# Lt spice question ?

Discussion in 'General Electronics Chat' started by curry87, Dec 28, 2010.

1. ### curry87 Thread Starter Member

May 30, 2010
101
0
Im trying to simulate a rc circuit and have set up transient analysis over 30 sec however the voltage across the cap never changes just stays the same as supply voltage 5v and doesn't show it charging.

rc = 100k
vss = 5v
c = 100uf

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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1,188
You must tell LTspice what you want.
So you need check this box Skip initial operating point solution
And in theory we assume that capacitor is charged after
T = 5*RC = 5 * 100K * 100uF = 50s

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3. ### Papabravo Expert

Feb 24, 2006
11,081
2,158
Or you can tell the supply to turn itself on at t=0 with a rise time and turn itself off at t=50 sec with a fall time. In essence you turn your voltage source into a pulse generator.

4. ### curry87 Thread Starter Member

May 30, 2010
101
0
How do i set the voltage across a cap to a particular starting voltage so that i can get a discharging voltage curve when i discharge the cap thought rc to gnd ?

5. ### SgtWookie Expert

Jul 17, 2007
22,194
1,761
Use an initial condition Spice directive.
You have labeled the node between R1 and C1 as vc, so the IC statement should look like:
Code ( (Unknown Language)):
1. .ic v(vc)=0
Use the .sp icon on the right side of the menu bar to create a spice directive.
Drop the text anywhere in the schematic, but preferably near the .tran statement.

6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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This might also work for you.

7. ### curry87 Thread Starter Member

May 30, 2010
101
0
I have some other spice questions thanks for the advice so far!

You know you can set the voltage source to change voltages at a particular time period how do you do this with a resistance value so that say every 1 secs the resistance of a resistor changes by an incremented value in a transient simulation ?

Also how would i do this if i wanted to test 5 different preset values of caps say 5uf ,100uf,1uf,15uf,48uf in a spice circuit so for the first 10 sec the cap is a 5uf then it changes to 100uf and so on instead of stepping the parameters using .step by an incremented value and being swamped by the data?

Feb 5, 2010
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