Assigning a function to an Electric Potential boundary

Hello,

I am currently making a model using the Electric Currents and the Bioheat Transfer physics. I want to assign a waveform to an electric potential: In stead of a continue 2000 V boundary I want the function [2000e^(-(t/tau-0.003))^2)] assigned to it. When I inserted this formula into the Electric potential field my simulation wouldn't finish, the convergence plot kept going up. It worked before I changed this so I am assuming this is the cause. So my question is: How do I assign a wavefunction to an electric potential?

Kind regards, Carolien