we all know from where we got a dc voltage ...but what about dc current
plz if you know a semple way tell me...

 

http://www.allaboutcircuits.com/vol_1/chpt_1/4.html

 

thanx joe ....but the page you gave, dosent answare my qoustion ....

 

In the same way that a DC voltage is always positive or negative, a DC current never changes polarity.

 

The DC current is created by the DC voltage whenever the circuit is completed. The DC current will always be directly proportional to the DC voltage and inversely proportional to the circuit series resistance.

 

frindes thingmaker and beenthere.....what your are saying is absloutly right
mybe my qustion is not clear and i m sorry for that ...
i mean as a source !!!! we all know that we can get a dc voltage source from battries ....so how we can establish a dc current source as well ....thats what i meant ...
thanx for your response ...thanx alot..

 

Transistors can be used to make current sources. Google up 'transistor current source' or look in Art of Electronics (by Horowitz & Hill) if you have it.

 

Voltage regulators can also be set up as constant current sources. Most voltage regulator datasheets contain an example of this.

 

Any DC voltage can be a source of DC current. As soon as AC voltage is rectified, it becomes DC, and thus a DC current source.

 

Any DC voltage can be a source of DC current. As soon as AC voltage is rectified, it becomes DC, and thus a DC current source.

That's not even close to a current source as in Norton's theorem's sense. I think that's what he was interested in, that is, constant current no matter what the load is (within its compliance limits, of course).

 

Transistors can be used to make current sources.
thats what i wont ...thanx eduard you get my point
i have heared about using op amps to make current source...i dont know if that possible!!!
thanx every bodys response

 

Pages 10 and 13 of National's old Op Amp Circuit Collection have examples of how op amps can be used as current regulators. http://www.national.com/an/AN/AN-31.pdf

 

Look at this equation: AC current + Bridge or center-tap rectifier + Capacitor = DC current.

But if you are looking for an ideal current source, I will let you know that doesn't exist. The closest thing is a HV DC supply with a current limiter.

 

The mains is an AC voltage source, not a current source.

And there's no ideal voltage source either, although voltage sources exist in nature.