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Garrettjoined October 22, 2016

posted Tuesday, November 29, 2016 | 3:54 pm PST

Hello. I am working on a circuit that is based off of the circuit on the following website: http://www.electroschematics.com/6899/12v-ldo-solar-charge-control/ . Near the bottom right of the circuit on the link, there is a variable resistor, the arrow of which comes from the base of a npn BJT. When trying to model this in systemvision, I cannot connect the BJT base to the variable resistor; it says it cannot connect different technologies and to add a converter block. When I add a rotational converter, it gives me the same message. I need the voltage across the resistor controlled per the link. The design I'm working on is called 'Charge Controller'.

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converter block

Hi Garrett,

maybe late, but I am new here, and nobody seems to know what is wrong, I had the same problem and found out what is happening.

you used a variable resistor, not wrong, but what you need is a potentiometer.

This pot had 4 connections, 3 black an 1 green.

The green connection is for adjusting the wiper setting and here you have to connect a converter block.

You can use the Quantity Source Constant block.

You will find that the output level can be adjusted, this level can be between 0 and 1, so if you want the potentiometer to be adjusted on 75% of his total resistance, you have to set the output to 0.75. From what direction, you have to find out yourself.

Easy if you know how to do it, that is the problem with SystemVision, it is too difficult for beginners and there is a lot of information that makes it more difficult, but no easy help function to guide you.

Too diffficult for beginners

I'm wondering if the difficulty peterkat2 mentions originates with SystemVision or with the details of the VHDL-AMS. Hardware description languages are not simple, especially when they set out to model the electrical domain plus mechanical, thermal, magnetic, etc. Before I started to use SystemVision I studied about the first half of The System Designer's Guide to VHDL-AMS by Peter J. Ashenden, Gregory D. Peterson and Darrell A. Teegarden. I keep that book handy as I run simulations. I refer to it throughout writing my own component models.

You need a simpler model

The problem here isn't the difficulty of the language or the tool, but rather the potentiometer model itself. The model with the "green arrow" is made to be a dynamically variable potentiometer, which means it can be changed over time during the simulation run. Something has to cause that change, and the green arrow allows for that to be any other discipline that we support (e.g. a pressure, position, temperature, etc.). This is really useful in some situations, but it is overkill for Garrett's application. In the original schematic on the website he referenced, the potentiometer terminal with the black arrow is just the electrical wiper terminal.It is assumed to have a fixed position during the simulation, representing the transistor bias setting specified by the designer, and assigned to the model as a constant value.

Which leads me to the solution ... we need another potentiometer model, one with fixed (but user specified) set-point parameter. I will make that model and put it in SystemVision's Analog Electronics Library very soon, hopefully later today. I'm sorry for the confusion, I should have made this simpler model first!

Mike Donnelly (SystemVision Model Developer)

New Potentiometer Model Available!

Following up on my previous comment, I added a new potentiometer model to the Analog Electronics Library. Here is a design that compares how you use both the old and the new potentiometer models: https://www.systemvision.com/design/test-potentiometers-0

As you can see, the original model (now called "Potentiometer - Variable" in the Library) can change with time, controlled by any of our "green terminal" quantity sources. In the example, I'm just using a sinusoidal source to vary the pot setting. For the new model, the user can just specify the fixed "wiper_setting" parameter. It can have any value between 0.0 and 1.0. Note that 0.0 means the wiper is at the bottom (i.e. at the "p2" terminal), and 1.0 means it is at the top (at the "p1" terminal).

I Hope you find this model easier to use for your circuit application. If you ever need to model a position sensor, a pressure sensor or any other sensor that uses a potentiometer as the electrical detector, and the sensed value changes over time, then you'll find the original model to be quite useful. You just connect it any mechanical, hydraulic, thermal, etc. section of your system using one of our converter models (e.g. displacement to quantity, pressure to quantity, temperature to quantity, etc.) found in our Technology Converters Library.

Hope this helps!

Mike