Help to create a time varying resistance in PSpice 

 Hello all,

    I am Shilpa and i wish to simulate a circuit where in a time varying resistance is required.The idea is to vary the resistance over time which will give a varying voltage output across the resistor terminals.I intend to feed the varying resistance values as a look-up table along with time. I am aware of accomplishing this by using the TABLE and ETABLE parts in PSpice but not very clear. Any help would be highly appreciable. Please DO REPLY..!!

Thanks and Regards,

Shilpa. 

 In PSpice you can access the TIME paramater during simulation, and you can use this in your expression. For example, one can create linearly increasing resistance by substituting resistance value by expression {((1000*TIME)+0.001)}. Here time would be current simulation Time.

You can use E or preferably G device to create varying load. You can define the expression or make it on the  basis of lookup table.

 Table can be defined as following

E<name> <(+) <node> <(-) node> TABLE { <expression> } = < <input value>,<output value> >

ET2 2 0 TABLE {V(A,B)} = (0,0) (3,1)

Above example would put 0 volts  between node 2 w.r.tground (0) node,when voltage between node (controlling node) A & B is 0 and it will put 1 voltage when voltage between A,B reaches 3

You can have as many as value pairs as your need. Avoid abrupt value changes value pairs to avoid convergence issues. 

 Thanks for ur reply. But what i dont understand is that in the ETABLE part can i use resistance and time values too? I mean according to the documentation E and G parts can be used to plot (V vs I) (V vs V) (I vs V) and (I vs I). But is there a possibility for (R vs V) or say (T vs R) or somethimg like that. Also the expression to caluculate time varying R in my case is:

                                               R= t/[-ln(V2/V1) * C]                       (1)

where, V1=2.8085V

             C=94.22F

              t= is varying.

I can also back calculate V2 by substituting for R as:

                                               V2=V1*exp[-t/(R*C)]                          (2)

My idea is to substitute T and R values and observe the corresponding change in voltage across its terminals. Or can i substitute V2 values over time and get R?

Is there any other way how can i do this in PSpice. I use 9.1 version.

I am also attaching my circuit where i need to vary R over time.

Thanks and Regards,

Shilpa

You can not use E device to define R & Time relation. You need to realize variable resistance using controlled sources. A simple "Resistance" can be realized by G device where current is linearly depend upon the voltage.

Please refer the application note http://www.cadence.com/rl/Resources/application_notes/modeling_resistors_appnote.pdf on how to model dependent resistance in PSpice.

Hope this helps.

 Hey thanks a lot. This was exactly what i was looking for..I am trying it in my circuit. Hope it works fine..The application note link is very useful. Thanks again..!