Hello !
I've got one question about controlled source.

When I have this situation :



I heard that there is a rule when I want to change controlled source into resistance or conductance. And it looked something like that.

 

I guess that in this case, the "equivalent" resistance will be positive. Because VCCS will absorb power in the original circuit.

 

I guess that in this case, the "equivalent" resistance will be positive. Because VCCS will absorb power in the original circuit.

Even if U is equal "- U" ?

 

The equivalent resistance will be negative resistance.

 

The equivalent resistance will be negative resistance.

My teacher said that it is still positivi but I don't know why.



EDIT:

Real example here is how it would look like for positiv current :



Which I see that it is equivalent to positive resistance. But this one ?



I don't know. Because I can't change how the controlled source is "picking up" the current. So for him it will be - I_E.

 

Show him these two circuits:

 

Real example here is how it would look like for positiv current :

View attachment 271651

Which I see that it is equivalent to positive resistance. But this one ?

View attachment 271652

I don't know. Because I can't change how the controlled source is "picking up" the current. So for him it will be - I_E.

But here you are using CCVS. So the situation will be different.

 

Show him these two circuits:



View attachment 271654

I'm sorry I may not understand this one.

I made something like that :



First one shows that it is positiv resistance, but the second one is when I switched the voltage source and now R2 is equal - Ur so U is also equal -U. But I know I should change how "U" arrow is pointing.

Because when I had this one :



And this one :



Whenever I_E is positive or negative the resistance is still negative :

https://tinyurl.com/25zndvzz

Maybe I'm wrong but If I change how this source is "picking" the currance. I mean here to change the direction of "i" then The resistance will be positive ? No matted if "i" is positive or negative ?

 

First, you showed us the circuit that contains VCCS. And now your second circuit contains CCVS. And you expect them to behave the same?

 

First, you showed us the circuit that contains VCCS. And now your second circuit contains CCVS. And you expect them to behave the same?

I expected that this rule I learned is works for both of them.

I picked the voltage on the circuit for VCCS. So I thought that when I pick it like this :


Same principle so I thought it would work the same. Just like for CCVS if the current is positive or not the resistance is positive or negative depending on how the CCVS is "taking" the current, or more likely which way.

For CCVS it was true like in the simulations but why isn't it true for VCCS? Same principle. But I don't know how to make a circuit with CCVS in the simulation so I can't check it.

 

https://tinyurl.com/264jjl5k

 

https://tinyurl.com/264jjl5k

But the first one the voltage the VCCS is picking like that :



But in the second one you've changed how the VCCS is picking the voltage like this :


Which is negative resistance.

But watch this :



The simulations shows the same :

https://tinyurl.com/25etxrtm


So why did you change the way VCCS is picking the voltage ?

 

PS.

It looks the same like CCVS, but maybe I'm wrong.
Is it still the same like the circuit I drawn ? With U = -Ur ?

 

Hello !
I've got one question about controlled source.

When I have this situation :

View attachment 271589

I heard that there is a rule when I want to change controlled source into resistance or conductance. And it looked something like that.

Hi,

I dont know if you want to think about this in more general terms or not, but the way you have that connected it is kind of simple.

If you have a dependent current source that is controlled by a voltage that is actually across the source, then we can write:

R=v/I(v)

where R is the proposed resistance and v is the voltage across it and I(v) is the current source dependent on the voltage v.

Now since the current source I(v) is related to the voltage usually by a constant factor, we can say that the current source is:
I(v)=a*v

or anything like that.
Substituting that into the first equation we get:
R=v/(a*v) which of course is R=1/a

so we can see that if a=4 then the resistance R is 1/4 for example.

If the dependent current source is more complicated then it wont work out so simple.
If I(v)=a*v^2+b*v+c then we end up with:
R=v/(a*v^2+b*v+c)

which can not be reduced to a single resistance unless of course the voltage v is constant, which would not make sense although i guess it is a possibility in a static circuit.

 

Hi,

I dont know if you want to think about this in more general terms or not, but the way you have that connected it is kind of simple.

If you have a dependent current source that is controlled by a voltage that is actually across the source, then we can write:

R=v/I(v)

where R is the proposed resistance and v is the voltage across it and I(v) is the current source dependent on the voltage v.

Now since the current source I(v) is related to the voltage usually by a constant factor, we can say that the current source is:
I(v)=a*v

or anything like that.
Substituting that into the first equation we get:
R=v/(a*v) which of course is R=1/a

so we can see that if a=4 then the resistance R is 1/4 for example.

If the dependent current source is more complicated then it wont work out so simple.
If I(v)=a*v^2+b*v+c then we end up with:
R=v/(a*v^2+b*v+c)

which can not be reduced to a single resistance unless of course the voltage v is constant, which would not make sense although i guess it is a possibility in a static circuit.

I don't know if I get it because I don't see te circuit and how it works but looking at this :



which is the same like this circuit :



The simulations says the same :

https://tinyurl.com/25etxrtm

So I thought that no matter what is the voltage "U". Even if it's U = -Ur the controlled source can be changed into positive resistance.

EDIT:

For example here :



So which one of this (bUr or Ur) is negative to get negative conductance

 

I don't know if I get it because I don't see te circuit and how it works but looking at this :

View attachment 271681

which is the same like this circuit :

View attachment 271682

The simulations says the same :

https://tinyurl.com/25etxrtm

So I thought that no matter what is the voltage "U". Even if it's U = -Ur the controlled source can be changed into positive resistance.

EDIT:

For example here :

View attachment 271684

So which one of this (bUr or Ur) is negative to get negative conductance

Hello again,

Are you looking for a Thevenin or Norton equivalent circuit here?
That would be similar except you would also find a source to go with it.