[![enter image description here][1]][1] I know that in a current source in series with just a capacitor (without the parallel resistor), it would charge until forever in an ideal case with the fomrula

I= (dV/dt)*C

but in this case since it is in parallel, the capacitor in t=0 will start with 1mA but it will be decreasing so I couldn't use the formula above and neither the Vc=Vi(1-e.....) either since no resistor in series or voltage source.

so I know the current will decrease (but can't know the slope since its variant) and that the voltage will increase until 1V( 1K*1mA). but I would like to know the formula for it so I could know the timing for other similar cases. THANKS SO MUCH for your time!

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    \$\begingroup\$ The current source and resistor can be replaced with their Thevenin equivalent voltage source and resistor. Then it becomes identical to a voltage source feeding an RC network. \$\endgroup\$ Jul 3, 2017 at 18:15

1 Answer 1


I couldn't use the formula above and neither the Vc=Vi(1-e.....) either since no resistor in series or voltage source.

1mA in parallel with 1 kohm in the fullness of time produces 1 volt, so change the current source (in parallel with the 1 kohm) to a 1 volt voltage source in series with 1 kohm: -

enter image description here

It's called source transformation and is related to Norton's and Thevenin's theorems.

  • \$\begingroup\$ @NormanCarrizo I'm sorry but you have evolved from your original circuit so much that the answer I have already given is at best obsolete and therefore you have wasted my time and other people's time reading and thinking. You can of course revert this question back to the original so that my answer and your reverted question are appropriate to each other. Then ask a brand new question about your new circuit because that is the more respectful thing to do. Remember this site is a showcase for Q and A and, in the long run, that's what is most important because other readers/learners benefit. \$\endgroup\$
    – Andy aka
    Jul 5, 2017 at 9:25
  • \$\begingroup\$ Ok, I have deleted the new things, I have posted that simpler circuit since I wanted to simulate the same effects of a transistor current and expected a general solution, but as I said I was wrong with trying to represent it the best, but please STOP DRAMA. I wasn't disrespectful at all I did appreciate your answer. \$\endgroup\$
    – N. C.
    Jul 6, 2017 at 12:49

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