Discussion Forum

Hi James,
The floating potential boundary condition is part of certain modules (AC/DC Module, MEMS Module and Plasma Module). If your 4.4 installation does not include any of those modules then that boundary condition would not be available to you. This may be the reason you're not seeing that BC. You can check which products are installed via File>Licensed and Used Products.
Best,
Jeff

Thanks Jeff!
Yeah I don't have the AC/DC module.

Is there a document somewhere that lists the differences in capabilities between the standard physics module and the AC/DC module?

James

Yes, the specification chart: www.comsol.com/products/specifications/
Please note on that page the tabs on the left that let you select between 11 different fields (AC/DC, Acoustics, etc).
Jeff

Even if you don't have the ACDC module you can by yourself the floating potential condition
usin the method suggested in the attached paper of the Femlab time (3.0)!
I haved used it many times and works well.
Nice to help you Roberto

Hi Roberto,

I too don't have the ACDC module. Would you be so kind as to describe a step by step procedure in order to incorporate this in COMSOL 5.0 (not Femlab)? I'm reading the user guide for the physic builder right now and it's far from being straightforward...

Thanks in advance for any help!

Best regards,
André.

Hello Andre,
At version 5.1 (the current one) and 5.0 (the previous one), the floating potential boundary condition is available right out of the box in the AC/DC Module, the MEMS Module and the Plasma Module.
Best regards,
Jeff

Hi Jeff,

Thanks for the info but I don't have the AC/DC module (nor plasma and MEMS). Although, I have access to electrical current (ec) and electrostatic (es)... Therefore, I don't know how to proceed without the AC/DC module.

Thanks for your help.

Best regards,
André.

It sounds like you only have access to those capabilities found in the core COMSOL Multiphysics package. Please consider having one of those modules added to your license so you don't have to reinvent a feature that is now implemented in the software.
Jeff

One big advantage that the AC/DC Floating Potential implementation has over a self-made one (like the one suggested by Roberto) is that it has a mode to treat disjoint boundaries as different electrodes. (There is a "Floating potential group" checkbox that needs to be checked to switch into this mode.) One can see in the Equation View, after running the solver, that it adds a separate potential and constraint for each electrode automatically. This makes it very simple for the user to create a large number of floating electrodes — as simple as selecting them all within a single Floating Potential node.

Implementing the equivalent using only the core Comsol, as far as I know, would entail creating a constraint for each electrode manually. In my case that would take an eternity, as I have a parametric geometry, some with possibly hundreds of electrodes. The best alternative would be to use the Java API and automate this process (the finalised geometry may be queried for adjacency information between boundaries to assign them to different electrode groups). While conceptually not too hard, this would take some work and would add a layer of awkwardness when it comes to merging changes done using the Comsol UI with the Java code. Comsol really needs a scripting language embedded. Until then, it's not so simple to replicate what the Floating Potential functionality does.

My suggestion was devoted to people that can't afford the
extracost to buy the license for the ACDC module.
If the case, simple electrostatics problems can be solved using Laplace Equations node
of Classical PDE module.
Also the sef-made floating-pot. procedure, even if laborious, is rather instructive on the usage
of weak contributions to add to a generic set of PDE equation(s).
Best regards Roberto