# NI Multisim Student Edition and LTSpice

Discussion in 'General Electronics Chat' started by thakid87, Oct 25, 2009.

1. ### thakid87 Thread Starter Active Member

May 23, 2009
122
0
Hey all.

I am trying to do the same labs that I've done in my classroom in either of these two programs. The circuits we are doing and analyzing are really simple as this is my first course in electronics.

I don't know if I'm doing something wrong or what. But when I put 3 resistors of different values in parallel in either program and simulate the circuit, I only get voltage readings out of the first 2. Another thing is that both programs ask for the circuits to have a ground in order to be simulated. I believe that the placement I am using is affecting the circuit is some way as I can't get any readings.

I don't know if I'm being clear enough. If anybody can direct me to some tutorials on making simple circuits to analyze in these two programs I'd appreciate it.

Thanks.

2. ### AdrianN Active Member

Apr 27, 2009
97
1
Both Multisim and LTSpice have SPICE engines. I recommend that you read about SPICE and then use GUI based programs. To my opinion, a great SPICE book is The SPICE Book by Andrei Vladimirescu. You will then understand that SPICE needs a reference node, Ground, from which the node count starts. That is why both programs ask for a ground point in your schematic. There are also example circuits in Multisim that should have been installed with your program.

For more help post your schematic.

3. ### thakid87 Thread Starter Active Member

May 23, 2009
122
0
I appreciate the help.

Attached is a picture depicting my very simple schematic. I get values for voltage at all resistors, but I only get amperage from the first two. As if no amperage is going through R3.

Thanks again.

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4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Perfect example of screwing around with a simulator before you understand the underlying circuit theory! Your circuit only has two "nodes"! By inspection, one is at 0V (the reference node), and the other is at 12V. There are four "branches", one through the battery, and three through the resistors.

You have three resistors, each of which has 12V across it. What is the current through each resistor? (Hint: you don't need to use a simulator; pencil and paper, or a simple four-banger calculator is sufficient). What is the total battery current?

5. ### thakid87 Thread Starter Active Member

May 23, 2009
122
0
Actually, I do. I just don't understand SPICE.

Total resistance = 546 ohms

Therefore, total current supplied by the voltage source is 22 mA.

Total current = 22 mA

I @ R1 = 12 mA
I @ R2 = 6 mA
I @ R3 = 4 mA
IT = 22 mA

Thanks!

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
OK, then you deserve a LTSpice example:

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7. ### thakid87 Thread Starter Active Member

May 23, 2009
122
0
So, how'd you get that information to come up like that? What do I need to do to get that?

Is it always preferable in these programs to place the ground in the middle? Or does it really matter?

8. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
The location of the ground on the net it is attached to is irelevant.

I believe the SPICE command is .op to compute the operating point

9. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
To get LTSpice to analyze a schematic that you have drawn, you have to select a type of analysis you want it to do. In the example above, I asked for a type of DC analysis which computes all of the node voltages and branch currents ".OP".

The ground symbol is universally understood no matter where it is drawn. It means literally, connect this node to Node 0.

Look at the schematic here. It shows another way to use the ground symbol, i. e, "connect all these together and call it Node 0".

This time I asked for a different type of analysis; namely take the applied voltage source V1, and instead of just doing the DC solution once (at 12V), I asked LTSpice to solve for the node voltages (trivial, since that is the independent variable of the analysis) and branch currents 1200 times in row, while stepping V1 from 0.01 to 12.00 in steps of 0.01V. This allows the plotting of a the branch currents vs the stepped voltage.

Notice the Red trace, where I wanted LTSpice to compute the parallel equivalent of three resistors, so I asked it to plot V(b)/(-I(V1)), which is the voltage across the resistors divided by the negative of the current that flows into the positive end of the voltage source, which is the "resistance".

Note that LTSpice uses the convention that positive current flow through a source is from the plus end to the minus end, so it has to be negated in this case.

Also note that when I asked LTSpice to plot something that has units of volts/amps, it automatically labeled the axis in Ohms. Also note the roundoff error in the Ohms reading...

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