Why is My LTSpice Thevenin Resistance 7.6 Ohms?

  • Thread starter Thread starter ntetlow
  • Start date Start date
  • Tags Tags
    Ltspice
Click For Summary

Discussion Overview

The discussion revolves around the calculation of Thevenin resistance in an LTSpice simulation involving resistors. Participants explore the values obtained from their simulations and seek clarification on the results, particularly focusing on the discrepancies in resistance values based on circuit configurations.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant initially states that the Thevenin resistance between a 3 ohm and a 6 ohm resistor is calculated to be 7.6 ohms, based on the sum of the 3 ohm resistor and the parallel combination of 20 ohms and 6 ohms.
  • Another participant requests screenshots of the circuit and simulation results to better understand the setup.
  • A participant later corrects their previous claim, stating that the correct value should be 6.6 ohms instead of 7.6 ohms, due to a change in the resistor value from 20 ohms to 9 ohms.
  • One participant mentions using a 1 volt source to measure Thevenin resistance and calculates it based on the current observed in the simulation.
  • Another participant acknowledges the correction and explains the configuration of the resistors in relation to the voltage source.
  • There is a suggestion to rename and attach the LTSpice file for further analysis.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the Thevenin resistance value, as there are conflicting claims regarding the correct configuration and resulting calculations. The discussion reflects uncertainty and corrections made by participants.

Contextual Notes

There are unresolved aspects regarding the specific circuit configuration and the implications of changing resistor values on the calculated Thevenin resistance. The discussion also highlights the importance of accurate netlist representation in simulations.

ntetlow
Messages
21
Reaction score
2
Attached is an ltcpice net file.
The thevenin resistance betweed the 3ohm and 6ohm resistor is 7.6 ohms. This works out as 3ohms plus the parallel value of 20||6 (4.6ohms).
My problem is - Why is this the thevenin resistance, could someone please explain?
 

Attachments

Engineering news on Phys.org
ntetlow said:
Attached is an ltcpice net file.
The thevenin resistance betweed the 3ohm and 6ohm resistor is 7.6 ohms. This works out as 3ohms plus the parallel value of 20||6 (4.6ohms).
My problem is - Why is this the thevenin resistance, could someone please explain?
My LTSpice software is on a different computer from the one I'm working on right now. Can you post a couple of screenshots of your circuit and your simulation results?
 
Thanks for the reply. Screenshot attached.

Neil
Screenshot (7).png
 
Last edited by a moderator:
Sorry, just realised that i've sent a different version of the ltspice, instead of 20ohms as above, please read 9 ohms(2 +3+4) making the thevenin resistance 6.6ohms
 
What voltage is ##V_1##?
 
V1 is 1 volt. it is used to measure the thevenin resistance. the screenshot shows the current
The current is 151.51mA, dividing 1 by 151.51mA gives a resistance of 6.6 ohms.
 
ntetlow said:
Sorry, just realised that i've sent a different version of the ltspice, instead of 20ohms as above, please read 9 ohms(2 +3+4) making the thevenin resistance 6.6ohms
That looks correct (I've deleted a couple replies of mine just now that were wrong). The 3 Ohm resistor in the ##V_1## source leg is in series with the 9//6 Ohm resistances.
 
Attach the LTspice.asc file to a post by renaming it as LTspice.asc.txt
Rename it to LTspice.asc then run it as LTspice program. See attached.
If you post a netlist, we must paste it as a command, then we see a different layout to you.
thev3.png
 

Attachments

  • Like
Likes   Reactions: berkeman
Yes, I see it now, just a matter of shifting things round, I wondered if that would do it.
Thanks for your help.

Neil
 
  • Like
Likes   Reactions: berkeman and Baluncore

Similar threads

  • · Replies 14 ·
Replies
14
Views
3K
  • · Replies 7 ·
Replies
7
Views
2K
Replies
26
Views
5K
Replies
1
Views
3K
Replies
9
Views
7K
  • · Replies 15 ·
Replies
15
Views
7K
  • · Replies 9 ·
Replies
9
Views
2K
  • · Replies 14 ·
Replies
14
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 17 ·
Replies
17
Views
3K