0:00:01.290,0:00:05.060 Let's now introduce the concept of[br]Thevenin and Norton Equivalent Circuits. 0:00:08.710,0:00:12.770 The idea here is that you can take[br]a complicated circuit, or complex circuit, 0:00:12.770,0:00:16.650 and reduce it to a model that[br]consists of only a voltage source. 0:00:16.650,0:00:18.600 An independent voltage source. 0:00:18.600,0:00:20.780 And a series resistance. 0:00:20.780,0:00:21.540 So, for example, 0:00:21.540,0:00:25.520 here we have the schematic of[br]an LM324 Operational Amplifier. 0:00:25.520,0:00:27.410 As you can see it's[br]relatively complicated. 0:00:27.410,0:00:30.750 It's got a number of transistors and[br]some capacitors, diodes and 0:00:30.750,0:00:33.800 some resistors would be[br]buried inside there also. 0:00:33.800,0:00:34.650 But, generally speaking, 0:00:34.650,0:00:40.000 when we're using OP Amps we're not[br]really concerned about what's inside. 0:00:40.000,0:00:41.480 The amplifier. 0:00:41.480,0:00:45.380 We simply want to know what's[br]going on between the A and 0:00:45.380,0:00:48.110 B terminals, the output voltage. 0:00:48.110,0:00:53.820 And what happens to that output voltage as[br]we then connect some sort of a load to it? 0:00:53.820,0:00:58.620 As we use that amplifier to To[br]perform some desirable function. 0:00:58.620,0:01:03.080 So the idea here is that we can[br]reduce this complex circuit 0:01:03.080,0:01:07.180 down to a single voltage source with[br]a single resistance in series within it. 0:01:07.180,0:01:12.150 And that a load connected[br]between terminals a and 0:01:12.150,0:01:15.390 b here We'll experience[br]the same voltage and 0:01:15.390,0:01:19.470 current relationships that that same[br]load connected to between the A and 0:01:19.470,0:01:21.800 B terminals of the amplifier[br]would experience. 0:01:21.800,0:01:27.790 It's something like the power[br]train in an automobile. 0:01:27.790,0:01:31.000 The automobile has an engine,[br]and it powered the engine, 0:01:31.000,0:01:37.060 it may be a 300 horsepower Engine at[br]the shaft but you don't experience 0:01:37.060,0:01:41.380 300 horsepower at the wheels because when[br]it goes through the transmission and 0:01:41.380,0:01:43.970 goes through the drive shaft you[br]come to the differential and 0:01:43.970,0:01:49.090 then out the rear axle to the wheel[br]bearings before you get to the actual 0:01:49.090,0:01:54.470 wheels you have losses typically[br]due to friction and vibration. 0:01:54.470,0:01:56.900 All along the drive train. 0:01:56.900,0:01:58.080 Such that, 0:01:58.080,0:02:01.730 the power at the wheels is different[br]than the power at the engine itself. 0:02:05.760,0:02:07.310 With a thevenin equivalent circuit, 0:02:07.310,0:02:10.020 we really don't care about what's[br]happening with the transmission. 0:02:10.020,0:02:13.830 we really don't even care about[br]how big the engine is inside. 0:02:13.830,0:02:18.750 All we care about is what are,[br]how much power 0:02:18.750,0:02:24.550 can we get at the wheels,[br]or in electrical terms, 0:02:24.550,0:02:30.570 what is the voltage, and as we start[br]requiring the circuit to drive a load, 0:02:30.570,0:02:35.710 how Is that load going to affect[br]the voltage at the terminals. 0:02:37.530,0:02:41.090 Why would it or how do we know that it[br]does, let's just take an example and 0:02:41.090,0:02:42.250 we're all very familiar with. 0:02:43.660,0:02:48.330 Any source as you start to draw current[br]from it as you connect the load, 0:02:48.330,0:02:53.060 any source We'll see a reduction[br]in the terminal voltage. 0:02:53.060,0:02:55.180 It may be minimal and negligible. 0:02:55.180,0:02:57.940 And example of one that is not minimal and[br]negligible, 0:02:57.940,0:03:02.300 the one that we can relate[br]to is a car battery. 0:03:03.330,0:03:05.110 Now in a car battery if you[br]have nothing connected, 0:03:05.110,0:03:07.760 you put your volt meter[br]across the terminal. 0:03:07.760,0:03:10.010 So we call that the open circuit voltage. 0:03:10.010,0:03:15.098 We measure the open circuit voltage You'll[br]measure something around 14.4 volts, 0:03:15.098,0:03:19.500 you'll turn on the lights and[br]you'll get a certain amount of light out. 0:03:20.530,0:03:23.460 The lights will burn at a certain[br]brightness and if you were 0:03:23.460,0:03:28.190 to measure the voltage you might detect[br]a relatively small voltage drop there 0:03:29.880,0:03:35.090 but with the lights on,[br]if you then Connect the starter motor, 0:03:35.090,0:03:38.920 engage the starter motor by putting on[br]the key, what happens to the lights? 0:03:38.920,0:03:40.870 The lights dim, don't they? 0:03:40.870,0:03:43.770 They dim because as the battery 0:03:43.770,0:03:46.950 is required to produce enough current[br]to drive not only the lights, but 0:03:46.950,0:03:52.510 also the starter motor,[br]which draws a large amount of current. 0:03:52.510,0:03:54.460 We see a voltage drop. 0:03:54.460,0:03:55.850 At the terminals. 0:03:57.080,0:04:01.970 That voltage drop is modeled[br]by the series resistance 0:04:01.970,0:04:06.510 that shows a voltage drop across that, as[br]current starts to flow from the battery. 0:04:09.610,0:04:16.560 So just in general or to summarize then[br]We're gonna have some actual circuit. 0:04:16.560,0:04:19.649 More complicated, less complicated,[br]we don't really care. 0:04:19.649,0:04:21.130 We don't care what's[br]going on inside there. 0:04:21.130,0:04:25.160 We simply want to know,[br]what are its terminal characteristics? 0:04:25.160,0:04:27.900 What happens if I connect[br]some load between A and B? 0:04:29.050,0:04:33.160 We're saying that we can model this[br]complex circuit With a simple circuit 0:04:33.160,0:04:38.112 consisting of a Thevenin voltage,[br]a voltage 0:04:38.112,0:04:42.130 supply, and a series resistance. 0:04:44.420,0:04:48.730 To create this model then, we need to[br]determine the values of the two components 0:04:48.730,0:04:54.300 D Thevenin and[br]R Thevenin V seven is nothing more than 0:04:54.300,0:05:00.180 the voltage you measure across the[br]terminals with no load connected to it. 0:05:00.180,0:05:02.330 We refer to that as the open[br]circuit voltage and 0:05:02.330,0:05:07.120 thus V seven is simply[br]the open circuit voltage. 0:05:08.280,0:05:11.910 Now to measure And we're gonna learn a[br]number of different ways of doing this but 0:05:11.910,0:05:16.740 conceptually Can be determined by shorting 0:05:18.140,0:05:23.220 the terminals A and D and[br]measuring the current that then flows 0:05:24.600,0:05:29.440 we'll refer to that current as the short[br]circuit current I short circuit 0:05:31.250,0:05:36.570 And we'll notice that I short[br]circuit is going to equal 0:05:36.570,0:05:40.620 the voltage drop V thevenin or the voltage[br]that is dropped across the resistance. 0:05:40.620,0:05:46.150 Or, I short circuit is going to equal[br]V thevenin divided by R thevenin. 0:05:47.190,0:05:49.030 Thus, R thevenin. 0:05:50.320,0:05:55.880 is equal to V7 divided[br]by I short circuit or 0:05:55.880,0:06:00.370 the open circuit voltage divided[br]by the short circuit current. 0:06:00.370,0:06:05.425 And in fact, we're going to use that in[br]the claim, that that is the definition of 0:06:05.425,0:06:10.580 [INAUDIBLE] So our task then, as we now[br]start looking at different circuits, and 0:06:10.580,0:06:15.350 determining their vth equivalent circuits,[br]our task is going to be to determine what 0:06:15.350,0:06:21.400 the vth voltage is, or the open circuit[br]voltage, and what the vth resistance is.