LT spice problem

Discussion in 'Homework Help' started by Bigcountry, Aug 1, 2014.

  1. Bigcountry

    Thread Starter AAC Fanatic!

    Jul 4, 2008
    The problem I am having is finding the voltage across the capacitor. I drew it right but my graph is not right. I am thinking my switching configurations are wrong. The problems says the circuit is steady state before the switch closes at t=0. Find v(t) for t >0.
    Last edited: Aug 1, 2014
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    You should have your transient analysis start with zero initial values.

    I'm not a LTSpice user, so I don't know the specifics.
  3. Papabravo


    Feb 24, 2006
    You have not connected the negative side of V1 to GND. Both sides of the capacitor are floating with respect to GND.

    Simple NOOB mistake -- we've all done it!

    Good Luck
    Bigcountry likes this.
  4. Bigcountry

    Thread Starter AAC Fanatic!

    Jul 4, 2008
    That fixed the problem. I get the exp. Curve I was searching.
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    The simplest way to simulate this circuit in the time domain (.TRAN) with C1 starting at zero volts is to dispense with the switch and simply connect V1 directly to the input of the circuit.

    To specify the method of simulation and to start with C1 initially discharged, right click anywhere in the schematic.
    Select Edit Simulation Command and left click.
    Specify how long you want to simulate for ( I suggest 1 s) by putting a value in Stop Time
    Check the Start external DC supply voltages at 0V box.

    When you run the simulation this way, the simulator first does an initial .DC solution with V1 set to 0V, meaning that C1 winds up with zero volts across it. It then steps V1 to 18V, and then does the Transient simulation run.