Why on Earth can't I simulate two capacitors?

Thread Starter

dannybeckett

Joined Dec 9, 2009
173
Hi all,

I'm using TINA-TI to simulate an incredibly simple circuit:
upload_2019-10-2_13-24-27.png

However when I try to do so, I get the warning:
upload_2019-10-2_13-24-54.png

Why?????
 

ci139

Joined Jul 11, 2016
711
The LT spice however initalizes the voltage difference in between the caps. as if circuit were started gradually from the initially shorted Pos. and Neg. supply rails (i guess assuming and some high leakage resistance being present)

but in real life you got the situation as undefined as with dummy Tina-ti unless you tile your cap up from different ones and use it as SUM capacitance with the inter-node voltages = "don't care" ← this shows the dummyness of the ti simulator as ? what if i use 2n2F+3n3F in series to get 1.32nF one -- "impossible ... Halt ! , Halt ! ... Panic! , Panic! . . . :eek:"
 

WBahn

Joined Mar 31, 2012
24,692
Maybe I should read the AAC forums before I ask a question next time.

For anyone who is interested in the answer to my question:

https://www.allaboutcircuits.com/textbook/reference/chpt-7/spice-quirks/
That only partially addresses/explains the issues.

For most of those circuits it has far less to do with "infinities" as it does with ambiguity. If you have two ideal inductors in parallel or two ideal capacitors in series, then the manner in which the DC current splits between the inductors or the DC voltage splits between the capacitors is indeterminate without knowing the complete history of the circuit. You can break the ambiguity by declaring initial conditions (voltages on troublesome nodes or currents into troublesome pins). If you add those (assuming your simulator supports them), then life becomes good again.

Many simulators have a few different ways to find the initial operating point of the circuit. You might explore than and see if any of them help. One common alternative method is the "power-up" analysis in which it is assumed that all device voltages and currents are zero. The power supplies are then brought up slowly and the circuit response is determined accordingly. This is generally pretty slow, but it is usually able to find a solution the simulator can work with.
 

MisterBill2

Joined Jan 23, 2018
3,869
Certainly all simulator programs are not the same, and some of them are really quite inadequate. It appears that you have discovered one of the inadequacies.
 
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