# Need a bit of help with basic circuitry/Winspice

Discussion in 'General Electronics Chat' started by rustygb, Oct 19, 2009.

1. ### rustygb Thread Starter New Member

Oct 19, 2009
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Hiya, I'm having some trouble with some circuit problems for my class. There's this particular diagram that has some confusing layouts. Here's an example:

The numbers are obviously nodes, but there's that one line from 4 to 3 is what's troubling me. I'm not sure how to represent this line in Winspice. I also can't figure out how to relate this extra wire to the circuit. I guess I could use an idea of where the current's moving. Can anyone enlighten me?

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2. ### MikeML AAC Fanatic!

Oct 2, 2009
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Put a 1uΩ or 1mΩ resistor from node 3 to node 4

3. ### SgtWookie Expert

Jul 17, 2007
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I disagree.

If the problem shows two different nodes, then they are not directly connected.

I see it like this:

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4. ### MikeML AAC Fanatic!

Oct 2, 2009
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I disagree. I see two different nodes connected by a wire. How much resistance does a wire have? The idea of numbering nodes is an artifact of Spice. Since the OP wants to map the problem onto a Spice derivative, then modelling the wire resistance with a fraction of an Ω is reasonable... besides, Spice wont tolerate a zero Ω resistor between two nodes.

5. ### rustygb Thread Starter New Member

Oct 19, 2009
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That was one thing I couldn't really figure out, whether or not that line was actually a wire. It's on the problem sheet, so shouldn't it be significant enough to actually be there? Does it just simplify to a more complicated parallel circuit?

6. ### SgtWookie Expert

Jul 17, 2007
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I know what you're saying - it sure looks like they're connected. But please follow along; Look a bit more closely at the problem.

The wire running between node 3 and node 4 is a proverbial "red herring". Since there are two different node numbers, they cannot be considered to be connected together.

I have been "bitten" by this very thing numerous times when I've had node number display turned off in simulations. Turning them on made it painfully obvious where my "so close, yet so far" connections weren't.

7. ### MikeML AAC Fanatic!

Oct 2, 2009
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I'm guessing that the instructor who gave this problem wasn't thinking about Spice at all, so the node numbers have nothing to do with Spice. The OP asked how he/she might map the problem onto Spice, realizing that a branch with zero resistance is a no-no.

8. ### SgtWookie Expert

Jul 17, 2007
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A PSPICE program will only assign a single node number to portions of a schematic that are connected together by wiring.

The wires certainly appear to be connected together, but you have to believe the node numbers, and realize that node 3 and node 4 are in fact not connected.

If you decide to connect them together with a low-value resistance, you will not arrive at a correct answer.

9. ### MikeML AAC Fanatic!

Oct 2, 2009
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If it is a wire, then the two lower left resistors are in parallel.

10. ### SgtWookie Expert

Jul 17, 2007
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Well, since there ARE node numbers, why would one think that they are irrelevant?
That's true. I'm not suggesting that there are any branches with zero resistances. What I'm suggesting is that the wire between node 3 and node 4 is not making a connection between the two nodes; and if our OP selected some arbitrary value to connect the two nodes together, they would change the circuit and arrive at a wrong solution to the problem.

I have to believe the node numbers over the wiring.

[eta]
The node numbers are generally placed at the most central location of the node. That is why I believe the bottom two resistors are node 3, and the vertical wire immediately above node 3 is not actually connected. If it were; node 4 would not be displayed, it would all be node 3.

Last edited: Oct 19, 2009
11. ### MikeML AAC Fanatic!

Oct 2, 2009
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You must have learned Spice before you learned how to read schematics

12. ### SgtWookie Expert

Jul 17, 2007
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No, and that's why I've been bitten lots of times!

I started fooling around with electronics back in the late 60's - back then we'd whittle our CPUs out of wood...

13. ### t06afre AAC Fanatic!

May 11, 2009
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The GUI is quite confusing if you ask me. Like wires running through the resistors Perhaps you have a setting in your program what shows a dot at every junction. This may help you to identify errors. Or perhaps get a better simulator program. My guess is that node 3 is connected to node 1. And in that case the numbering is correct.

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14. ### SgtWookie Expert

Jul 17, 2007
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Why do you think that nodes 1 and 3 are connected by wire?

They have different node numbers; therefore they are not directly connected.

15. ### t06afre AAC Fanatic!

May 11, 2009
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Can you post the netlist. The netlist will look something like this. I do not know which file name your program use. But the file should be in plain text

*SPICE Netlist generated by Advanced Sim server on 19.10.2009 18:06:38
*Schematic Netlist:
R1 NetR1_1 NetR1_2 1K
R2 NetR1_2 NetR2_2 2k
R3 NetR3_1 NetR1_2 1K3k
R4 0 NetR3_1 4K
R5 0 NetR1_2 5k
R6 NetR1_2 NetR2_2 6k
V1 NetR1_1 0 5
.SAVE 0 NetR1_1 NetR1_2 NetR2_2 NetR3_1 V1#branch @V1[z] @R1 @R2 @R3
.SAVE @R4 @R5 @R6 @R1[p] @R2[p] @R3[p] @R4[p] @R5[p] @R6[p] @V1[p]
*PLOT OP -1 1 A=NetR1_1
*Selected Circuit Analyses:
.OP
.END