PartSim Blog

DC Source for Transient Sim

Recently a Partsim user asked about a circuit that wasn't working.

The user was expecting to see a typical RC curve across the capacitor, but saw nothing but a few pico amps of current and noise. So, what is the issue?

Spice calculates the DC steady state circuit values before it runs a simulation. The simulation is started with those values. In this case, that DC steady state condition would have C1 charged to 13.5V, with no current.

If you want to see a transient charge, you must use a transient voltage source.

This circuit replaces the DC source with an AC source. It uses a Pulse in order to delay the source rise value, so we get a clear turn on, and the expected RC curve.

And, the result - An RC current curve just like we learned in school.

Play with the simulation here

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