voltage source/current source?

[gauthamtechie]

When voltage exists, current flows and hence a "voltage source" will carry current when wires are used to make a closed circuit, right? Then what is the meaning of Voltage source and current source when both co-exist?:?::arrow:

 

[Pjdd]

A voltage source has a low internal resistance so that its output voltage does not change or changes by a small amount when different loads are connected to it. A pure voltage source, which exists only in theory, has zero internal resistance.

A current source has a high internal resistance so that the output current does not change or changes only by a small amount when different loads are connected to its output. A pure current source, which does not exist in practice, has an infinite internal resistance.

 

[phatcreators]

when we should use current source in a circuit ?
&
when we should use voltage source in a circuit ?

thanks

 

[Pjdd]

There are many different kinds of applications for each type of source and it's impossible to mention them all. I'll give just a few examples:

A voltage source is needed when the output voltage has to change as little as possible with different loads, as with a power supply or the output of an audio amplifier.

A current source is appropriate when the output current should remain the same with changes in load. An example would be a wire temperature sensor where the resistance changes in proportion to the temperature. Or in some biasing circuits.

 

[phatcreators]

There are many different kinds of applications for each type of source and it's impossible to mention them all. I'll give just a few examples:

A voltage source is needed when the output voltage has to change as little as possible with different loads, as with a power supply or the output of an audio amplifier.

A current source is appropriate when the output current should remain the same with changes in load. An example would be a wire temperature sensor where the resistance changes in proportion to the temperature. Or in some biasing circuits.

thanks a lot

but would you name some more of them for me ?

thanks

 

[LvW]

When voltage exists, current flows and hence a "voltage source" will carry current when wires are used to make a closed circuit, right? Then what is the meaning of Voltage source and current source when both co-exist?:?::arrow:

*A voltage source does not "carry a current" - rather it is regarded as the cause of current in a closed loop whereby the amount of current is determined by the impedance characteristic of that loop.
*What do you mean with "co-exist"?
*The other answers depend on your goal:
1.) What is the purpose of the circuit you like to connect to a source (voltage or current) ?
2.) Do you want to model, for example, an active part (or understand an existing model) for simulation purposes? Here, sometimes a combination of ideal voltage and current sources are used for correct modelling. For example, the heart of a bipolar transistor or FET model are controled current sources.
Does this information help?

 

[goldsmith]

Dear Friends
Hi
In fact the transistor is a current source that you can control its maximum current with base current as well as. all of the current sources will made with transistor . when we need to use a high impedance microphone , we have to use a high impedance preamplifier ( such as current source )( my mean is a buffer ) . because if the input impedance of preamplifier , become low , the created voltage by microphone will decrease , and it is not good. we can use a circuit that used a transistor , that its input impedance is high . the out put power will be constant ( approx ) by using some reasonable loads.
( it can be considered as a current source) .
Best Wishes
Goldsmith

 

[Raza]

Voltage Source:
An Ideal Voltage source:
The concept of an ideal voltage source is pretty simple.
* An ideal voltage source is a voltage source that maintains the same voltage across the source's terminals no matter what current is drawn from the terminals of the source or what current flows into the terminals.
Ideal Current Source:
* An ideal current source is a simple model for many current sources. It is reminiscent of the ideal voltage source - but with voltage and current interchanged.
The idea of using ideal sources is something that some one may rebel at. After all, there is no such thing as an ideal source anywhere in the world.
Then why we use them ? (A natural question striking mind)
We use them when we make the circuit analysis.
But in general we refer the voltage source as the source which has a specified Voltage on its terminal. A best example of the Ideal Voltage source is a BATTERY (nearest to Ideal Voltage source).

 

[gauthamtechie]

---------- Post added at 17:04 ---------- Previous post was at 17:03 ----------

*A voltage source does not "carry a current" - rather it is regarded as the cause of current in a closed loop whereby the amount of current is determined by the impedance characteristic of that loop.
*What do you mean with "co-exist"?
*The other answers depend on your goal:
1.) What is the purpose of the circuit you like to connect to a source (voltage or current) ?
2.) Do you want to model, for example, an active part (or understand an existing model) for simulation purposes? Here, sometimes a combination of ideal voltage and current sources are used for correct modelling. For example, the heart of a bipolar transistor or FET model are controled current sources.
Does this information help?

yes! ... and i made a mistake by saying "co-exist", because i thought any source that supplies voltage will carry current. . As you've answered and from the discussions, i've understood that voltage source only has constant voltage & current depends on impedence of load..and similarly current source dispenses current and voltage develop will also depend on load...
it looks like there are situations where we may need to keep the current/voltage constant irrespective of the changes in signal/load.. so there arises this concept of voltage and current sources.. And further for the purpose of analysis of circuits, with for e.g transistors ., we may use "current source" concepts too..is that right?

 

[LvW]

Yes, I think you have summarized all aspects in a correct way.

 

[Raza]

Yes now you have the good concept and had a good summary (in almost all aspects).

 

[gauthamtechie]

thanks a lot everyone!! And please check this thread too! Your inputs will be valuable
https://www.edaboard.com/threads/247152/#post1057418

 

[ROCKET SCIENTIST]

Wow such a nice post... Learned many things from it... I also got confusions about V and I sources but now I'm clear.. But one question.. Like independent voltage source (battery) is there any independent current source you can name??

 

[Ratch]

RS,

Any voltage source together with a large resistance in series makes a current source. A perfect current source would be capable of supplying infinite voltage if necessary, and have a infinite internal resistance.

Ratch

 

[goldsmith]

Dear Ratch
Hi
I don't think it is correct that :

Any voltage source together with a large resistance in series makes a current source. A perfect current source would be capable of supplying infinite voltage if necessary, and have a infinite internal resistance.

Current source has an infinite resistance ( or very high ) but it's current due to the each load is constant .
Best Wishes
Goldsmith

 

[FvM]

I don't think it is correct that

I can perfectly agree with the quoted statement. It doesn't doubt constant current.

There's no problem to represent a real current source by a voltage source with series resistance. I understand however, that question #13 is asking for a physical process, that is generating a current rather than a voltage "by nature". Perhaps a van-de-graaf generator can be considered as such. https://en.wikipedia.org/wiki/Van_de_Graaff_generator

 

[Ratch]

goldsmith,

Current source has an infinite resistance ( or very high ) but it's current due to the each load is constant .

Which it will be if the resistance of the current source is much larger than the load.

FvM,

I understand however, that question #13 is asking for a physical process, that is generating a current rather than a voltage "by nature". Perhaps a van-de-graaf generator can be considered as such. https://en.wikipedia.org/wiki/Van_de_Graaff_generator .

Or a Widlar current source? https://en.wikipedia.org/wiki/Widlar_current_source

Ratch

 

[FvM]

Or a Widlar current source?

Yes. I was reading the term "independent" current source as description of a pyhsical device, that act's as a current source. A transistor current source depends on an external supply voltage, it's not sourcing current on it's own.

 

[LvW]

........................
I understand however, that question #13 is asking for a physical process, that is generating a current rather than a voltage "by nature".

Yes, I have the same understanding ("by nature"). Therefore, the following remark.

Perhaps you will consider it as pure academic, however: I think even the Widlar source is a voltage source with an internal resistance that is very large.
Of course, that's no surprise and you may believe me - I know, that we call such a circuit "current source".

I stress this point because I am afraid the question in post#13 concerns a source that cannot exist: A source that delivers a current without a corresponding driving voltage.
Is this the meaning of "independent" ?
Or did I misinterpret the question #13 ?

 

[gauthamtechie]

i don't know the answer ,. i think #13 is plainly asking an equivalent .. Like Voltage source: battery :: current source : _______ ??? so is there any example u could point to and say: " This thing is a current source" just like we can call a torch battery as a " voltage source"

also help in this post will be much appreciable : the link : https://www.edaboard.com/threads/247152/#post1057418 ..
thank you all !