Niikos,
Try decoupling from the load.
If you need more help, I suggest you post your code.
- B O E

To get a proper time domain behavior that looks like a exponential rise/fall you need to make sure you get enough time points. The best way to achieve this is by using the $bound_step system task. If you use a linear rise from the transition operator as input and just map the linear value to the equivalent exponential value you should be able to calculate arguments to the $bound_step task that control the step size to be sufficiently small during transition while switching off this step size control when it has reached its final value.

Cheers,
Marq