Author Topic: LTspice current flow directions  (Read 14070 times)

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Offline KM4FERTopic starter

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LTspice current flow directions
« on: March 09, 2015, 05:47:37 pm »
Now I understand that in circuit analysis you can pretty much choose any direction you want for current flow and the math works out as long as you keep the +'s and -'s correct but using certain conventions just seem more intuitive to me than others.  Unfortunately LTspice doesn't seem to use nice conventions.  For example:



I've annotated the schematic with LTspice's choices for direction of current flow and they sure aren't the way I would have chosen.  Why not?  Well when you look at the graph you see that an increasing voltage results in a decreasing current through V1 and R3 but an increasing current through R1.  This just seems to contradict Ohm's and Kirchoff's laws until you stop and think about it awhile.

So, are LTspice's directions due to the way I drew the schematic or is there a way I can change them so the graph comes out the way I would expect with current following voltage?

Thanks...
earl...
 

Offline MatthewEveritt

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Re: LTspice current flow directions
« Reply #1 on: March 09, 2015, 05:50:04 pm »
I tend to be lazy and just plot the negative (right click on the signal name at the top of the plot and add a - to the front.)
 

Offline c4757p

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Re: LTspice current flow directions
« Reply #2 on: March 09, 2015, 05:51:44 pm »
Just rotate the resistors around.
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Online IanB

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Re: LTspice current flow directions
« Reply #3 on: March 09, 2015, 05:53:32 pm »
What happens if you flip R3 left to right on the schematic?
 

Offline KM4FERTopic starter

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Re: LTspice current flow directions
« Reply #4 on: March 09, 2015, 07:01:50 pm »
OK, testing shows that how you rotate/mirror the resistors does make a difference in how LTspice assigns current direction.  I guess resistors must have a directional attribute after all ( joke, joke ).

And I agree that putting the minus sign in the label of the waveform in the graph is the easiest way to change the looks of the graph.

thanks everybody.

 


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