Voltage Controlled Current Source PSPice Homework Problem

Thread Starter

Allen S

Joined Feb 7, 2015
7
Hello,

I have enrolled in a circuits class this semester and, although I am not majoring in electrical engineering, am very fascinated with the class thus far. Our instructor assigns weekly homework problems with the option of additionally completing said homework on PSpice for extra credit. I have been playing around with PSPice for the past few weeks and have been looking everywhere for tutorials on the program. I have one problem in particular that has been giving me trouble with drawing the correct schematic. I have attached a URL to the problem and a second URL to my attempt. I am getting readings but not the correct ones. Could somebody please inform me on what needs to be changed in a voltage controlled current source in order for the reading to be correct?

Thank you greatly,

Allen

http://tinypic.com/r/6srgps/8

http://tinypic.com/r/28is4kw/8
 

MikeML

Joined Oct 2, 2009
5,444
I dont know what is going on with PSpice, but here is what LTSpice shows:

Si.gif

Oh, and would you mind putting your attachments here, instead of that third party site that just wants to show me ads.
 

Thread Starter

Allen S

Joined Feb 7, 2015
7
I dont know what is going on with PSpice, but here is what LTSpice shows:

View attachment 80063

Oh, and would you mind putting your attachments here, instead of that third party site that just wants to show me ads.
Sure! I am new to this forum and am a bit unfamiliar as to how to display pictures. The reading you are getting through LTSpice are the correct ones, but I am not sure why PSpice is displaying weird current and voltage values. I am not sure if it is because I messed up some kind of value in the part itself but I replaced the one I had prior with a new one and am still getting the same values.
 

WBahn

Joined Mar 31, 2012
24,709
Just for a sanity check, try replacing the dependent current source with a 5Ω resistor and see if that gives you the correct voltage on the top node.

Post the netlist for your circuit, maybe we can see something wrong there.

It's interesting that if I assume that the two currents at the top of your schematic are the currents flowing downward in the two resistors, that the current that has to be flowing through the current source on the right is 1.739A.
 

Thread Starter

Allen S

Joined Feb 7, 2015
7
Hey guys,

I grounded my circuit and I am now getting correct results. We have not yet gone over what happens when a circuit is grounded. Could somebody please explain what this does? Also, is there a difference between gnd_earth and and_analog?

Thank you
 

Thread Starter

Allen S

Joined Feb 7, 2015
7
I also have one more question. Is there a way for wires to cross without forming a node? I have another problem with a voltage controlled current source that needs to be connect to a wire on the opposite end of the circuit, but getting there is impossible without crossing a few wires that do not play a part in the vccs.
 

Jony130

Joined Feb 17, 2009
4,983

WBahn

Joined Mar 31, 2012
24,709
I do not see any ground symbol (AGND) on your PSpice diagram, and this is why PSpice show wrong result.
Good catch. It's been so long since I've used PSpice that I forgot that it will let you simulate a circuit without a reference node. The simulators I've been using all squeal if you try to do this.
 

WBahn

Joined Mar 31, 2012
24,709
Hey guys,

I grounded my circuit and I am now getting correct results. We have not yet gone over what happens when a circuit is grounded. Could somebody please explain what this does? Also, is there a difference between gnd_earth and and_analog?

Thank you
Most (all?) simulators assign voltages to nodes and currents to branches and then play with those to get the circuit to converge. The very concept of the voltage at a node requires that there be a reference node against which each node voltage is referred. When there is no reference, each node voltage is only known to within a constant offset, and that's not good enough for the simulator. As for the specific details of why that isn't good enough (i.e., why the simulator doesn't produce the correct results except having each node voltage offset by the same amount) is dependent on the algorithm used by the simulator and I don't know those details of the internals well enough to answer that question.

While I didn't catch that you were missing the reference node in your schematic, I almost certainly would have caught it when I saw your netlist because that is one of the things that I unconsciously look for is where Node 0 is in your netlist as that helps orient me mentally to your netlist. I've caught my own errors in this regard several times.

A "ground" in general is just a reference node. Most simulators use Node 0 as the reference node and they don't care what you call it or how you intend for it to be used -- you could arbitrarily put your ground at the junction of a resistor and an inductor and it wouldn't care. From a practical matter, we often have different "grounds" for different purposes and those often deal with either safety or with noise performance. In either case, if you want that purpose to be modeled well in the simulation, you have to model the connections between those different "grounds". You still have to pick one node as your global reference node and it does not have to be any of your "grounds" (although it almost always is).
 

Thread Starter

Allen S

Joined Feb 7, 2015
7

WBahn

Joined Mar 31, 2012
24,709
See those big blue dots? Those are connection dots. In order to avoid them you must be sure to route any wire that you don't want to connect to a node so that it does not cross that node at any of the endpoints of components or other connection points for that node. In this case, your routed your wires from R3 across the endpoints of R4 and R1 and so it connected them. Route the top wire a bit higher and the bottom wire a bit lower (i.e., move them away from R1 and R4) and you will be fine.
 

Thread Starter

Allen S

Joined Feb 7, 2015
7
See those big blue dots? Those are connection dots. In order to avoid them you must be sure to route any wire that you don't want to connect to a node so that it does not cross that node at any of the endpoints of components or other connection points for that node. In this case, your routed your wires from R3 across the endpoints of R4 and R1 and so it connected them. Route the top wire a bit higher and the bottom wire a bit lower (i.e., move them away from R1 and R4) and you will be fine.
That got it! To everyone who commented on this forum, thank you very much for aiding me in these problems. I took away a lot from all of this. I will definitely be using this website a lot this semester.
 
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