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Capacitor charged with constant current - SPICE
Capacitor charged with constant current - SPICE
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Old 23rd November 2017, 01:28 PM   #1
reedcat is offline reedcat  United States
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Default Capacitor charged with constant current - SPICE

I am trying to simulate very simple circuit where a capacitor of 1uF is charged with constant current source of 1mA. According to my calculations the Voltage across the cap should reach 10V in 10 msec, however SPICE time transition simulation fails to confirm it. What am I doing wrong?

The schematic and sim results are attached. The voltage is as measured across the cap.
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File Type: jpg spice-cap-current.jpg (14.1 KB, 47 views)
File Type: jpg spice-sim.jpg (73.9 KB, 47 views)
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Old 23rd November 2017, 01:37 PM   #2
udok is offline udok  Austria
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You need to select "Start external DC supply voltage at 0" in the simulation dialog.
Without this, you see the voltage as if the 1mA current source is on forever.
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Old 23rd November 2017, 01:59 PM   #3
reedcat is offline reedcat  United States
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Yay! The setting fixed the plot, and yes, my calculations were correct.
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Old 23rd November 2017, 03:12 PM   #4
Mark Johnson is offline Mark Johnson  United States
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Capacitor charged with constant current - SPICE
I think you will find that the Help File section which discusses the LTSPICE directive ".IC" (Initial Conditions), is incredibly useful. For this one example, and more importantly, for some future day when you decide to try .TRANsient analysis of bistable and multi-stable circuits. Such as the Jung-Didden Super Regulator (and many others).
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