Electrical Sources
Why Are Sources Important?
What Is An Ideal Voltage Source?
What Is An Ideal Current Source?
Using Ideal Sources
What If The Source Isn't Ideal?
Problems
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Why Are Sources Important?

Electrical sources, in practice, include things like:

• batteries,
• the ever-present wall plug,
• power supplies (like those little things you use with your calculator and other things)
• and many other kinds of electrical devices that deliver electrical energy.
They are the starting point for any circuit you design.  You have to have an electrical source in any circuit if it is going to do anything.
Goals For This Lesson

Your goals for this lesson include the following:

Given a circuit with a source,
Be able to model the source as an ideal source, and
Be able to determine when an ideal source model is appropriate.

What Is An Electrical Source?

You probably suspect an electrical source has to be something that gives you a voltage or a current, and you would be correct.  In fact, there are two different kinds of ideal electrical sources - voltage sources and current sources.  In this lesson, we'll look first at voltage sources, but we don't want to overlook current sources.  They will be discussed after ideal voltage sources.

What Is An Ideal Voltage Source?

Looking at voltage sources first, let's consider the concept of an ideal voltage source.  When you buy a battery you buy a nine (9) volt battery, or a 1.5 v battery, or a twelve (12) volt car battery - or some other battery that has a specified voltage.   Clearly those batteries - electrical sources - are assumed to have a certain voltage - whatever it is - that doesn't change much.

When you buy a power supply for your calculator or a telephone answering machine or some other device you need to look at the voltage you need for the power supply.  They usually come in a few specified voltages.  It seems clear that many sources are designed to give you some particular voltage and to attempt to maintain a constant voltage.

Below we'll consider simple models for voltage sources that maintain a constant voltage and we'll take a look at how you can represent that kind of device.  A voltage source that can maintain a constant voltage - no matter what you do to it, like drawing a lot of current, or putting it in a situation where current flows through it - is an ideal voltage source.  That's what we are going to examine in this lesson.  We'll leave it for another lesson to look at sources that are not ideal.  So, let's answer that question above.  What is an ideal voltage source?  And there's another question implied.  Why worry about an ideal voltage source since nothing like that exists in the real world.  We'll take those questions in order.

Ideal Voltage Sources

The concept of an ideal voltage source is pretty simple, and it was really embedded in the previous discussion.

• An ideal voltage source is a voltage source that maintains the same voltage across the source's terminals no matter what current is drawn from the terminals of the source or what current flows into the terminals.
That's it in a nutshell.  If the source is a DC Source, we can plot a voltage current plot for an ideal voltage source.  The plot is shown below.  However, we need to define terms.  Here is a circuit symbol for an ideal voltage source.  In this symbol, we assume the following.
• The voltage across the terminals is denoted as Vt.
• The load current flowing from the source to a load (presumably a load is attached when the source is in a circuit) is denoted as IL.
• With those definitions, here is the source symbol.  It's just a circle with polarity indicated.

And, here is the plot of terminal voltage against load current.

Given the discussion above, we can say:

• Vt = constant, no matter what the load current is.
That's pretty much the description of the ideal voltage source.  It's not too complex, but it is an important concept.  In the next section we'll look at how you can put this concept to use.  For the rest of this section we'll look at ideal current sources starting next.
What Is An Ideal Current Source?

An ideal current source is a simple model for many current sources.  It is reminiscent of the ideal voltage source - but with voltage and current interchanged.  Here is the story.

• There is a special circuit symbol for an ideal current source.  See below.
• IL = constant, no matter what the terminal voltage is.
• The plot of load current against terminal voltage is similar to the plot for an ideal voltage source, but voltage and current are interchanged.  Here is the plot.

Notice that an ideal current source is somewhat similar to an ideal voltage source.  However, when you use an ideal source - usually when doing circuit analysis- there is a significant difference in the analysis.  However, that's getting ahead of the story.  We first have to worry about how you would "use" an ideal source, when we know that there is no such thing as an ideal source, i.e. a source that is "perfect" in some way.

Using Ideal Sources

The idea of using ideal sources is something that you may rebel at.  After all, there is no such thing as an ideal source anywhere in the world.  You can't pull an ideal source off the shelf in the lab, so why are we even talking about them?  The answer to that question is that you use ideal sources when you have a non-ideal (a real source) source in a circuit.  There are two important things to note.

• There are some sources that are very good sources and that can be modelled as ideal sources.  (And when that happens, be grateful.)  Some situations like that include the following.
• A power supply in the lab.  Many times you connect a power supply to some electronic circuit, for example, and when you connect the circuit you find that the output voltage from the power supply doesn't change measurably.  (After all, power supply designers try to make that happen!)  In that case, the power supply might be considered to be an ideal source - at least as long as you are working on that particular circuit.
• There are many sources that do not perform ideally.  However, it has proven to be possible to construct models of real sources, and those models often contain ideal source in combination with other ideal elements (like resistors, etc.).  Thevinin and Norton equivalent circuits are examples of models of real sources that can account for loading effects (i.e. drawing enough current from the source to change the output voltage) and they are widely used in circuit analysis.  You will even find that manufacturers give you parameter values for Thevinin and Norton equivalents on the front panel of many instruments like function generators.
You often have situations in which the sources that you use can be approximated with ideal sources.  Shown below is a bridge circuit powered by a battery.  Often a battery maintains a pretty constant voltage across the terminals, so you may be able to replace the battery with an ideal voltage source when you analyze the circuit.

Here's the circuit with an ideal voltage source substituted for the battery.  At this point, you may know how to do the analysis so you're ready to go.  If you don't know how to do the analysis, you'll get there in these lessons.  (Click here to go to the first lesson on circuit analysis.  However, you may want to spend more time looking at sources, particular sources that are not ideal.)

More On Using Sources - AC Sources

You've used AC sources many times.  Every time you plug anything into a wall plug you are using an AC source.  That source - the wall plug - is designed to be a good voltage source - to maintain the voltage without change - as current is drawn from the plug.  The power company makes a lot of effort to ensure that you have a reliable, known voltage at the wall plug.  You, and all the manufacturers that make things to plug into wall plugs, depend on that voltage being what it is claimed to be.

It's important to understand how that source is used because it is much like the way many other sources are used.  You've plugged many things into wall plugs.  Have you ever considered how those plugs are wired.  The diagram below depicts how wall plugs are wired.  (We haven't used the standard code for colors for house wiring since that involves a wire that is white that wouldn't show up here.)

Two wires carry the voltage to the plug - shown in red and blue above..  The voltage difference between these two wires is approximately:

170 * sin(2p60t).

Notice that the three wall plugs illustrated below are actually wired in parallel so that the same voltage appears across the three plugs.  Because they are in parallel and the voltage is the same for each plug we are actually counting on the source that drives this circuit to be a good voltage source - as close to an ideal voltage source as possible.  Every article that gets plugged into the wall plug is designed to operate on the voltage that appears across the plug, so it is reasonable to expect the same voltage at every plug.

Note that we are not claiming that the voltage is always the same at every time in the situation above.  Rather, for any given time, the voltage across all of the plugs is what we want it to be for that particular time.  As time goes on, for the wall plug, the voltage we want to have varies sinusoidally.  (Click here if you want to examine some information about sinusoidal signals.)

Whenever you wire things in the lab you'll find the same kind of thing happening.  Here's an illustration of using a +5 volt supply to power three integrated circuit logic chips.  Each chip needs five (5) volts.  Each chip draws current.  The power supply should be designed to provide 5 volts (the red lead(s)) no matter how much current is drawn up to some limit.  It's designed to be as close as possible to an ideal voltage source as it can be.

Conclusions

There are some conclusions to be drawn here.  Many devices - from large appliances to IC chips - are designed to operate at specific voltages.  The sources the supply power to these devices therefore have to be designed to supply constant voltages so they will be as close to ideal voltage sources as the designer can get them.  The flip side of all this is that when you want to use these devices you need to ensure that you apply the correct voltage.  You do that by connecting the devices in parallel - and not in series.

There are undoubtedly other lessons about sources that you will learn in your electrical engineering career (and some may be bitter and hard-learned indeed).

What If The Source Isn't Ideal?

That's a tough problem.  You know that there are sources that aren't ideal.  In the lab voltages change when you attach components.  In your car, the lights dim when you start the car with the lights on.  These are examples of non-ideal sources in the real world.

To deal with non-ideal sources you can use Thevenin Equivalent Circuits or Norton Equivalent Circuits.  In either case you will find that a component of the equivalent circuit is an ideal source - even though the equivalent is used to represent a non-ideal source.  Go to that lesson to find our how that all comes about.

Problems
Links to Other Lessons on Sources