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Course: Electrical engineering>Unit 2

Lesson 1: Circuit elements

Ideal sources

Introduction to the voltage source and current source. Created by Willy McAllister.

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• Hello.
What software did you use to do the videos?
Thanks
• I use Sketchbook (Autodesk, Pro version) for drawing. The software to capture the screen plus my voice is called Camtasia. My tablet is a Wacom Bamboo, but you can use anything. My microphone is a Samson C01U USB. A few videos have animations, which I created with D3js. My computer is a Macbook Pro.
• How can current be constant it is changing everywhere due to resistance
• IMHO it can. :)

(my personal) Definitions

Voltage sources : Deliver fixed voltage regardless of current needed.

Current sources : Deliver fixed current regardless of voltage needed.

:)

Resistance change? No problem for the sources, their power delivery systems (power control circuit/regulator) ensures that it will works just fine. Example : electrical lab power power supply (http://my.rs-online.com/web/c/test-measurement/bench-power-and-power-measurement/bench-power-supplies/).

Since P = VI

A voltage source will supply the needed power by varying the supply current, and fixing the supplied voltage, depending on the total resistance encountered. A current source will do the same by varying the supply voltage, and fixing the supplied current, depending on the total resistance encountered.

hope that helps.
• According to the plot of the current and voltage it appears they can have negative values? What does it mean to have negative voltage or current?
(1 vote)
• Voltage is a relative measurement, just like measuring elevation is a relative measurement. When you measure elevations the habit is to say sea level = 0 elevation. If you go up a mountain you get positive elevation. If you take a submarine under the ocean you have negative elevation (also called "depth"). Voltage works the same way.

Current can be positive or negative. If you look at the symbol for the ideal current source you see it has an arrow. That defines which way positive current flows. If you assign a value of I = -2mA to the ideal current source, it means 2mA flows in the opposite direction of the arrow.
• I have a question.

At about , you start talking about graphing the lines of the ideal sources. You mention that the voltage can fluctuate even though the current stays constant, and the current can fluctuate but the voltage stays constant. How is this possible?
In my understanding (which is unfortunately limited), the two are linked. The greater the voltage, the greater the current, because there is more charge pushing and therefore the charge moves faster. This must be wrong if one can change while the other stays constant. Did I misunderstand the definition of voltage?
(1 vote)
• You are mixing together the concept of an ideal source (v or i) with the concept of a resistor. These are two different animals.

A resistor obeys Ohm's Law, v = i R. If R is constant, then v and i scale up and down together.

An ideal source is a mathematical model of a thing that holds either voltage or current constant. It is not a resistor, so Ohm's Law does not have to be true.

A battery is an example of a real voltage source. Over a reasonable range of currents, the voltage of a battery does not change. A small AA battery producing 1.5v will hold that voltage constant for currents between 0 and say 20ma. At higher currents the voltage may droop down a little, but not too much. The ideal mathematical model for a voltage source does a pretty good job describing a battery, as long as you don't try to draw a giant current out of it.
• What applications would be needed for either constant voltage or current source?
• Hello Dreamer,

A lithium ion battery charger is a good example that contains both a Constant Current (CC) and a Constant Voltage (CV) source. See a data sheet here:

When charging begins the current is initially limited to a safe value via a circuit operating as a CC source. For this particular cell look for a CC of 1375 mA in the data sheet. Later in the charging cycle the voltage will reach 4.2 volts. When this happens the charger will switch to CV mode and hold at 4.2 volts until the current drops below 100 mA. You can see these curves in the "charge characteristics" section of the data sheet.

For another answer you may want to look here:

https://www.passdiy.com/project

In particular, take a look at the DIY op-amps:

https://www.passdiy.com/project/amplifiers/diy-op-amps

Please don't get discouraged. It takes years of study to understand all of the implication on this website.

Perhaps you could build one of these amplifiers. The ZEN would be an interesting amplifier to construct as some versions are built using a CC source.

Happy soldering!

APD

P.S. While we are talking about audio you may want to look at these links:

http://sound.whsites.net/index2.html

• what does the term constant means in the circuit ?
(1 vote)
• The word "constant" means "never changing, always the same". A double-A battery puts out a constant voltage of 1.5V.
• Do current sources produce an emf like voltage sources? Do they produce electric field in a wire? Or does the usual electromagnetism no longer apply here?
(1 vote)
• current sources produce an emf?
> All charge interaction (generation/flow/change) WILL produce emf.

produce electric field in a wire? >
> if there is a current flow in the wire, there IS an electric field, regardless of source type.

the usual electromagnetism no longer apply here?
> electromagnetism applies everywhere, here is included.

p/s : just because it was not mentioned, doesn't mean it is not there. For the sake of syllabus, simplicity is preferred. When we deal will the real world, we cannot ignore earth gravity in our experiment/machine due to its magnitude. But this doesn't mean that there is no gravity effects from the sun and moon. It is there, just a question of scale/applicability/device accuracy/sensitivity.
Eg. LHC + CERN team have to calculate earth rotation speed in their experiments. But in our school lab, assuming earth is static seems applicable for the scale we work at. Good to know though. :)