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I was playing with qucs and noticed that capacitors are not simulated as I expected. I designed simplest circuit that illustrates that case.

AC Source: 2 Hz (2 times a a second) Capacitor: 1pF Simulation: 1 second

I expected to see different graphs on Probe 1 and Probe 2 BUT they are same.

enter image description here

Is it correct results? If not how do I get it to show correct results? .. or how should I use qucs to simulate such circuit? (e.g. should I put some resistors)

PS I added 10 MOhm in parallel and it worked. Thanks @DaveTweed

enter image description here

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  • \$\begingroup\$ Why do you think they should be different? How much current is flowing through the capacitor, and how much voltage is across it? (Hint: If you want to simulate how a real voltmeter or oscilloscope would behave, put a 10Mohm resistor in parallel with PR2.) \$\endgroup\$
    – Dave Tweed
    Commented Aug 9, 2017 at 15:54
  • \$\begingroup\$ What results are "correct" depends on what circuit you want to simulate. If you want to simulate a circuit with resistors (or with a resistive load), put resistors. If you want to simulate a circuit without resistors, don't put resistors. \$\endgroup\$
    – The Photon
    Commented Aug 9, 2017 at 15:58

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It's because the "probe" or voltmeter PR2 has (by design) an extremely high input resistance. Even the tiniest current (in fact no current at all) flowing through the capacitor will "move the needle" of the meter.

For a more practical result, place a 1M resistor over the terminals of PR2, and change capacitor C1 to 80nF.

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