PSpice - Time varying resistance

Discussion in 'General Electronics Chat' started by otispunkmeyer, Jun 20, 2013.

  1. otispunkmeyer

    Thread Starter New Member

    Jun 20, 2013
    Hi there

    I am trying to produce a resistance that varies with time during a simulation within PSpice. However, after much searching about it appears that this isn't possible, at least not directly.

    A few things I have seen show that it might be possible to use a G or E block thingy and a controlling voltage to mimic the effect. I have no idea how to even get started on that!!

    What I have is a circuit that models a spark discharge into a load. This is done by having a capacitor with initial conditions, a switch (in place of a spark gap) and 3 different loads that switch in at different times to mimic the wave forms we see in real life. I suspect, in real life, that the second resistance that is switched actually alters for the duration of the current flow. I believe it increases with time. The increase might not be linear, but a linear increase will suffice for now. The circuits are linked if anyone wants to take a gander.

    I thought I had found some answers on line but a lot of these things appear to be sweeps where the resistance is changed incrementally for a series of model runs. I am after a resistance or "resistance" to change dynamically during a single simulation.

    I would be extremely grateful for any help or for being pointed in the right direction of some websites and examples. I will continue to search google!

  2. MrChips


    Oct 2, 2009
    A FET is a variable resistor whose resistance is controlled by the voltage applied to the gate.

    You can obtain a controlling voltage by using a 555 timer circuit and tapping off the voltage at the timing capacitor (pin-6 of the 555 timer circuit).
  3. otispunkmeyer

    Thread Starter New Member

    Jun 20, 2013
    is that able to act over nano-seconds? The whole simulation covers about 2 micro-seconds. The varying resistance will be acting only for 500 ns or so.

    Also where do I find this FET in the parts list... Typically its not just FET! lol