETA PHASE )
VO Offset
VA amplitude
FREQ frequency

Your (dc) offset is 4V, which is the

``````transient time 0 value
``````

There is no extra DC value specified.

You may calculate the output amplitude and phase by hand and compare it to the simulation results. Is the red curve the output signal, which is the transient reponse?.

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yes this is true… I have seen manual… the red one is output voltage of resistor… my point is the current of cap should gradually decrease to zero…by time constant of 1msec… this can take time 4-5*time constant…

Concerning dc, please see my previous post. op calculation yields a steady state. Then transient simulation starts with the steady state values. There is no voltage jump, so no dc cap current.

Concerning ac, the ac current will not disappear, of course.

If you want to see the influence of the voltage offset jumping from 0 to 4V, you will have to provide such a jump. Set VO to 0, give TD a small value, place another voltage source in series which jumps from 0 to 4 V at around TD (using a pulse source with long duration and repetition times for example).

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Thank you very much. That’s it…

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