Circuit Components

So far every model has been a flat list of equations. One advantage of Modelica is that it supports component-based modeling — you build reusable parts and wire them together. This tutorial models an RC circuit from first principles using connectors, inheritance, and connect equations.

Key Modelica concepts

• connector Pin — defines an electrical interface with voltage (across variable) and current (flow variable). Kirchhoff’s current law is enforced automatically at every connection point.
• extends TwoPin — Resistor, Capacitor, and VoltageSource all inherit the two-pin interface, avoiding duplicated boilerplate.
• connect(a, b) — equates across variables and sums flow variables. The topology of your circuit is declared, not hand-wired with equations.

Things to try

• Change the resistance R = 2.5 to a smaller value and watch the capacitor charge faster.
• Add an inductor component modeled after the Resistor (hint: v = L * der(i)) and wire it in series to build a full RLC circuit.
• Try adding a second resistor in parallel with the capacitor — you only need a new instance and two connect statements.
• Notice how the flat RLC model from the previous tutorial and this component model produce equivalent results — the component approach scales better to complex systems.