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Created July 28, 2020
Last modified August 04, 2020
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I couldn't resist finding out what your analog computer can do! The results are impressive, the Time-Domain plot from the "ExploreAnalog" circuit (below) clearly shows 3 classic damping levels for a linear positional control system. I discovered which of the potentiometers controls the losses due to movement (eg viscosity, friction) and declared a parameter for simulation control of its K value.


I had to make some changes which are indicated on the diagram in blue.

I also tried it again with the 1Hz square input (not part of "ExploreAnalog") and the output "displacement" now follows the input more closely - with the correct choice of damping. The Frequency Domain simulation produces amplitude and phase plots.

by EF82
August 05, 2020

Oh my gosh! YOU should be teaching the class, not me. Thanks so much for your efforts. I can't wait to see what you came up with.

Even if the students need guidance, just seeing something this advanced working will give them a real sense of growth from basic series/parallel bulbs to transistors/op-amps.

Carol Strong UAH

by strongc
August 05, 2020

Pleased to be of help - hope you will be able to use your circuit for teaching. Lucky students who will get to work with this great device!

by EF82
August 08, 2020

Believe it or not, though difficult, this Analog Computer circuit was easier to handle with real elements on a breadboard. I think I will provide the students with your solution so they can see the depth of this program's abilities and the willingness of you and your team to help out. I'm thinking of having them do a "plan your own" circuit using an Op-Amp instead.

Every day that I work on preparing circuits for this coming semester, I am learning new techniques.

I am hitting a snag with a transistor circuit that works great on a breadboard, but again, I'm apparently missing something in my CircuitLab setup. Having the 2 resistors at the beginning of the hardware circuit gives some options to create different response curves for the same transistor easily by switching out the 1K ohm with a 3.3K ohm, for example. I'll attach the 1K ohm setup here, but I am not desperate for a solution. I found an easy transistor lab in the collection of the circuits that you provide that will work just fine for my group... It's just got me curious.

by strongc
August 10, 2020


I found two points here: 1] the interplay between simulation and reality, 2] a latent "gotcha"(!) with the potentiometer.

1) I suspect that your real transistor 2N3904 will be diverging significantly from its simulated counterpart, at the high base and collector currents ("high", that is for a real small signal transistor). 1k ohm in the base connection gives about 4.2mA base current. Multiply by gain (140 simulated) and you have nearly 600mA demand for collector current (Ic), against the real device's absolute max Ic of 200mA. The simulation will happily handle current levels which the real device cannot. In this type of situation the real device's gain falls off sharply.

In the 2N3904 datasheet, maximum gain is at Ic around 1 - 10mA, this is an indicator of where the real device is "comfortable". I suggest having higher base and collector resistances to run the device nearer to its comfort-zone. I would aim for a base current of 10mA/140, about 70uA.

Also, in my view, this will get students more "used" to the environment of small-signal transistors. I'm assuming that you are looking at the transistor as constant current source, and are not yet dealing with the concept of collector saturation.

2) Potential short circuit: transistor to supply V1. When the potentiometer R is at one extreme (K=0) the transistor is connected directly across the supply V1. Again, the simulation will blithely carry on with its abstract calculations. Meantime, a real 2N3904 trying to copy would be toast! (600mA at 5v Vce > 3Watts package dissipation; implies a (real) junction temperature of 600degC, about 1,100degF!!). What I try and remember(!) to do in this type of potentiometer arrangement is add a fixed resistor in series, enough to protect the connected circuit. 47 ohm would limit Ic to about 100mA.

Hope all this use useful "grist" to the teaching mill.


I apologise if I have given the impression of being in a "Team". I just make the occasional "drive-thru" of the Q&A forums and pitch in if I can contribute something useful. My responses are complicated by a transatlantic time-lag. I only found this question because I have your interesting circuit tagged: very useful for me to explore the boundary areas between reality and simulation.

by EF82
August 11, 2020

Your timing is just fine, team or no. Delayed responses don't bother me one bit as I tend to work odd hours. As the semester starts, all my classes are late afternoon (joys of being low on the totem pole) and Mondays and Wednesdays I force myself to fitness training early morning.

Your responses for the Transistor circuit have really made me think about the previous hardware version of the lab that never worked quite right. I'm betting that I was sending too much current through/voltage across the transistor we chose. That's why I was so glad to come across the newer layout that I sent you.

But you're right, the layout might be okay, but the limits of the transistor chosen have to be resolved and an additional resistor could be part of the solution. I'll play with your suggestions and see what I come up with. It may take me a bit to resolve this as online classes begin on Wednesday and I won't need this handled until later in the semester.

Thanks again for your thoughtful input. It gives me options for the students to play with and learn from.

Carol Strong UAH

by strongc
August 11, 2020

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