WEBVTT

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Let's now introduce the concept of
Thevenin and Norton Equivalent Circuits.

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The idea here is that you can take
a complicated circuit, or complex circuit,

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and reduce it to a model that
consists of only a voltage source.

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An independent voltage source.

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And a series resistance.

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So, for example,

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here we have the schematic of
an LM324 Operational Amplifier.

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As you can see it's
relatively complicated.

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It's got a number of transistors and
some capacitors, diodes and

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some resistors would be
buried inside there also.

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But, generally speaking,

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when we're using OP Amps we're not
really concerned about what's inside.

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The amplifier.

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We simply want to know what's
going on between the A and

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B terminals, the output voltage.

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And what happens to that output voltage as
we then connect some sort of a load to it?

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As we use that amplifier to To
perform some desirable function.

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So the idea here is that we can
reduce this complex circuit

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down to a single voltage source with
a single resistance in series within it.

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And that a load connected
between terminals a and

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b here We'll experience
the same voltage and

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current relationships that that same
load connected to between the A and

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B terminals of the amplifier
would experience.

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It's something like the power
train in an automobile.

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The automobile has an engine,
and it powered the engine,

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it may be a 300 horsepower Engine at
the shaft but you don't experience

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300 horsepower at the wheels because when
it goes through the transmission and

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goes through the drive shaft you
come to the differential and

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then out the rear axle to the wheel
bearings before you get to the actual

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wheels you have losses typically
due to friction and vibration.

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All along the drive train.

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Such that,

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the power at the wheels is different
than the power at the engine itself.

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With a thevenin equivalent circuit,

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we really don't care about what's
happening with the transmission.

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we really don't even care about
how big the engine is inside.

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All we care about is what are,
how much power

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can we get at the wheels,
or in electrical terms,

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what is the voltage, and as we start
requiring the circuit to drive a load,

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how Is that load going to affect
the voltage at the terminals.

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Why would it or how do we know that it
does, let's just take an example and

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we're all very familiar with.

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Any source as you start to draw current
from it as you connect the load,

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any source We'll see a reduction
in the terminal voltage.

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It may be minimal and negligible.

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And example of one that is not minimal and
negligible,

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the one that we can relate
to is a car battery.

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Now in a car battery if you
have nothing connected,

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you put your volt meter
across the terminal.

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So we call that the open circuit voltage.

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We measure the open circuit voltage You'll
measure something around 14.4 volts,

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you'll turn on the lights and
you'll get a certain amount of light out.

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The lights will burn at a certain
brightness and if you were

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to measure the voltage you might detect
a relatively small voltage drop there

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but with the lights on,
if you then Connect the starter motor,

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engage the starter motor by putting on
the key, what happens to the lights?

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The lights dim, don't they?

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They dim because as the battery

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is required to produce enough current
to drive not only the lights, but

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also the starter motor,
which draws a large amount of current.

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We see a voltage drop.

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At the terminals.

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That voltage drop is modeled
by the series resistance

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that shows a voltage drop across that, as
current starts to flow from the battery.

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So just in general or to summarize then
We're gonna have some actual circuit.

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More complicated, less complicated,
we don't really care.

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We don't care what's
going on inside there.

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We simply want to know,
what are its terminal characteristics?

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What happens if I connect
some load between A and B?

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We're saying that we can model this
complex circuit With a simple circuit

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consisting of a Thevenin voltage,
a voltage

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supply, and a series resistance.

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To create this model then, we need to
determine the values of the two components

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D Thevenin and
R Thevenin V seven is nothing more than

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the voltage you measure across the
terminals with no load connected to it.

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We refer to that as the open
circuit voltage and

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thus V seven is simply
the open circuit voltage.

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Now to measure And we're gonna learn a
number of different ways of doing this but

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conceptually Can be determined by shorting

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the terminals A and D and
measuring the current that then flows

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we'll refer to that current as the short
circuit current I short circuit

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And we'll notice that I short
circuit is going to equal

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the voltage drop V thevenin or the voltage
that is dropped across the resistance.

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Or, I short circuit is going to equal
V thevenin divided by R thevenin.

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Thus, R thevenin.

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is equal to V7 divided
by I short circuit or

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the open circuit voltage divided
by the short circuit current.

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And in fact, we're going to use that in
the claim, that that is the definition of

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[INAUDIBLE] So our task then, as we now
start looking at different circuits, and

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determining their vth equivalent circuits,
our task is going to be to determine what

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the vth voltage is, or the open circuit
voltage, and what the vth resistance is.