Periodic Stimulation of Electric Currents Interface

Hello,

I am trying to simulate an electrode with a periodic biphasic square pulse through a periodic piecewise function to stimulate it. The piecewise goes between 1, -1 and 0 and I multiply it by my input current to simulate this stimulation. I am doing this in a time-dependent study using the electric currents interface. The convergence continues to oscillate up and down until eventually it says the solution wasn't converged. Is this oscillation in the convergence due to the oscillation of the stimulation? What does an oscillating convergence mean? Does the electric currents interface only converge if a DC signal is given for a current source?

Thanks,
Trevor Davis