Variable resistors in QUCS

While designing a 3-band equalizer, using QUCS (view previous post about QUCS here), the need for variable resisors came up. As the current versions of QUCS still don’t support EVALUE components, it was impossible to implement voltage controlled resistors (i.e. the solution suggeted here). However, one equation based element supplied with the software is the equation defined device, available in the nonlinear components library. This device couples the charge (first time derivative of current) passing through the component and voltage between the component nodes. You can read more about the numerical implementation of the component in the QUCS documentation. A 2 node resistor is defined by Ohms law

R = \frac{V}{I}

Defining, then, a resistor using such a component would simply require typing I1 = V1/R in the current equation, where I1 is the current passing through the 1st (and in the below figure, the only) branch of the equation defined device, V1 is the voltage between the device ports and R is defined by the equation on the right hand side of the device, as seen below.

The keen reader noticed that in the case that the resistor value is zero, division by zero is to be expected. In many applications, a 3 port variable resistor is required. Basically, a 3 node resistor is comprised of two resistors with a total resistance of R, shown below. Note that a, in this case, is between 0 and 1.

The schematic model for the variable resistor is comprised of a two branched equation defined device. The number of branches is a property set by right or double clicking the component.

Finally, the two resistors are defined by inserting the equations as previously shown. To complete the model, the parameters r (the small factor between 0 and 1) and R, should be parameterized. This is done by going to the symbol editor (press F9) and double clicking the writing SUB1. In the dialog box, fill in the two paramters r and R.

Enjoy!

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