About Connecting Electrical Circuits to Physics Interfaces
This section describes the various ways electrical circuits can be connected to other physics interfaces in COMSOL Multiphysics. If you are not familiar with circuit modeling, it is recommended that you review the Theory for the Electrical Circuit Interface.
In general electrical circuits connect to other physics interfaces via one or more of three special circuit features:
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External I vs. U 
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External U vs. I 
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External I-Terminal 
In the AC/DC Module, the physics features supporting the connection to an electrical circuit are the Terminal and Floating Potential features in electric field interfaces and the Coil and Lumped Port features in magnetic field interfaces.
These features either accept a voltage measurement from the connecting noncircuit physics interface and return a current from an Electrical Circuit interface or the other way around.
The “External” features are considered “ideal” current or voltage sources by the Electrical Circuit interface. Hence, you cannot connect them directly in parallel (voltage sources) or in series (current sources) with other ideal sources. This results in the error message The DAE is structurally inconsistent. A workaround is to provide a suitable parallel or series resistor, which can be tuned to minimize its influence on the results.