I don't understand why I'm getting this Thevenin resistance in my LTSpice circuit

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ntetlow
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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?
 

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  • #2
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?
 
  • #3
Thanks for the reply. Screenshot attached.

Neil
Screenshot (7).png
 
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  • #4
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
 
  • #5
What voltage is ##V_1##?
 
  • #6
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.
 
  • #7
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.
 
  • #8
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
 

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  • #9
Yes, I see it now, just a matter of shifting things round, I wondered if that would do it.
Thanks for your help.

Neil
 
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1. What is Thevenin resistance and how is it calculated in LTSpice?

Thevenin resistance is a concept from Thevenin's theorem, which states that any linear electrical network with voltage and current sources and only resistances can be replaced at terminals A-B by an equivalent voltage source Vth (Thevenin voltage) in series with a resistance Rth (Thevenin resistance). In LTSpice, you can calculate Thevenin resistance by zeroing out all independent voltage sources (replace them with short circuits) and independent current sources (replace them with open circuits), then calculating the resistance seen from the terminals of interest.

2. Why does the calculated Thevenin resistance in my LTSpice simulation differ from my expected theoretical value?

This discrepancy can occur due to several reasons such as the presence of non-linear components like diodes or transistors that aren't accounted for in the theoretical analysis, incorrect model parameters, or errors in how the circuit is constructed in the simulation. Ensure all components are modeled correctly and that the simulation settings match the theoretical assumptions.

3. How do I simulate to find the Thevenin resistance in LTSpice?

To simulate and find Thevenin resistance in LTSpice, first, remove all power sources as described (voltage sources replaced by wires, current sources by open circuits). Then, connect a test voltage source (1V for simplicity) across the terminals where you want to find the Thevenin resistance. Use the ".op" directive to perform an operating point analysis and measure the current flowing through the test source. Thevenin resistance is then calculated as the voltage of the test source divided by this current (V/I).

4. What common errors might lead to incorrect Thevenin resistance results in LTSpice?

Common errors include not properly setting the initial conditions, using incorrect or idealized models for components that behave differently in reality, and misconfiguring the circuit connections. Additionally, overlooking the effects of dependent sources or failing to account for all active sources when replacing them can also lead to erroneous results.

5. How can I verify the accuracy of my Thevenin resistance calculation in LTSpice?

To verify the accuracy of your Thevenin resistance calculation in LTSpice, cross-check your simulation results with hand calculations or a simplified version of the circuit. Additionally, you can validate your simulation by comparing it with practical measurements if a physical counterpart of the circuit is available. Incrementally building and testing the circuit in simulation can also help isolate and correct errors in complex circuits.

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