# Current / Voltage source

Discussion in 'General Electronics Chat' started by MartinV, Jun 2, 2011.

1. ### MartinV Thread Starter New Member

Jun 2, 2011
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I am new to this part of physics (electrotechnics) and I am finding it a bit confusing to get the point of these components of a circuit. So I would be very grateful if someone could explain these (even shortly) their functions.

2. ### #12 Expert

Nov 30, 2010
18,076
9,690
A constant voltage source, also known as a voltage regulator, attempts to provide a steady voltage as the load changes. It is said to have low impedance because changing the current that flows does not change the voltage very much.

A constant current source attempts to always have the same current flowing through it, no matter how much voltage is applied, as long as there is enough voltage to operate the current source, but not enough to destroy it. A constant current source can have very high impedance and is sometimes used as a load, in place of a resistor, because it allows enough current to operate the idle current of the amplifier, but refuses to change current when AC is applied to it.

Good start?

3. ### MartinV Thread Starter New Member

Jun 2, 2011
22
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How does the voltage source affect the current flow? I read something about about the voltage source making potential differences that make the electrons (current) move opposite of the electrostatic field, and I was quite confused by that.

Dec 26, 2010
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There are real-world systems, such as just described, which approximate to current and voltage sources. On the other hand, in circuit analysis these items are often considered as idealised abstractions.

These may be found as elements of problems requiring solution, or else they may used as part of models describing the operation of other circuits. If you continue to study this subject you are likely to encounter Thevenin (voltage) and Norton (current) equivalent circuits for networks, as well as a variety of models for semiconductor devices, which generally use so-called controlled sources, whose output depends on conditions elsewhere in the circuit.

5. ### MartinV Thread Starter New Member

Jun 2, 2011
22
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The shameful part is that I've met with Thevenin's method but I've been using it schematic. I wanted to understand these components deeply. I wanted to know how will the multiple voltage sources will contribute to the changing of the current intensity, and how will they affect each other.

6. ### #12 Expert

Nov 30, 2010
18,076
9,690
Speaking of "components of a circuit" (post #1) what component has this "electrostatic field" (post #3) of which you speak? How many "multiple voltage sources" are we talking about? How are they connected that they would change the current intensity (post #5)?

I am completely lost right now.

Dec 26, 2010
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I don't think that there is all that much to be confused about. The choice of positive and negative terminology was originally made arbitrarily, before the electron was identified as the carrier of electric current. Later on, the electron was discovered and turned out to have a negative charge according to the already entrenched convention - it was too late to change it.

We simply have to accept that the electron has a negative charge. Opposite charges attract, and like charges repel. Positive, then, is what attracts electrons, namely a relative lack of them.

8. ### MartinV Thread Starter New Member

Jun 2, 2011
22
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The electric field is a part of definition, so forget that part.
Here's an example. If we know the amount of Volts of E10 (voltage source) and the ohms of the resistor R10 we can easily get the amount of I10. Is that right?
(Look at the picture below)

http://imageshack.us/photo/my-images/191/circuitx.jpg/

9. ### MartinV Thread Starter New Member

Jun 2, 2011
22
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My question is, if we say E=I*R, does that mean that the current through the voltage source (E) is the same as the current entering that "branch" from the near ones?

Apr 30, 2011
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No. The circuit contains 4 other voltage sources and 9 other resistance elements that must be accounted for.

Apr 30, 2011
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Yes. It's the only path.

12. ### MartinV Thread Starter New Member

Jun 2, 2011
22
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What do you mean by "the only path"?

Dec 26, 2010
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We need to be very clear here. The OP must understand that, for his circuit, one cannot say that E10 = I10/R10

14. ### MartinV Thread Starter New Member

Jun 2, 2011
22
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What is OP, and do you mean E10=I10*R10?

Apr 30, 2011
1,568
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Last edited: Jun 2, 2011
16. ### MartinV Thread Starter New Member

Jun 2, 2011
22
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So we look at that path (the one including R10 and E10) as it was isolated?

The node method should do the job here, right?

Apr 30, 2011
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No because it isn't. It's part of a network. You must use one of the network analysis techniques. Read the Wikipedia article section on choice of method.

18. ### MartinV Thread Starter New Member

Jun 2, 2011
22
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Yes I know that now, since you edited your previous post

19. ### MartinV Thread Starter New Member

Jun 2, 2011
22
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Thanks guys, after tormenting you a bit we got to the wanted question/answer.