# what does a dependent power supply look like?!

Discussion in 'Homework Help' started by PG1995, May 22, 2011.

1. ### PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
Hi

I was wondering what a dependent power supply, either voltage or current, looks like. I have seen a 'general' power supply in the lab (see link #2). Actually I was looking at the example problem 1.7 in the given link #1. It has a dependent current which is rated 0.2I. When I is 5A, the current supply by the dependent source is 1A. But how does it sense that the current supplied by the "independent" source on the left is 5A. There should be some way; I don't think it works using telepathic means!

1: example problem from the book:
http://img716.imageshack.us/img716/2527/dependentsupply.jpg

2: the power supply I've seen in the lab:
http://img696.imageshack.us/img696/8841/powersupplyg.jpg

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
I have never heard of a dependent power supply. I am in doubt that such exists.

BTW, judging from the links this should have been in the homework forum.

****************************

OK, I see what you were referring to.

I have to agree, that is clear as mud. I am willing to bet the text book used the phrase earlier to explain a concept in the authors mind, look for the key words in the chapter in front of that test. It isn't part of the standard terminology. The only thing I can think of is a current source or sink, that or a power supply set for providing a constant current source or sink. My leaning is toward the former.

What is the name of the text book, and revision?

Last edited: May 22, 2011
3. ### PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
Fundamental of Electric Circuits - Alexander Sadiku

Dec 26, 2010
2,147
300
These are mathematical abstractions, used as part of exercises in circuit theory. In practice, devices like transistors and FETs can approximate to them, as can amplifiers made using such devices. A real controlled source always requires some sensing element to detect the controlling voltage or current, but this is not necessary for a theoretical analysis problem.

Computer simulation models for frequently employ controlled sources, but again these are mathematical simplifications of processes occurring e.g. in transistors.

Some types of power supply can be constructed to develop an output related to an external reference, but these are not common lab items.

Last edited: May 22, 2011
PG1995 likes this.
5. ### PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
Thanks, Bill, Adjuster. Adjuster, you reply was very helpful. I like to understand things from 'physical' world point of view. So, is it safe to assume that in real 'physical' world no such 'dependent' device exists as shown in the link? It's just mathematical abstraction. Please let me know. Thanks.

6. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,442
1,628
Yep, dependent sources are complete mathematical abstractions, excepting of course if you look hard enough you could probably buy something that does the same function off EBay.

7. ### PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
Hi Ernie

Would you please use your googling skills to find such something for me? I'm just curious!

Dec 5, 2009
5,201
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It depends

9. ### PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
Now, you guys are confusing me!

10. ### PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
I was thinking that how a transistor could would imitate the behavior of a dependent power supply. Could you please tell of some simple circuit which uses such a configuration where a transistor is acting like a dependent source? Thanks.

11. ### jegues Well-Known Member

Sep 13, 2010
735
43
A BJT will act like a current controlled current source when in its active mode, with Ib as the controlling current, and Ic as the controlled current.

12. ### PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
Hi, again,

I was reading this thread again and I think I need your help. I tried to find it myself without any success. I wanted to see a simple circuit which implements a transistor (preferably BJT) to emulate mathematical dependent current source. I understand it's not considered good to post in somewhat old thread but starting a new thread wasn't a good idea either. Thank you.

13. ### Hi-Z Member

Jul 31, 2011
157
17
I don't know if this is the sort of thing you're looking for:

http://en.wikipedia.org/wiki/Current_mirror

Current mirrors are common building blocks in analogue ics, and I would think they demonstrate the idea of a "dependent current source" within the transistor model.