# SparkFun Forums

### Using LTSpice to check problem solutions#201252

By dougm52
#201252
I have an older version of a circuits book with problems I would like to work as a review. The problem is that some publishers are no longer including answers in the back of the book, and when provided online tend to disappear when a new version of a book comes out. So I want to use LTSpice as a means of checking my work.

https://www.dropbox.com/s/77lfeeq02x087 ... 3.png?dl=0

In the hopefully attached LTSpice image link, R3 is to have 2A current. Now I can start from the left and figure out the voltage across R2 which is the desired answer. V(n003): 12.8 voltage is the correct answer, but LTSpice requires me to enter I(I1): 5.8 device_current to get the correct DC simulation values which means I have to solve the problem before I can use LTSpice to check my answer.

It seems like I should be be able to either sweep I1 linearly or force the current through R3 to be 2A to get the desired answer. Can either or both of these be done?

V(n001): 30 voltage
V(n002): 14 voltage
V(n003): 12.8 voltage
I(I1): 5.8 device_current
I(R4): -0.6 device_current
I(R3): -2 device_current
I(R2): 6.4 device_current
I(R1): 1.4 device_current
I(V1): -2 device_current

### Re: Using LTSpice to check problem solutions#201254

By dougm52
#201254
I determined that I could set I1 to 10A, then sweep it linearly by 0.1A, display the plots of the current through R3 and the voltage across R2, set cursors on the two plots, move the cursor on R3 until I get 2A, then set the cursor to the R2 node to get the voltage. At this point I'm a bit puzzled about why a swept source has to have a fixed voltage set in the component before it can simulate.

### Re: Using LTSpice to check problem solutions#201268

By redwire
#201268
This circuit example is to teach super-position theory, which makes solving this by hand, not too hard.
I had no troubles doing .tran .1 analysis instead of LTSpice trying to solve for a DC operating point.
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