PSpice - Time varying resistance

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

1. otispunkmeyer Thread Starter New Member

Jun 20, 2013
2
0
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.

https://www.dropbox.com/s/o066jvs40zxp8l8/Discharge.sch

https://www.dropbox.com/s/t8mdidpdqmf2fqi/Discharge2.cir

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!

Thanks

2. MrChips Moderator

Oct 2, 2009
12,446
3,361
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
2
0
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