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enter image description here

While simulating using .op analysis for the forward bias of a diode in LTspice, it showing the source current Iv1 in negative sign. But, all the other currents are in positive sign. What does it mean?

enter image description here

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    \$\begingroup\$ I do not think these numbers are validated to KCL. It is just a matter of the graphical orientation of the symbol. For sanity check, try disconnect R1, rotate it 180 degrees and connect it again. Now, I believe you will change the sign of R1's current. \$\endgroup\$ Feb 3, 2020 at 7:16
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    \$\begingroup\$ 0V-15V = -15V.... \$\endgroup\$
    – tuskiomi
    Feb 3, 2020 at 7:37
  • \$\begingroup\$ @Bonnevie OP asks about the voltage source, not the resistor. Probably OP (accidently?) rotated R1 such that it has the (expected) positive current. \$\endgroup\$
    – Huisman
    Feb 3, 2020 at 7:46
  • \$\begingroup\$ @tuskiomi Your equation is correct, but makes no sense in this case. OP probably edited the schematic (incorrectly). The schematic as it is shown will result in an error: Unknown parameter "-15v" \$\endgroup\$
    – Huisman
    Feb 3, 2020 at 7:49
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    \$\begingroup\$ @tuskiomi it would if OP would have surrounded it with curly braces \$\endgroup\$
    – Huisman
    Feb 3, 2020 at 8:03

4 Answers 4

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Spice follows the passive sign convention: the positive current direction through a current or voltage source is from the positive node to the negative node.
Hence the negative sign.

You might find a resistor is sometimes also showing an unexpected sign. Probably Bonnevie is pointing that out.
Just rotate the resistor 180 degrees, or, download a resistor symbol with current arrow from ltwiki.org

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The indications depend on how the LTSpice reads the voltage or current (R1.1 to R1.2) or (R1.2 to R1.1).

Here is a good source of some definitions.

As pointed out by @Bonnevie, you can ignore it.


For the source current, the LTSpice treats all sources and resistors in the same at unlike how we write Kirchoff loop equations. To see the real direction of the current, place the current probe on the supply and observe the direction shown on the current probe when you place the mouse pointer on the voltage source.


Before rotating R1: enter image description here

After rotating R1 by 180 degrees:
enter image description here

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    \$\begingroup\$ But the question is about the source current, not the resistor current. \$\endgroup\$ Feb 3, 2020 at 17:18
  • \$\begingroup\$ Would like to see in LTspice tomorrow soon \$\endgroup\$
    – User323693
    Feb 3, 2020 at 18:33
  • \$\begingroup\$ The real source current direction can be seen when we see the current probe direction as well. Will capture it soon after I get hands on my PC \$\endgroup\$
    – User323693
    Feb 3, 2020 at 18:41
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Making the circuit more simpler,

enter image description here

we get,

enter image description here

You see the current flows out of the voltage source positive side, but gets a negative sign. This convention has a good intention, that is, positive power (IV) means consuming power, but negative power means delivering power. For example, the resister away gets positive power.

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  • \$\begingroup\$ Try to rotate the resistor 180 degrees, and you should get a negative resistor current also. \$\endgroup\$
    – G36
    Feb 3, 2020 at 15:18
  • \$\begingroup\$ @G36, true, but the power calculation still ends up positive if you use the convention, P=I_1to2 * V_1to2 (1, 2 are the two nodes of a resistor). \$\endgroup\$
    – X J
    Feb 3, 2020 at 15:30
  • \$\begingroup\$ @G36 Moreover, the question isn't about the resistor. \$\endgroup\$
    – Huisman
    Feb 3, 2020 at 17:41
  • \$\begingroup\$ @Huisman Yes, i know. electronics.stackexchange.com/questions/470755/… \$\endgroup\$
    – G36
    Feb 3, 2020 at 17:52
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By convention in electrical measurements, power is positive for loads and power is negative for generators or sources. This is done by defining load current as positive and supply current as negative and both with positive voltages. It is the same for the grid where RMS power is technically negative for AC generators due to convention.

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