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.