Direction of current flow in LTSpice

Discussion in 'Analog & Mixed-Signal Design' started by rds01, Dec 5, 2018 at 5:58 PM.

  1. rds01

    Thread Starter New Member

    Wednesday
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    I am trying to simulate a simple circuit in LTSpice -- with resistors and a DC voltage source. I am noticing some of the currents appearing as negative and others as positive. Is there a way to display the direction of current flow in a circuit? I tried to look online but am unable to find any relevant posts. Would very much appreciate if you can share if this is possible or not. Thank you!
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
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    hi rds,
    Welcome to AAC.
    One method to set the plot current direction thru a component, is to rotate the component 180deg', Cntrl + R

    As the circuit creator you need to know the expected direction of the current flow.

    E
     
    Last edited: Dec 6, 2018 at 4:15 AM
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    In Spice-land even resistors are polarised! Flip one over and the current direction reverses. Hover the cursor over the resistor in the schematic to see Spice-current direction.
     
  4. Bordodynov

    Well-Known Member

    May 20, 2015
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    You can make new symbols for the resistor (for example, marked with a positive pole). Then if a current flows into this pin, then the quantity is positive, and if it flows out, then the quantity is negative.
    2018-12-06_16-23-23.png
     
  5. rds01

    Thread Starter New Member

    Wednesday
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    Thank you so much for your time and help -- much appreciate it. I was trying to see if there was a way to get LTSpice to post arrows indicating the current flow. If I hover the mouse over the resistor, I only see the 'Hand' symbol. Should I be doing something else to see the current direction? Thank you again.

    upload_2018-12-6_10-52-9.png
     
  6. ericgibbs

    Moderator

    Jan 29, 2010
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    hi rds,
    Post your asc file so that we can run it.
    You must run the sim first to get a voltage or current value.
    E
     
  7. rds01

    Thread Starter New Member

    Wednesday
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    Yes, I did run the sim. The -12.5 A is the current value post-calculation. I will attach the .asc file shortly. Thanks.
     
  8. rds01

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    Wednesday
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    Attached my .asc file. As you see, it is very simple -- just a DC source and a resistor for now. Thanks again!
     
    • try.asc
      File size:
      433 bytes
      Views:
      6
  9. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    Try this.
    You were using an .op , this has a .tran.
    E
     
    • try2.asc
      File size:
      439 bytes
      Views:
      7
  10. rds01

    Thread Starter New Member

    Wednesday
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    Thank you, I ran the try2.asc with the .tran. I didn't see any difference in the output. Are you able to see a current direction in the output? Can I request you to post an image of that?

    upload_2018-12-6_11-17-45.png
     
  11. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    This what I see.
    E


    EDIT:
    You know that you have to Hover the cursor over the component.???

    AA1 06-Dec-18 19.28.gif
     
  12. rds01

    Thread Starter New Member

    Wednesday
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    Wow! That would be so nice... Does that appear just by hovering the mouse or as part of the output itself.

    Did I download a different version of LTSpice? I downloaded it from Analog Devices...

    Or, should I be turning some switch On for this to appear... I am guessing there is something I am likely overlooking.

    Thanks again.

    upload_2018-12-6_11-32-5.png
     
  13. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    You know that you have to Hover the cursor over the component to show the meter symbol, and keep it hovered, else it disappears.

    It has been on every version I have used for the past few years.
     
  14. crutschow

    Expert

    Mar 14, 2008
    20,224
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    Post an image of what your cursor looks like when hovering over the resistor after you've run the transient analysis as you did in post #5.
     
  15. rds01

    Thread Starter New Member

    Wednesday
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    Wanted to reply earlier but looks like there is a limit of 5 posts per hour and so had to wait this long to submit my Post.

    My apologies... Yes, it does appear when I hover the mouse over in the .tran version.

    Thank you again.
     
  16. ebp

    Well-Known Member

    Feb 8, 2018
    2,221
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    I used to add a tiny dot next to pin 1 of all symmetric (non-polar) passive components like resistors, caps and inductors in my schematic libraries. It allows orientation to be seen easily without littering the screen and hard copy with pin numbers that aren't of any general use whatever or symbols like + and - that imply some sort of polar nature that doesn't exist.
     
  17. WBahn

    Moderator

    Mar 31, 2012
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    In virtually all (I know of know exceptions) SPICE simulators, current AT a component pin is defined as conventional current ENTERING the pin. This was the convention used by most datasheet writers at the time (and is still the case for many/most today). For a two-terminal device, current THROUGH the component is defined as the current at pin #1.
     
  18. rds01

    Thread Starter New Member

    Wednesday
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    Folks, thanks again for all your time and responses. Much appreciate them!
     
  19. ericgibbs

    Moderator

    Jan 29, 2010
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    hi rds,
    I guess you noticed the current is flowing he 'wrong' way thru the resistor, in post #11, using Conventional current direction.?
    Flip the resistor 180deg.
    E
     
  20. rds01

    Thread Starter New Member

    Wednesday
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    Yes, I noticed that and I had to change the orientation... Thank you.
     
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