1 00:00:02,500 --> 00:00:07,835 Hello, this is Doctor Ferri. This particular lesson is a lab demo on 2 00:00:07,835 --> 00:00:14,040 RC circuits. And just to set it up, I want to explain 3 00:00:14,040 --> 00:00:16,490 what we're looking at. We're going to be looking at an RC 4 00:00:16,490 --> 00:00:19,140 circuit. We'd like to be able to look at the 5 00:00:19,140 --> 00:00:22,880 transient response. In order to do that, and measure it, we 6 00:00:22,880 --> 00:00:25,220 have to keep generating a transient response. 7 00:00:25,220 --> 00:00:27,730 It's hard to catch it if we only see it once. 8 00:00:27,730 --> 00:00:32,130 So, we're going to keep generating and triggering a transient response. 9 00:00:32,130 --> 00:00:37,730 And the way we're going to do it, if you look on the left, it's as if I have two 10 00:00:37,730 --> 00:00:41,020 circuit sources. One with the polarity this way and the 11 00:00:41,020 --> 00:00:46,150 other with the polarity that way. So, if I'm looking at it, when it's, when 12 00:00:46,150 --> 00:00:50,556 the switch is connected this way, I will have one voltage source. 13 00:00:50,556 --> 00:00:56,660 And when the switch is connected this way, I will have the opposite voltage 14 00:00:56,660 --> 00:01:01,910 source. And in order to generate that in a 15 00:01:01,910 --> 00:01:06,840 physical system, I can use a function generator to give me a square wave. 16 00:01:06,840 --> 00:01:13,470 So looking at the left, a square wave is equivalent to, just switching back and 17 00:01:13,470 --> 00:01:19,150 forth. And then, you generate the square wave. 18 00:01:19,150 --> 00:01:22,610 So, the function generator is going to generate a square wave, and then we'll be 19 00:01:22,610 --> 00:01:25,000 looking at the voltage across the capacitor. 20 00:01:28,110 --> 00:01:36,510 The lab demo is on RC Circuits. This experiment is to demonstrate the use 21 00:01:36,510 --> 00:01:39,900 of some very common instruments used in electrical engineering. 22 00:01:39,900 --> 00:01:44,470 And to use those instruments to measure the characteristics of an RC circuit. 23 00:01:47,159 --> 00:01:50,630 If we have a circuit and we want to measure voltages, we have to be able to 24 00:01:50,630 --> 00:01:53,710 input that into our, our measurement device. 25 00:01:53,710 --> 00:01:59,370 And in this case, we'll be using an, a national instruments myDAQ, which is the 26 00:01:59,370 --> 00:02:03,710 data aquisition board. These wires allow us to output voltage 27 00:02:03,710 --> 00:02:07,710 from the myDAQ or circuit. So, these two are going to be output 28 00:02:07,710 --> 00:02:11,689 voltages. And it also allows us, if you look at the 29 00:02:11,689 --> 00:02:14,110 side of this, and that leaves different channels. 30 00:02:14,110 --> 00:02:16,120 So, these are my output voltages to my circuit. 31 00:02:16,120 --> 00:02:21,400 Like it acts like a source, and these are two input voltage channels. 32 00:02:21,400 --> 00:02:25,230 And those allow me to measure two different wave forms, voltage wave forms. 33 00:02:31,510 --> 00:02:38,470 So, for example, let me set this up so that, I want to output. 34 00:02:38,470 --> 00:02:43,814 So, that's my source voltage. I'm outputting a voltage, and then I want 35 00:02:43,814 --> 00:02:46,098 to measure it. This is channel zero. 36 00:02:46,098 --> 00:02:52,186 I want to measure the exact same thing that I'm outputting. 37 00:02:52,186 --> 00:02:58,080 So, if I go to my computer, I'm running software on the computer. 38 00:02:58,080 --> 00:03:03,400 And the software allows me to take those measured signals and to display them on 39 00:03:03,400 --> 00:03:06,830 what's known as an oscilloscope. And I'm also using a function generator. 40 00:03:06,830 --> 00:03:13,160 A function generator generates a signal. I can output a, a sine wave, a triangular 41 00:03:13,160 --> 00:03:17,476 wave, a square wave, and the oscilloscope allows me to measure it. 42 00:03:17,476 --> 00:03:24,956 Since I've got the circuit set up, let me go ahead and run these. 43 00:03:24,956 --> 00:03:26,890 So, this is outputting a 100 Hertz sine wave. 44 00:03:26,890 --> 00:03:32,650 That's clicked, that's a sine wave. Let me change this to 2 volts peak to 45 00:03:32,650 --> 00:03:39,910 peak. And then if I run the oscilloscope, let 46 00:03:39,910 --> 00:03:43,720 me adjust the edge. The trigger and the edge, so that it 47 00:03:43,720 --> 00:03:48,550 doesn't move around so much. The oscilloscope is displaying that sine 48 00:03:48,550 --> 00:03:53,370 wave across this scale, the horizontal scale. 49 00:03:53,370 --> 00:03:58,360 It's a time-based time division of five milliseconds per division, so that's 5 50 00:03:58,360 --> 00:04:02,770 milliseconds per division. We've got a period of about, looks like 51 00:04:02,770 --> 00:04:07,610 two divisions, so 10 milliseconds. Well, 10 milliseconds and 100 Hertz, it 52 00:04:07,610 --> 00:04:11,430 matches. 100 hertz has a 10 millisecond period. 53 00:04:11,430 --> 00:04:18,370 And the vertical is the voltage, so this is set at one volt per division. 54 00:04:18,370 --> 00:04:22,490 And I've got two volts, peak to peak, which is what I asked for. 55 00:04:22,490 --> 00:04:26,710 Now I can take the function generator, and I can adjust the frequency, and I'm 56 00:04:26,710 --> 00:04:32,160 measuring the output signal. So, I'm, I'm just measuring my source 57 00:04:32,160 --> 00:04:34,626 signal. I can change the amplitude. 58 00:04:34,626 --> 00:04:48,140 Okay. I'm going to set that back to 2, and I 59 00:04:48,140 --> 00:04:52,830 can also just adjust the DC offset. So, I can raise or lower it. 60 00:04:52,830 --> 00:05:06,490 Go ahead and set that back to zero. So, oscilloscopes and function generators 61 00:05:06,490 --> 00:05:10,230 are very, very common. And a lot of times, they come in big, 62 00:05:10,230 --> 00:05:14,740 their own self contained instruments and they could cost thousands of dollars. 63 00:05:14,740 --> 00:05:17,280 This one I'm using is actually relatively cheap. 64 00:05:17,280 --> 00:05:25,088 It's $175, or in that range. Around $200 which is cheap in comparison. 65 00:05:25,088 --> 00:05:31,180 So, I'm going to build an RC circuit, and actually, let's look at the RC circuit I 66 00:05:31,180 --> 00:05:36,270 want to build. Let's start with a capacitor, this 67 00:05:36,270 --> 00:05:38,350 capacitor, this is what a real capacitor looks like. 68 00:05:38,350 --> 00:05:41,550 They actually come in a lot of different shapes and sizes. 69 00:05:41,550 --> 00:05:45,282 This is a common one and this has a code on there, 104. 70 00:05:45,282 --> 00:05:55,570 The 104 stands for 1,0, and then four zeros following times 10 to the minus 12, 71 00:05:55,570 --> 00:05:59,430 and that's in farads. If I convert that to microfarads, it 72 00:05:59,430 --> 00:06:04,830 would be 0.1 microfarads. So, the code 104 corresponds to 0.1 73 00:06:04,830 --> 00:06:11,989 microfarads. I want to put that in a circuit I've got 74 00:06:11,989 --> 00:06:22,930 a, a source voltage, a resistor, and series with capacitor. 75 00:06:22,930 --> 00:06:28,010 And I'm going to call this V sub c is my voltage I'm trying to measure. 76 00:06:28,010 --> 00:06:32,480 So, I'm going to build this, this circuit. 77 00:06:32,480 --> 00:06:38,265 The source is from the myDAQ, and I'm going to be measuring this voltage. 78 00:06:38,265 --> 00:06:52,771 So the first thing it does is, from the source goes to resistor. 79 00:06:52,771 --> 00:07:06,570 And then, from the resistor, it goes to the capacitor. 80 00:07:06,570 --> 00:07:11,550 And then, from the capacitor, it goes back to the source. 81 00:07:11,550 --> 00:07:14,890 So, what I've just hooked up here is the source voltage. 82 00:07:14,890 --> 00:07:19,330 And I've also hooked up one of my channels in my oscilloscope to measure 83 00:07:19,330 --> 00:07:22,210 that source voltage. So, I just want to see what the source 84 00:07:22,210 --> 00:07:26,980 voltage looks like. Now with the other channel, I want to 85 00:07:26,980 --> 00:07:37,920 measure the voltage across the capacitor. So I'm taking this wire, and measuring 86 00:07:37,920 --> 00:07:46,420 across that capacitor. So, the orange wire's there correspond to 87 00:07:46,420 --> 00:07:53,500 measuring of V sub c. And they are going to be recorded on 88 00:07:53,500 --> 00:07:56,940 channel one on my oscilloscope. So, going back to my oscilloscope and 89 00:07:56,940 --> 00:07:58,970 function generator, let me set up this to run. 90 00:07:58,970 --> 00:08:12,440 Let me set it up with a square wave, and let me run this. 91 00:08:12,440 --> 00:08:18,700 What I'm seeing is the green, which is my source voltage, that's on channel zero. 92 00:08:18,700 --> 00:08:21,930 Channel one, let me enable that. It's in blue. 93 00:08:21,930 --> 00:08:26,860 It is my voltage across capacitor. They look like they're almost on top of 94 00:08:26,860 --> 00:08:32,551 one note here, so let me change my screen right here. 95 00:08:32,551 --> 00:08:50,670 Now, I want to change my frequency. Oh, I need to change this to a1 Now I see 96 00:08:50,670 --> 00:08:55,130 a difference of a 0. Channel zero is the source voltage, 97 00:08:55,130 --> 00:09:01,802 channel one is the capacitor voltage. And I see that I get the characteristic 98 00:09:01,802 --> 00:09:06,730 exponential growth on here. If I start from this voltage and I go up 99 00:09:06,730 --> 00:09:11,000 to this voltage, then this is what we see on typically on RC circuits. 100 00:09:11,000 --> 00:09:14,500 A response of an RC circuit. If I want to measure the time constant, 101 00:09:14,500 --> 00:09:17,565 let me expand this time scale a little bit bigger. 102 00:09:17,565 --> 00:09:21,135 A little bit easier to see. I want to measure this time constant. 103 00:09:21,135 --> 00:09:25,990 To measure a time constant, use the oscilloscope. 104 00:09:25,990 --> 00:09:29,660 I can use these cursors. The cursors, I set at a certain point 105 00:09:29,660 --> 00:09:35,939 here, and that's at this time division. And then, I go over and set this one. 106 00:09:38,420 --> 00:09:43,470 Now, recall that the time constant is roughly about 2 3rd of the way there. 107 00:09:43,470 --> 00:09:47,852 So if I look at that, this is, this is roughly about 2 3rd of the way. 108 00:09:47,852 --> 00:09:51,740 That DT tells me the time difference between the first cursor and the second. 109 00:09:51,740 --> 00:09:59,650 That's a hundred microseconds, a hundred microseconds is 0.1 milliseconds. 110 00:09:59,650 --> 00:10:06,030 And 0.1 milliseconds is the, the value of RC. 111 00:10:06,030 --> 00:10:11,639 This particular case, the R is a thousand, and the capacitor is 0.1 112 00:10:11,639 --> 00:10:14,920 microfarad. And the combination is 0.0001, or 1 mili, 113 00:10:14,920 --> 00:10:22,850 0.1 milliseconds. And that's what I'm measuring right here. 114 00:10:22,850 --> 00:10:28,276 So this experiment, we've looked at typical instruments that electrical 115 00:10:28,276 --> 00:10:32,250 engineers use. We've built an RC circuit on there, and 116 00:10:32,250 --> 00:10:35,190 we've shown how to measure the time constant of an RC circuit. 117 00:10:35,190 --> 00:10:40,090 If I change the R or the C in this circuit, I'm going to be changing that 118 00:10:40,090 --> 00:10:43,718 time constant. Thank you. 119 00:10:43,718 --> 00:10:48,250 In summary, we've looked at an oscilloscope to measure and record 120 00:10:48,250 --> 00:10:52,440 voltage signals versus time. A function generator allows you to input 121 00:10:52,440 --> 00:10:57,760 voltage, voltage signals into a circuit. Inputting a square wave into the circuit 122 00:10:57,760 --> 00:11:01,280 allows you to capture the RC circuit transient behavior and to measure the 123 00:11:01,280 --> 00:11:05,780 time constant. In the next lesson, we will look at 124 00:11:05,780 --> 00:11:07,190 analyzing RL circuits.