Why Are Sources Important?
Electrical sources, in practice, include things like:
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batteries,
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the ever-present wall plug,
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power supplies (like those little
things you use with your calculator and other things)
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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.
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.
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