Back to Forums
9 posts / 0 new
#1 Apr 14, 2020
AthinaPapa
Offline
Last seen: 1 year 3 months ago
Joined: 2020-04-14 18:08

#### Variation of resistor value with time

Hello all,

I am new to this forum and I hope this is the right environment to ask my question.

I am running a transient analysis and I want the value of a resistor to be varied through out the simulation.

What I need in more detail is:

R=30Ohm until t=200nsec

Then R is decreased linearly to 0hm between t=200nsec and t=250nsec

And finally R starts increasing exponentially to 30 Ohms from t=250nsec until t=500nsec.

Is this possible to do? I am familiar with parametric sweep but I am not able to come up with a way to use for this example.

Wed, 2020-04-15 13:57
retiredEE
Offline
Last seen: 1 week 18 hours ago
Joined: 2018-03-21 12:19
You can emulate this time variable resistor's value with two voltage sources in series; a PULSE source and an EXP source.
V1 2 1 EXP 0 30 250n 50n 500n 1m
V2 1 0 PULSE 30 0 200n 50n 50n 1m 1

Using the value of V(2,0) times the current through an E source could simulate the response of your time varying resistor but not knowing your circuit configuration I couldn't say for certain.
E1 node1 node2 VALUE={I(E1)*V(2,0)}
Wed, 2020-04-15 15:34
AthinaPapa
Offline
Last seen: 1 year 3 months ago
Joined: 2020-04-14 18:08

Thank you for your reply! Using the two voltage sources in series seems like something that could work.

To provide a bit of a background to what I am trying to do: The resistor I am referring to is the collector resistance of a BJT and is connected in series with the BJT.  The BJT is turned on by a transient current and this variation of Rc is meant to reproduce the thermal effects in the transistor.

I am not sure though, about how I should use and connect the E source you mention. Since we are interested in the current why not using a current dependant source?

Thu, 2020-04-16 07:36 (Reply to #3)
AthinaPapa
Offline
Last seen: 1 year 3 months ago
Joined: 2020-04-14 18:08

So, coming back to my last reply, I figured out how the E source should be used.

I can't however find a way to use I(E) as the gain of the E source and I always get an error. is there any example available on how to set the current of the source itself as the gain?

Thu, 2020-04-16 10:04
retiredEE
Offline
Last seen: 1 week 18 hours ago
Joined: 2018-03-21 12:19

I don't understand how you are setting up your circuit.  Is the BJT an NPN or PNP?  How is R(t) tied into it?  Would you post a netlist or an image of your circuit if using Capture to create your schematic.

Thu, 2020-04-16 13:05 (Reply to #5)
AthinaPapa
Offline
Last seen: 1 year 3 months ago
Joined: 2020-04-14 18:08

This my original circuit, where Q5 is the NPN BJT I am simulating and Rtc is the collector resistor, which has a constant value. I5 is a transient pulse which turns on the BJT and F5 is simulating a regenerative feedback mechanism.

Based on your suggestion, I modified the circuit accordingly:

I used the two voltage sources and connected them to the E source, which is placed at the position of the resistor. I can't figure out though how to change the gain of the EE source and set it equal to I(E).

Thu, 2020-04-16 15:07
retiredEE
Offline
Last seen: 1 week 18 hours ago
Joined: 2018-03-21 12:19
Replace E1 with an EVALUE part from the ABM library.
Change the EXPR property from V(%IN+, %IN-) to V(%IN+, %IN-)*(I(E^@REFDES)).
Connect IN+ to VR and IN- to 0.
Connect OUT+ to F5 and OUT- to Q5.

I noticed that V1 of V10 and V2 of V11 are set to 0.1.  I think they should be set to 0.
Thu, 2020-04-16 15:37
AthinaPapa
Offline
Last seen: 1 year 3 months ago
Joined: 2020-04-14 18:08

That worked great! Thank you very much, I really appreciate the help.

Thu, 2021-03-18 17:02
mehreensiddique
Offline
Last seen: 1 day 1 hour ago
Joined: 2020-05-09 12:20

Awesome article, it was exceptionally helpful! I simply began in this and I'm becoming more acquainted with it better! Cheers, keep doing awesome! walmartone wire