[Script Info]
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Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:01.29,0:00:05.06,Default,,0000,0000,0000,,Let's now introduce the concept of\NThevenin and Norton Equivalent Circuits.
Dialogue: 0,0:00:08.71,0:00:12.77,Default,,0000,0000,0000,,The idea here is that you can take\Na complicated circuit, or complex circuit,
Dialogue: 0,0:00:12.77,0:00:16.65,Default,,0000,0000,0000,,and reduce it to a model that\Nconsists of only a voltage source.
Dialogue: 0,0:00:16.65,0:00:18.60,Default,,0000,0000,0000,,An independent voltage source.
Dialogue: 0,0:00:18.60,0:00:20.78,Default,,0000,0000,0000,,And a series resistance.
Dialogue: 0,0:00:20.78,0:00:21.54,Default,,0000,0000,0000,,So, for example,
Dialogue: 0,0:00:21.54,0:00:25.52,Default,,0000,0000,0000,,here we have the schematic of\Nan LM324 Operational Amplifier.
Dialogue: 0,0:00:25.52,0:00:27.41,Default,,0000,0000,0000,,As you can see it's\Nrelatively complicated.
Dialogue: 0,0:00:27.41,0:00:30.75,Default,,0000,0000,0000,,It's got a number of transistors and\Nsome capacitors, diodes and
Dialogue: 0,0:00:30.75,0:00:33.80,Default,,0000,0000,0000,,some resistors would be\Nburied inside there also.
Dialogue: 0,0:00:33.80,0:00:34.65,Default,,0000,0000,0000,,But, generally speaking,
Dialogue: 0,0:00:34.65,0:00:40.00,Default,,0000,0000,0000,,when we're using OP Amps we're not\Nreally concerned about what's inside.
Dialogue: 0,0:00:40.00,0:00:41.48,Default,,0000,0000,0000,,The amplifier.
Dialogue: 0,0:00:41.48,0:00:45.38,Default,,0000,0000,0000,,We simply want to know what's\Ngoing on between the A and
Dialogue: 0,0:00:45.38,0:00:48.11,Default,,0000,0000,0000,,B terminals, the output voltage.
Dialogue: 0,0:00:48.11,0:00:53.82,Default,,0000,0000,0000,,And what happens to that output voltage as\Nwe then connect some sort of a load to it?
Dialogue: 0,0:00:53.82,0:00:58.62,Default,,0000,0000,0000,,As we use that amplifier to To\Nperform some desirable function.
Dialogue: 0,0:00:58.62,0:01:03.08,Default,,0000,0000,0000,,So the idea here is that we can\Nreduce this complex circuit
Dialogue: 0,0:01:03.08,0:01:07.18,Default,,0000,0000,0000,,down to a single voltage source with\Na single resistance in series within it.
Dialogue: 0,0:01:07.18,0:01:12.15,Default,,0000,0000,0000,,And that a load connected\Nbetween terminals a and
Dialogue: 0,0:01:12.15,0:01:15.39,Default,,0000,0000,0000,,b here We'll experience\Nthe same voltage and
Dialogue: 0,0:01:15.39,0:01:19.47,Default,,0000,0000,0000,,current relationships that that same\Nload connected to between the A and
Dialogue: 0,0:01:19.47,0:01:21.80,Default,,0000,0000,0000,,B terminals of the amplifier\Nwould experience.
Dialogue: 0,0:01:21.80,0:01:27.79,Default,,0000,0000,0000,,It's something like the power\Ntrain in an automobile.
Dialogue: 0,0:01:27.79,0:01:31.00,Default,,0000,0000,0000,,The automobile has an engine,\Nand it powered the engine,
Dialogue: 0,0:01:31.00,0:01:37.06,Default,,0000,0000,0000,,it may be a 300 horsepower Engine at\Nthe shaft but you don't experience
Dialogue: 0,0:01:37.06,0:01:41.38,Default,,0000,0000,0000,,300 horsepower at the wheels because when\Nit goes through the transmission and
Dialogue: 0,0:01:41.38,0:01:43.97,Default,,0000,0000,0000,,goes through the drive shaft you\Ncome to the differential and
Dialogue: 0,0:01:43.97,0:01:49.09,Default,,0000,0000,0000,,then out the rear axle to the wheel\Nbearings before you get to the actual
Dialogue: 0,0:01:49.09,0:01:54.47,Default,,0000,0000,0000,,wheels you have losses typically\Ndue to friction and vibration.
Dialogue: 0,0:01:54.47,0:01:56.90,Default,,0000,0000,0000,,All along the drive train.
Dialogue: 0,0:01:56.90,0:01:58.08,Default,,0000,0000,0000,,Such that,
Dialogue: 0,0:01:58.08,0:02:01.73,Default,,0000,0000,0000,,the power at the wheels is different\Nthan the power at the engine itself.
Dialogue: 0,0:02:05.76,0:02:07.31,Default,,0000,0000,0000,,With a thevenin equivalent circuit,
Dialogue: 0,0:02:07.31,0:02:10.02,Default,,0000,0000,0000,,we really don't care about what's\Nhappening with the transmission.
Dialogue: 0,0:02:10.02,0:02:13.83,Default,,0000,0000,0000,,we really don't even care about\Nhow big the engine is inside.
Dialogue: 0,0:02:13.83,0:02:18.75,Default,,0000,0000,0000,,All we care about is what are,\Nhow much power
Dialogue: 0,0:02:18.75,0:02:24.55,Default,,0000,0000,0000,,can we get at the wheels,\Nor in electrical terms,
Dialogue: 0,0:02:24.55,0:02:30.57,Default,,0000,0000,0000,,what is the voltage, and as we start\Nrequiring the circuit to drive a load,
Dialogue: 0,0:02:30.57,0:02:35.71,Default,,0000,0000,0000,,how Is that load going to affect\Nthe voltage at the terminals.
Dialogue: 0,0:02:37.53,0:02:41.09,Default,,0000,0000,0000,,Why would it or how do we know that it\Ndoes, let's just take an example and
Dialogue: 0,0:02:41.09,0:02:42.25,Default,,0000,0000,0000,,we're all very familiar with.
Dialogue: 0,0:02:43.66,0:02:48.33,Default,,0000,0000,0000,,Any source as you start to draw current\Nfrom it as you connect the load,
Dialogue: 0,0:02:48.33,0:02:53.06,Default,,0000,0000,0000,,any source We'll see a reduction\Nin the terminal voltage.
Dialogue: 0,0:02:53.06,0:02:55.18,Default,,0000,0000,0000,,It may be minimal and negligible.
Dialogue: 0,0:02:55.18,0:02:57.94,Default,,0000,0000,0000,,And example of one that is not minimal and\Nnegligible,
Dialogue: 0,0:02:57.94,0:03:02.30,Default,,0000,0000,0000,,the one that we can relate\Nto is a car battery.
Dialogue: 0,0:03:03.33,0:03:05.11,Default,,0000,0000,0000,,Now in a car battery if you\Nhave nothing connected,
Dialogue: 0,0:03:05.11,0:03:07.76,Default,,0000,0000,0000,,you put your volt meter\Nacross the terminal.
Dialogue: 0,0:03:07.76,0:03:10.01,Default,,0000,0000,0000,,So we call that the open circuit voltage.
Dialogue: 0,0:03:10.01,0:03:15.10,Default,,0000,0000,0000,,We measure the open circuit voltage You'll\Nmeasure something around 14.4 volts,
Dialogue: 0,0:03:15.10,0:03:19.50,Default,,0000,0000,0000,,you'll turn on the lights and\Nyou'll get a certain amount of light out.
Dialogue: 0,0:03:20.53,0:03:23.46,Default,,0000,0000,0000,,The lights will burn at a certain\Nbrightness and if you were
Dialogue: 0,0:03:23.46,0:03:28.19,Default,,0000,0000,0000,,to measure the voltage you might detect\Na relatively small voltage drop there
Dialogue: 0,0:03:29.88,0:03:35.09,Default,,0000,0000,0000,,but with the lights on,\Nif you then Connect the starter motor,
Dialogue: 0,0:03:35.09,0:03:38.92,Default,,0000,0000,0000,,engage the starter motor by putting on\Nthe key, what happens to the lights?
Dialogue: 0,0:03:38.92,0:03:40.87,Default,,0000,0000,0000,,The lights dim, don't they?
Dialogue: 0,0:03:40.87,0:03:43.77,Default,,0000,0000,0000,,They dim because as the battery
Dialogue: 0,0:03:43.77,0:03:46.95,Default,,0000,0000,0000,,is required to produce enough current\Nto drive not only the lights, but
Dialogue: 0,0:03:46.95,0:03:52.51,Default,,0000,0000,0000,,also the starter motor,\Nwhich draws a large amount of current.
Dialogue: 0,0:03:52.51,0:03:54.46,Default,,0000,0000,0000,,We see a voltage drop.
Dialogue: 0,0:03:54.46,0:03:55.85,Default,,0000,0000,0000,,At the terminals.
Dialogue: 0,0:03:57.08,0:04:01.97,Default,,0000,0000,0000,,That voltage drop is modeled\Nby the series resistance
Dialogue: 0,0:04:01.97,0:04:06.51,Default,,0000,0000,0000,,that shows a voltage drop across that, as\Ncurrent starts to flow from the battery.
Dialogue: 0,0:04:09.61,0:04:16.56,Default,,0000,0000,0000,,So just in general or to summarize then\NWe're gonna have some actual circuit.
Dialogue: 0,0:04:16.56,0:04:19.65,Default,,0000,0000,0000,,More complicated, less complicated,\Nwe don't really care.
Dialogue: 0,0:04:19.65,0:04:21.13,Default,,0000,0000,0000,,We don't care what's\Ngoing on inside there.
Dialogue: 0,0:04:21.13,0:04:25.16,Default,,0000,0000,0000,,We simply want to know,\Nwhat are its terminal characteristics?
Dialogue: 0,0:04:25.16,0:04:27.90,Default,,0000,0000,0000,,What happens if I connect\Nsome load between A and B?
Dialogue: 0,0:04:29.05,0:04:33.16,Default,,0000,0000,0000,,We're saying that we can model this\Ncomplex circuit With a simple circuit
Dialogue: 0,0:04:33.16,0:04:38.11,Default,,0000,0000,0000,,consisting of a Thevenin voltage,\Na voltage
Dialogue: 0,0:04:38.11,0:04:42.13,Default,,0000,0000,0000,,supply, and a series resistance.
Dialogue: 0,0:04:44.42,0:04:48.73,Default,,0000,0000,0000,,To create this model then, we need to\Ndetermine the values of the two components
Dialogue: 0,0:04:48.73,0:04:54.30,Default,,0000,0000,0000,,D Thevenin and\NR Thevenin V seven is nothing more than
Dialogue: 0,0:04:54.30,0:05:00.18,Default,,0000,0000,0000,,the voltage you measure across the\Nterminals with no load connected to it.
Dialogue: 0,0:05:00.18,0:05:02.33,Default,,0000,0000,0000,,We refer to that as the open\Ncircuit voltage and
Dialogue: 0,0:05:02.33,0:05:07.12,Default,,0000,0000,0000,,thus V seven is simply\Nthe open circuit voltage.
Dialogue: 0,0:05:08.28,0:05:11.91,Default,,0000,0000,0000,,Now to measure And we're gonna learn a\Nnumber of different ways of doing this but
Dialogue: 0,0:05:11.91,0:05:16.74,Default,,0000,0000,0000,,conceptually Can be determined by shorting
Dialogue: 0,0:05:18.14,0:05:23.22,Default,,0000,0000,0000,,the terminals A and D and\Nmeasuring the current that then flows
Dialogue: 0,0:05:24.60,0:05:29.44,Default,,0000,0000,0000,,we'll refer to that current as the short\Ncircuit current I short circuit
Dialogue: 0,0:05:31.25,0:05:36.57,Default,,0000,0000,0000,,And we'll notice that I short\Ncircuit is going to equal
Dialogue: 0,0:05:36.57,0:05:40.62,Default,,0000,0000,0000,,the voltage drop V thevenin or the voltage\Nthat is dropped across the resistance.
Dialogue: 0,0:05:40.62,0:05:46.15,Default,,0000,0000,0000,,Or, I short circuit is going to equal\NV thevenin divided by R thevenin.
Dialogue: 0,0:05:47.19,0:05:49.03,Default,,0000,0000,0000,,Thus, R thevenin.
Dialogue: 0,0:05:50.32,0:05:55.88,Default,,0000,0000,0000,,is equal to V7 divided\Nby I short circuit or
Dialogue: 0,0:05:55.88,0:06:00.37,Default,,0000,0000,0000,,the open circuit voltage divided\Nby the short circuit current.
Dialogue: 0,0:06:00.37,0:06:05.42,Default,,0000,0000,0000,,And in fact, we're going to use that in\Nthe claim, that that is the definition of
Dialogue: 0,0:06:05.42,0:06:10.58,Default,,0000,0000,0000,,[INAUDIBLE] So our task then, as we now\Nstart looking at different circuits, and
Dialogue: 0,0:06:10.58,0:06:15.35,Default,,0000,0000,0000,,determining their vth equivalent circuits,\Nour task is going to be to determine what
Dialogue: 0,0:06:15.35,0:06:21.40,Default,,0000,0000,0000,,the vth voltage is, or the open circuit\Nvoltage, and what the vth resistance is.