L3 5 1Thevenin Introduction

Introduces Thevenin Equivalent Circuit technique

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

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

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

An independent voltage source.

And a series resistance.

So, for example,

here we have the schematic of
an LM324 Operational Amplifier.

As you can see it's
relatively complicated.

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

some resistors would be
buried inside there also.

But, generally speaking,

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

The amplifier.

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

B terminals, the output voltage.

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

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

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

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

And that a load connected
between terminals a and

b here We'll experience
the same voltage and

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

B terminals of the amplifier
would experience.

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

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

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

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

goes through the drive shaft you
come to the differential and

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

wheels you have losses typically
due to friction and vibration.

All along the drive train.

Such that,

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

With a thevenin equivalent circuit,

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

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

All we care about is what are,
how much power

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

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

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

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

we're all very familiar with.

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

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

It may be minimal and negligible.

And example of one that is not minimal and
negligible,

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

Now in a car battery if you
have nothing connected,

you put your volt meter
across the terminal.

So we call that the open circuit voltage.

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

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

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

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

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

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

The lights dim, don't they?

They dim because as the battery

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

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

We see a voltage drop.

At the terminals.

That voltage drop is modeled
by the series resistance

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

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

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

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

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

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

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

consisting of a Thevenin voltage,
a voltage

supply, and a series resistance.

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

D Thevenin and
R Thevenin V seven is nothing more than

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

We refer to that as the open
circuit voltage and

thus V seven is simply
the open circuit voltage.

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

conceptually Can be determined by shorting

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

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

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

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

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

Thus, R thevenin.

is equal to V7 divided
by I short circuit or

the open circuit voltage divided
by the short circuit current.

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

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

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

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