1
\$\begingroup\$

I am learning about capacitors and decided to run a simulation on Multisim to understand them a bit more. Whenever I connect the capacitor to a DC source and connect an ammeter the result is quite high negative current. What is the significance of this? and why would it happen since its supposed to be no current at all since dV/dt = 0? Negative current

\$\endgroup\$
5
  • \$\begingroup\$ What is the initial voltage across C1 when the simulation begins? If V(C1)=0 at start of simulation, dV/dT will be very large. \$\endgroup\$
    – MarkU
    Dec 29, 2014 at 2:31
  • \$\begingroup\$ The initial voltage is 10 V and it does rise very fast so there should be a large amount of current. However I am not sure if you are able to see the oscilloscope but the red line is the voltage across (10 V) with no change and there is still current flow. \$\endgroup\$
    – AlanZ2223
    Dec 29, 2014 at 2:34
  • \$\begingroup\$ at 360 ms the current should of vanished, as the rise of the voltage applied has long stabilized \$\endgroup\$
    – AlanZ2223
    Dec 29, 2014 at 2:35
  • 1
    \$\begingroup\$ "Current" does not "flow." Charge does. ;) \$\endgroup\$
    – fuzzyhair2
    Dec 29, 2014 at 3:13
  • \$\begingroup\$ I fixed it boss \$\endgroup\$
    – AlanZ2223
    Dec 29, 2014 at 4:39

2 Answers 2

2
\$\begingroup\$

What is the "correct" direction of current? It´s a matter of definition. And that´s exactly what we see in simulation programs.

I think, in SPICE based programs (at least in PSpice) the value of the current is considered as positive if it flows from the positive to the negative terminal through the source.

\$\endgroup\$
0
\$\begingroup\$

If the capacitor and voltage source are modeled as ideal, you probably shouldn't have a switch in the circuit. Discontinuities in capacitor voltage are not allowed. Try putting a resistor in series with the voltage source, even if it's just 0.1 ohms.

You might also try taking the scope out of the circuit to make sure there isn't anything weird going on there.

\$\endgroup\$
3
  • \$\begingroup\$ What I would just like you to clarify is that this is clearly an abnormal condition within the simulator and in actuality there would be no such thing as negative current or any sort of current. Correct? \$\endgroup\$
    – AlanZ2223
    Dec 29, 2014 at 4:34
  • \$\begingroup\$ In a physical DC circuit containing only a 10 volt DC source and a 10 millifarad capacitor, there should be basically no DC current. Any that exists would be leakage current and should be far less than 1 milliamp. There would definitely not be a negative current through the capacitor because the only source of DC current is a positive voltage source. So yes, this looks like something wrong with the simulation. If the colors of the wires are supposed to indicate voltage, then it seems like all of the voltage is dropped across the current meter, which shouldn't happen. \$\endgroup\$
    – Adam Haun
    Dec 29, 2014 at 5:43
  • \$\begingroup\$ Now that you brought the different color wire to my attention I deleted them and connected circuit once again and I measured pA. The wires signified something and that was what gave me the weird measurement \$\endgroup\$
    – AlanZ2223
    Dec 29, 2014 at 5:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.