Creating Custom User Models in uSimmics (formerly QucsStudio) [2026]

uSimmics (formerly QucsStudio) supports three distinct approaches for building custom component models: Equation Components, VerilogA, and C++. This guide explains each method with practical examples, helping you choose the right approach and implement it correctly for accurate device simulation.

What You’ll Learn
Overview of Modeling Approaches
1. Equation Component
1. Example: Simple Resistor Model
2. VerilogA Model
1. Example: Custom Transistor Model
3. C++ Model
Choosing the Right Approach
Summary
Related Articles

What You’ll Learn

The three user model approaches in uSimmics (formerly QucsStudio) and when to use each
How to define a simple resistor model using the Equation Component
How to implement a custom transistor model in VerilogA
How to place a .va file in a project and load it into a schematic
How to select between approaches based on complexity, accuracy, and development effort

Overview of Modeling Approaches

uSimmics (formerly QucsStudio) offers three primary methods for creating user-defined component models. Each serves a different range of complexity and accuracy requirements.

Approach	Difficulty	Typical Use Case
Equation Component	Beginner	Linear models, basic electrical behavior
VerilogA Model	Intermediate–Advanced	Nonlinear and frequency-dependent models
C++ Model	Advanced	High-performance numerical algorithms, custom solvers

1. Equation Component

The Equation Component lets you define component behavior by typing mathematical expressions directly into the schematic. This is the fastest approach for simple linear models and initial prototyping.

Example: Simple Resistor Model

Goal: Model current flow through a resistor using I = V/R

Step 1: Place the Equation Component

Launch uSimmics (formerly QucsStudio) and open the schematic editor.
In the Components panel, expand the nonlinear components section.
Drag an Equation Component onto the canvas.

Step 2: Define the Equations

Double-click the component to open its property editor.
Enter the following in the Equation field:
I = V/R
I: computed current (output)
V: voltage (input variable)
R: resistance (constant)
Add auxiliary equations as needed, for example V = 5 and R = 100.

Step 3: Complete the Circuit and Simulate

Add a voltage source (V) and a ground (GND) to close the circuit.
Click Simulate to run the simulation.
Verify in the results window that I equals V/R as expected.

2. VerilogA Model

VerilogA (Analog Hardware Description Language) is the right choice for components with nonlinear or frequency-dependent characteristics that standard library components cannot capture.

Example: Custom Transistor Model

Goal: Describe a transistor with a custom current-voltage (I-V) relationship

Step 1: Write the VerilogA File

Create a file with a .va extension in any text editor and enter the following code:

module CustomTransistor(n1, n2, n3);
  inout n1, n2, n3;
  electrical n1, n2, n3;
  parameter real Vth = 0.7;   // Threshold voltage — transistor turns on above this value
  parameter real K = 1.0e-3;  // Device constant — scales collector current magnitude

  analog begin
    if (V(n2, n3) > Vth) begin
      I(n1, n2) <+ K * (V(n2, n3) - Vth)^2;
    end else begin
      I(n1, n2) <+ 0;
    end
  end
endmodule

Terminal assignments:
– n1: Base
– n2: Collector
– n3: Emitter

Model description:
– Vth: Threshold voltage. Collector current flows only when V(n2, n3) exceeds this value.
– K: Device-specific scaling constant.
– The analog begin ... end block defines the collector current expression.

Step 2: Add the .va File to the Project

Copy the .va file into the uSimmics (formerly QucsStudio) project folder.
Restart uSimmics (formerly QucsStudio) or reload the project.
Confirm the .va file appears in the Content panel.

Step 3: Place the VerilogA Model in the Schematic

Select the .va file in the Content panel.
Move the cursor to the schematic canvas and click to place the model.
Connect it to other standard components and run the simulation.

3. C++ Model

C++ models are appropriate when a component requires advanced mathematical algorithms or performance-intensive numerical computation, such as integrating an existing C++ library into the simulation. The code structure follows the same external-file pattern as VerilogA. Refer to the official uSimmics (formerly QucsStudio) documentation for the full implementation details.

Choosing the Right Approach

Situation	Recommended Approach
Quick linear model for initial prototyping	Equation Component
Nonlinear or frequency-dependent behavior	VerilogA
Custom numerical algorithms or external libraries	C++

Summary

uSimmics (formerly QucsStudio)’s three user model approaches cover a wide spectrum of modeling needs, from elementary circuit concepts to cutting-edge device physics. Start with the Equation Component to validate your design intent, then move to VerilogA or C++ as accuracy requirements increase. Regardless of approach, model parameters should always be validated against measured device data to ensure simulation fidelity.