qucssimulationvoltage divider
How do I measure the characteristics of a voltage divider in QUCS?
Below is what I have tried thus far, using Qucs 0.0.15 on Ubuntu 10.04
I want to measure how the voltage at the center of a voltage divider changes based on R2 in the schematic below. I am attempting to simulate this in QUCS using a DC simulation and a parameter sweep.
This is how I have it set up below, but the output is constant from what I can tell, which does not make sense.
SMIIW...
The term 'sweep' in this context refers to a change from one value to another.
So a \$V_{SWEEP}\$ of 3-8V would be a change from 3V to 8V over a predefined time.
A DC Sweep would therefore be a change in voltage of a DC source. An AC sweep could be a change in an AC voltage over time, or more commonly, a change in AC frequency over time.
Any adjustable parameter can be the subject of a sweep - for instance turning a 100KΩ potentiometer from one extreme to the other would be a sweep of 0Ω-100KΩ
If you look at this picture:
(source: infn.it)
You can see that \$V_{SWEEP}\$ goes from 0V to 1.5V, and the output (V2.1) is calculated for each point along that sweep as the output from the circuit if the input were what \$V_{SWEEP}\$ was at that time.
At first approximation, you can model a DC self-commutated motor as a resistor in series with a voltage source. The resistor is fixed, and represents the winding resistance. The voltage source is proportional to rotation speed. You can find the resistance easily enough by applying a fixed voltage to the motor and measuring the stall current. That is the current with the motor externally held fixed (not allowed to rotate). The resistance from Ohm's law is simply the applied voltage divided by the measured current.
There are various ways to find the rotation speed to voltage scale factor, which represents the motor acting like a generator. Since this is a permanent magnet DC motor, you can simply measure the open circuit voltage at some known rotation rate. You can try at a few different rotation rates, but it should be pretty linear. Another way to measure the speed factor is to apply a fixed known voltage and measure both the current and the rotation speed. From the current you can compute how much voltage is dropped by the resistance. The remainder is made up by the voltage source.
For the next order approximation, add a inductor in series with the resistor and the voltage source. This is the real inductance of the windings. It is harder to measure well. You can get a rough idea by again holding the motor fixed and measuring it like it were a inductor with the previously measured resistance in series.
There are more nuances, especially as commutation occurs as a function of rotation angle, but you can get a lot done with the basic resistor and speed-dependent voltage source alone.
Best Answer
Like SPICE simulators there's a difference between simulation parameters and reference designators.
Here, the reference designator for the lower resistor is R2. The R value of R2 is always 50 ohms. To use a parameter, you must assign it to the parameter name surrounded by curly braces. So the R value of resistor R2 should be
{R2}to have resistor R2's R value set to the parameter R2.To make things a bit more clear, change the parameter sweep param to RLOW and the value of R2 to
{RLOW}. The simulation should work as expected and you'll get the correct results (yeah, I realize that my voltage source isn't at the same level as the OP so the actual values will be different).