## Monday, July 21, 2014

### Stiff loop evaluation

Been a long time since I have posted. One of the reasons has been that I have made change after change in the solver. With every change, the circuit that I am working on gets fixed but a previously tested one breaks. Even now I am not totally sure until I test them all. But I will start posting code anyway.

To begin with, the stiff loop evaluation. The concept until now has been:
1. Sort out the loops into stiff loops and non-stiff loops.
2. With stiff loops, there are those loops that were non-stiff until the previous iteration and became stiff. So these will have a non-negligible current because a diode may have turned off causing the current to be slightly negative.
3. So the currents of these stiff loops will have to be re-evaluated to brought back to values that correspond to their being stiff sloops with large resistances. A failure to do so will cause these loops to disrupt the calculation of the other stiff loops.
4. Simple way to do this is to have another function. The code is below.

This is the function that calls the stiff equation solver which has not changed from before (click "view raw" link below the code box to see code in a new window):

There is a bit of guessing here as well. Out of "n" loops, let us suppose "m" loops (m<n) have turned stiff in the previous iteration. The remove_stiffness function makes the stiff loops into a upper triangular matrix with the first stiff branch of every stiff loop being present only in that loop. However, as the number of stiff branches can be larger than the number of stiff loops, the last few stiff branches may appear in multiple loops. As an example, take a look at these loops:

stiff_forward no no no no no no stiff_reverse reverse reverse no no reverse no reverse reverse no reverse
no stiff_forward no no no no no stiff_forward forward forward no no forward no forward forward no forward
no no stiff_forward no no no no stiff_reverse no reverse no no no reverse reverse reverse reverse no
no no no stiff_reverse no no no stiff_reverse no reverse no no no reverse reverse reverse reverse no
no no no no stiff_reverse no no stiff_reverse no no forward forward no no reverse reverse no reverse
no no no no no stiff_reverse no stiff_forward no no reverse reverse no no forward forward no forward
no no no no no no stiff_reverse stiff_reverse no no no no no no no no no no

All the loops are stiff and the 8th branch appears in all the stiff loops. And for all you know, this 8th branch could be the branch that became recently stiff. So there is a possibility that these newly formed loops could disrupt the calculation of other new formed loops. So thus there is the initialization of newly formed stiff loop currents to zero.

The current of a stiff loop is decided only by the voltage in that loop and the resistance of that loop. The loop has no dynamics di/dt. So repeatedly calculating the loops will not change the loop currents unless the currents of one of the other stiff loops has changed. Which is why if the newly formed stiff loops are recalculated a number of times (thrice for now), the values should stabilize. In any case without a di/dt, they should not blow up out of bounds.

Following this, when it comes to calculating loop currents from branches, there were two parts to that function. The code is below (click "view raw" link below the code box to see code in a new window):

As can be seen, the entire block of code that would compute currents of stiff loops has been commented out. The reason is that if the currents of stiff loops are being calculated from the stiff equation solver, there is no need to repeat the process. On the contrary repeating the process has the capacity to disrupt the loop current values.