External I vs. U
The External I vs. U () feature connects an arbitrary voltage measurement (for example, a circuit terminal or circuit port boundary or a coil domain from another physics interface) as a voltage source between two nodes in the electrical circuit. The resulting circuit current from the first node to the second node is typically coupled back as a prescribed current source in the context of the voltage measurement.
Node Connections
Specify the two Node names for the connecting nodes for the voltage source. The first node represents the positive reference terminal.
External Device
Enter the source of the Voltage. If circuit or current excited terminals or circuit ports are defined on boundaries or domains or a multiturn coil domains is defined in other physics interfaces, these display as options in the Voltage list. Also select the User defined option and enter your own voltage variable, for example, using a suitable coupling operator. For inductive or electromagnetic wave propagation models, the voltage measurement must be performed as an integral of the electric field because the electric potential only does not capture induced EMF. Also the integration must be performed over a distance that is short compared to the local wavelength.
Except when coupling to a circuit terminal, circuit port, or coil, the current flow variable must be manually coupled back in the electrical circuit to the context of the voltage measurement. This applies also when coupling to a current excited terminal. The name of this current variable follows the convention cirn.IvsUm_i, where cirn is the tag of the Electrical Circuit interface node and IvsUm is the tag of the External I vs. U node. The tags are typically displayed within curly brackets {} in the Model Builder.
Connecting to Electrical Circuits
Nonlocal Couplings and Coupling Operators in the COMSOL Multiphysics Reference Manual