# Current source!

Discussion in 'Homework Help' started by eman12, Mar 26, 2009.

1. ### eman12 Thread Starter Active Member

Oct 26, 2007
41
0
Hello guys.

I have a few questions about sources!

1:What is an independent Current source? I do know its definition but I do not know what it is in real life?

2:If we short a voltage source the whole current will pass through it and its voltage goes down to zero, right? So what will happen if we short a current source?
3:What is the Thevenins equivalent of a current source (say 5 amperes) when it is shorted?

Thanks a bunch

2. ### hitmen Active Member

Sep 21, 2008
159
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1. Independent current src gives the same current regardless of the rest of the circuit

2. If we short a current source, it is an open circuit

3. not possible without rest of circuit

3. ### eman12 Thread Starter Active Member

Oct 26, 2007
41
0
Thanks sir,

But I think I need more help to understand 1, 2 and 3.
what is your meaning by the response number 3?
Here's a pic of what I told about the question number 3:

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4. ### hitmen Active Member

Sep 21, 2008
159
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If I am not wrong, there is nothing left. All the current will flow through the shorted area

5. ### eman12 Thread Starter Active Member

Oct 26, 2007
41
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But you told that it would be an open circuit?!

6. ### eman12 Thread Starter Active Member

Oct 26, 2007
41
0
Is that a Hard Question?

7. ### hgmjr Moderator

Jan 28, 2005
9,030
214
All the 5A current source in your diagram will flow through the short circuit and there will be no voltage dropped across the short circuit. Hence there will be no current flowing in any passive load you connect to terminals A and B.

hgmjr

8. ### eman12 Thread Starter Active Member

Oct 26, 2007
41
0
Thanks,

yea that's right.
Can you please response my other questions?

9. ### hgmjr Moderator

Jan 28, 2005
9,030
214
As for the last question in your post concerning the Thevenin Equivalent circuit for the ideal current source with a short circuit across it, you should be able to imagine what voltage and what resistance you would expect to observe looking back into terminals A and B. I believe that will yield the answer.

hgmjr

10. ### eman12 Thread Starter Active Member

Oct 26, 2007
41
0
I was refering to this question:
":What is an independent Current source? I do know its definition but I do not know what it is in real life?
"
I think regardless to the current source amperage , the Thevenin's equivalent would be a zero resistor in series with a zero voltage source . Right?

Last edited: Mar 28, 2009
11. ### swty_todd Active Member

Aug 3, 2008
82
0
yeah even i was wondering...
Like a battery is an example of an independent voltage source , i am hoping to get an example of an independent current source

12. ### eman12 Thread Starter Active Member

Oct 26, 2007
41
0
I think a Battery is a power source, not only a voltage source.

Hope an expert here could help us to make a right sight into current source!

13. ### hgmjr Moderator

Jan 28, 2005
9,030
214
You can take a look at the information in Wikipedia's section on the topic of current source. Scroll down to the section on ideal current sources.

hgmjr

14. ### swty_todd Active Member

Aug 3, 2008
82
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yes battery is a power source ie, a current source as well as a voltage source but it is an independent voltage source and a dependent current source.
I went through wikipedia and other sites before posting here and they werent much useful.

15. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
An "Ideal Current Source" doesn't exist. If you could buy a 1A Constant current source at the store, next to the AA Batteries, some things in electronics would be a bit easier.

Current sources usually live in circuits as current mirrors. See Bill Mardsen's post on LED circuits for examples of "realized" current sources, made both with 317 adjustable regulators, and transistors.

16. ### eman12 Thread Starter Active Member

Oct 26, 2007
41
0
Thanks,

But why there is no an indipendent current soure in real life? Why we can make independent voltage sources but not a independent current sources?

In a book I read that a battery with a high resistance resistor in series with it acts like an current source, is that true?
Unfortunately I have no idea of an current source and that cases me to have several problems solving the exercises involving current sources.

17. ### Wendy Moderator

Mar 24, 2008
20,765
2,536
Mainly because a constant current source is an artificial concept. Since we can make stable voltages with electronics, and convert voltage to current pretty accurately, we can make really stable current regulators (AKA constant current sources) with electronics.

It also doesn't hurt that bipolar junction transistors (as in NPN or PNP) make pretty simple constant current sources.

18. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
That will limit the short circuit draw, but it will not provide constant current.

For driving an LED, using a resistor in series with a battery will limit current below a certain value.

An "Ideal Current Source" forces that current through the load, no matter what it is, as such, it needs to be capable of infinite voltage, and thus, infinite power.

If you have an Ideal 1A Current Source, and connect it to a 1Ω load, the source will have 1V across it. If you connect it to a 1MΩ (MegOhm) load, 1A will still flow, but the current source will have 1 Million Volts across it.

This is why ideal current sources do not exist in reality. They can be approximated with transistors, but only up to certain voltages/currents. At the point where required voltage is higher than available voltage, the current is no longer constant, and becomes lower due to lack of voltage.