LTspice dc transients

[normad]

I started using Ltspice because most of the users here recommendded it.. now im trying to view the transient curve of a voltage of a capacitor when a dc voltage is applied..

for this i used a voltage source with a pulse of 5 volts and a 10 k resistor in series with a 1n capacitor.. but in the process of charging the capacitor the curve is a line instead of the exponential curve which is given by the equation. please help. im trying to understand dc transients using this..

thanks in advance

 

[ericgibbs]

I started using Ltspice because most of the users here recommendded it.. now im trying to view the transient curve of a voltage of a capacitor when a dc voltage is applied..

for this i used a voltage source with a pulse of 5 volts and a 10 k resistor in series with a 1n capacitor.. but in the process of charging the capacitor the curve is a line instead of the exponential curve which is given by the equation. please help. im trying to understand dc transients using this..

thanks in advance

hi,
Post your LTspice *.asc file.

 

[normad]

here it is thanks

 

[MikeMl]

You will have to use the .IC statement

 

[normad]

im sorry im new to this.. what is the .ic statement?

 

[ericgibbs]

im sorry im new to this.. what is the .ic statement?

hi.
Its 'Initial Conditions' for transient operations.

 

[ericgibbs]

hi normad,
Look at this edited version of your asc file.

 

[normad]

thank you eric!!!
it worked.. but could you explain to me the significance of having a small rise and fall time? isnt it 0 for an original pulse?

 

[ericgibbs]

thank you eric!!!
it worked.. but could you explain to me the significance of having a small rise and fall time? isnt it 0 for an original pulse?

hi,
A rise time of '0' could be considered as 'dc'...

When setting the rise time and step times this determines the LTS calculation and plot intervals.
More calc/plot points the higher the resolution.

If you have coarse rise/steps you will get a coarse calc and plot interval.

 

[normad]

but wouldnt a capacitor act in a similar manner when connected to a dc source? im confused so y does the result change when theres a 0 rise time?

 

[ericgibbs]

but wouldnt a capacitor act in a similar manner when connected to a dc source? im confused so y does the result change when theres a 0 rise time?

hi,
Look at this image, mine is the left image, yours the right.
How would you expect to see a 60uSec rise time in a 10 seconds plot.?

The plot would have to be about 10s/.000066 longer in the X axis. [about 150,000 longer]

Do you follow.?

 

[normad]

hmm.. yeah i think i understand that.. in this configuration you've left the rise time as zero.. so you get the transient effect regardless of the rise time.. am i correct?

 

[ericgibbs]

hmm.. yeah i think i understand that.. in this configuration you've left the rise time as zero.. so you get the transient effect regardless of the rise time.. am i correct?

hi,
As I have set the Stop Time at 100uS and Time Step at 1uS, LTS is smart enough to realise that a fast Rise/Fall time is required in order to give the required plot.

Why dont you experiment with different Rise/Fall times and see the effect.

 

[normad]

i think i understand thanks a lot for your time..

 

[MikeMl]

im sorry im new to this.. what is the .ic statement?

Sorry it took me so long to post this, but here is what I thought you were trying to do:

I figured you wanted to hook a DC source (battery ) to your RC network, and then watch the capacitor charge. When doing a Transient solution (.TRAN), any Spice first does a DC solution to figure the operating point of the circuit before doing the transient solution. Unfortunately, that causes the capacitor to "precharge" to 10V, so when the transient solution
happens, the capacitor is already charged, so nothing happens but a "flat line".

To see the "charging" of the capacitor, it is necessary to suppress the default behavior by using the .IC V(out) = 0 statement, which forces the DC pre-solution to leave the capacitor uncharged, enabling you to see the transient as the capacitor charges through the resistor.