1 00:00:00,570 --> 00:00:04,160 Welcome back. This is Dr Ferri and we're continuing in 2 00:00:04,160 --> 00:00:10,140 our course on Linear Circuits. We're almost finished with this module. 3 00:00:10,140 --> 00:00:12,825 We wanted to finish it out looking at some applications. 4 00:00:12,825 --> 00:00:17,250 In this particular module, we'll look at a lab demo, applications of capacitors. 5 00:00:18,950 --> 00:00:24,020 So far in this module, we have covered a lot of theoretical background on how to 6 00:00:24,020 --> 00:00:28,149 analyze circuits with capacitors and inductors in them. 7 00:00:28,149 --> 00:00:32,530 So at this point, we want to look at some physical applications of these quantities 8 00:00:32,530 --> 00:00:38,000 of capacitors and inductors. Let's start out with capacitors. 9 00:00:38,000 --> 00:00:41,820 Now we use capacitors all the time, in the field of electrical and computer 10 00:00:41,820 --> 00:00:44,630 engineering. We use them in circuits because we like 11 00:00:44,630 --> 00:00:48,305 the behavior that they give to circuits. We like the reactive behavior. 12 00:00:48,305 --> 00:00:51,485 We like the transient that they, that they cause in circuits. 13 00:00:51,485 --> 00:00:59,100 Now we also use them as miniature batters to hold charges. 14 00:00:59,100 --> 00:01:03,490 For example in charge pumps they're used this like little miniature batteries 15 00:01:03,490 --> 00:01:10,380 they're also used in memory in computers. The main memory or DRAM in a computer has 16 00:01:10,380 --> 00:01:14,520 capacitors in them. Every bit is determined. 17 00:01:14,520 --> 00:01:19,100 Every bit of information is determined by whether a capacitor is charged or 18 00:01:19,100 --> 00:01:21,830 discharged. You've got a one if it's charged. 19 00:01:21,830 --> 00:01:26,930 And a zero if it's discharged. But those are all ECE applications. 20 00:01:26,930 --> 00:01:32,040 Let's look outside the field of ECE. In that case, the most common application 21 00:01:32,040 --> 00:01:37,779 is as sensors, capacitive sensors. Capacitive sensors work by varying the 22 00:01:37,779 --> 00:01:40,990 capacitance. If you look at this image here of a 23 00:01:40,990 --> 00:01:45,300 capacitor, this is what we've looked at before when we first started talking 24 00:01:45,300 --> 00:01:48,950 about capacitance. We showed that the capacitance is 25 00:01:48,950 --> 00:01:53,649 determined by this formula here. So, depends on the permittivity, the 26 00:01:53,649 --> 00:01:58,400 cross-sectional area of the plates, and the distance here between the plates. 27 00:01:59,590 --> 00:02:03,030 If you vary any one of those, you vary the capacitance. 28 00:02:03,030 --> 00:02:07,140 Another effect is by looking at the fringe effect, which is where the 29 00:02:07,140 --> 00:02:12,980 electric field bends around the outside or the edges of the capacitor. 30 00:02:12,980 --> 00:02:17,870 So as a common application of varying capacitance is a fluid level sense. 31 00:02:17,870 --> 00:02:24,060 Fluid level sensor displaces the material between the plates from air to fluid. 32 00:02:24,060 --> 00:02:30,240 So as a fluid level grows, then the material between these plates changes 33 00:02:30,240 --> 00:02:36,626 which means that it's going to change this, this permittivity capacity. 34 00:02:36,626 --> 00:02:42,497 Or this permittivity constant. So in this lab demo, we will look at some 35 00:02:42,497 --> 00:02:45,730 physical examples and applications of capacitance. 36 00:02:48,540 --> 00:02:57,230 Let's take a look at a capacitive keypad. Some capacitive touch screens and keypads 37 00:02:57,230 --> 00:03:01,820 have a hard surface, like this one. A finger close to the surface may act as 38 00:03:01,820 --> 00:03:04,775 a ground to reduce the local charge in the capacitors. 39 00:03:04,775 --> 00:03:11,550 Or your finger may change the fringing effects by near the edge of the plates as 40 00:03:11,550 --> 00:03:14,062 a result the effective capacitance changes. 41 00:03:14,062 --> 00:03:21,016 Now some, some keypads have a squishy surface when you press down, and in these 42 00:03:21,016 --> 00:03:25,740 cases pressing down changes the distance between the plates. 43 00:03:27,080 --> 00:03:30,840 A similar phenomena happens in capacitive microphones. 44 00:03:30,840 --> 00:03:35,290 Let me show you one here. This is a capacitive microphone. 45 00:03:35,290 --> 00:03:38,560 It's a very cheap microphone. It's omnidirectional. 46 00:03:38,560 --> 00:03:43,710 That means it's sensitive to sound that hits it in along this surface right here. 47 00:03:45,230 --> 00:03:47,480 Let me take one apart so you can see the inside. 48 00:03:54,930 --> 00:04:04,160 So I took off the outer covering there. And the next that goes on is one of the 49 00:04:04,160 --> 00:04:13,480 plates that capicator. So that's a capacitor plate. 50 00:04:13,480 --> 00:04:19,420 Then you've got a spacer. It's a red ring, o-ring, is a space in 51 00:04:19,420 --> 00:04:24,680 there. And then to top it off, you've got the 52 00:04:24,680 --> 00:04:33,590 second plate right here. Now inside this plate, inside the round 53 00:04:33,590 --> 00:04:35,460 part. The outer edge of this round part is kind 54 00:04:35,460 --> 00:04:37,740 of hard. It keeps it, the spacer. 55 00:04:37,740 --> 00:04:44,420 But the inside is a very thin, metallic flexible membrane. 56 00:04:44,420 --> 00:04:50,039 So, when the sound waves hit this, that flexible membrane on the inside here. 57 00:04:51,700 --> 00:04:56,980 On the inside there goes in and out. And that means that you're going to have 58 00:04:56,980 --> 00:05:01,750 the distance between the plates change. And as the distance gets smaller, the 59 00:05:01,750 --> 00:05:08,103 capacitance increases. It's hard to measure capacitance 60 00:05:08,103 --> 00:05:12,690 directly. So instead, we measure how a circuit 61 00:05:12,690 --> 00:05:19,390 response changes when we change the capacitance. 62 00:05:19,390 --> 00:05:21,620 To demonstrate that, let's look at this circuit. 63 00:05:22,860 --> 00:05:28,310 This is an RC circuit, and let me show you what's going on here. 64 00:05:28,310 --> 00:05:34,096 We've got an RC, a resistor and capacitor In series with one another. 65 00:05:34,096 --> 00:05:41,314 Along this rail here is the ground. And this is the the voltage source right 66 00:05:41,314 --> 00:05:43,945 here. So the voltage source goes into the 67 00:05:43,945 --> 00:05:48,320 resister which is in series with the capacitor, and that's connected to 68 00:05:48,320 --> 00:05:52,220 ground. I want to look at the voltage source, so 69 00:05:52,220 --> 00:05:57,640 my oscilloscope is in these green wires is showing the voltage source. 70 00:05:57,640 --> 00:06:01,500 And that's between, you know it's connected to the voltage source and to 71 00:06:01,500 --> 00:06:05,040 ground. And I also want to look at the voltage 72 00:06:05,040 --> 00:06:09,270 across the capacitor. So it's on this side of the capacitor and 73 00:06:09,270 --> 00:06:10,265 then to ground. [SOUND]. 74 00:06:10,265 --> 00:06:15,040 Now I'm showing this capacitor here. I don't have it hooked up right now. 75 00:06:15,040 --> 00:06:19,730 I wanted to be kind of flexible with it. It's, it's connected to this other 76 00:06:19,730 --> 00:06:22,560 capacitor right there, because it's in the same hole. 77 00:06:23,830 --> 00:06:28,990 This hole right here, so this, the two capacitors are connected right here, and 78 00:06:28,990 --> 00:06:33,580 then on the other side [SOUND]. I'm going to touch it to the other leg of 79 00:06:33,580 --> 00:06:37,180 the other capacitor. So when I have it loose, it's open loop, 80 00:06:37,180 --> 00:06:39,790 and it's, it has no affect on the circuit. 81 00:06:39,790 --> 00:06:44,360 When I touch the capacitor's legs like this, that means I'm increasing the 82 00:06:44,360 --> 00:06:48,500 capacitance. Because remember, capacitors in parallel 83 00:06:48,500 --> 00:06:52,090 add. So I increase the capacitance, decrease 84 00:06:52,090 --> 00:06:56,650 it, increase, decrease it. So it's as if I'm touching something, I'm 85 00:06:56,650 --> 00:06:59,140 touching a touch pad. I'm increasing the capacitance 86 00:06:59,140 --> 00:07:02,140 momentarily while I touch it, then I let go. 87 00:07:02,140 --> 00:07:04,799 So let's look at the response, let's look at the screen. 88 00:07:06,120 --> 00:07:11,860 The screen shows the oscilloscope, response. 89 00:07:11,860 --> 00:07:18,620 In green is shown the voltage source. So I'm showing this with a square wave, 90 00:07:18,620 --> 00:07:22,950 because what I'm really interested in doing is triggering the transient 91 00:07:22,950 --> 00:07:27,450 response. So I have to have a change in the voltage 92 00:07:27,450 --> 00:07:29,825 to this circuit to be able to see that transient response. 93 00:07:29,825 --> 00:07:36,780 Now if I increase the capacitance by touching that other capacitor to it, you 94 00:07:36,780 --> 00:07:41,790 can see, suddenly the change. The time constant increases, while the 95 00:07:41,790 --> 00:07:46,590 time constant is RC, So increasing the capacitance increases that time constant. 96 00:07:47,790 --> 00:07:52,570 The way I can detect the change in capacitance, is I can put a separate 97 00:07:52,570 --> 00:07:58,400 circuit in there with a comparator. A comparator is a circuit I can build 98 00:07:58,400 --> 00:08:03,700 that, that checks when this voltage of the blue. 99 00:08:06,630 --> 00:08:12,850 So checks when this blue voltage, or blue signal, the voltage crosses capacitor, 100 00:08:12,850 --> 00:08:15,510 reaches a certain level. For example, I might want to see when it 101 00:08:15,510 --> 00:08:19,020 reaches this level that's indicated by the red line. 102 00:08:19,020 --> 00:08:22,804 And as soon as it reaches that level, then I trigger a pulse. 103 00:08:22,804 --> 00:08:28,570 So, if I look at the time difference between when the voltage source goes 104 00:08:28,570 --> 00:08:34,380 high, that's the green line going high. And the blue line crossing that, that 105 00:08:34,380 --> 00:08:39,620 threshold line, it's longer time delay than in this case. 106 00:08:39,620 --> 00:08:45,330 And by that shifted time delay, I can tell When this capacitance is increased. 107 00:08:45,330 --> 00:08:48,225 In other words I can tell when the keypad has been touched. 108 00:08:48,225 --> 00:08:53,650 And touchscreens work, capacitive touchscreens work in the same way, but 109 00:08:53,650 --> 00:08:59,870 they also have spacial configurations. So you have to figure out where you have 110 00:08:59,870 --> 00:09:02,970 touched it on the touch pad, not just when you touched it, but where you 111 00:09:02,970 --> 00:09:08,060 touched it. Another device that uses varying 112 00:09:08,060 --> 00:09:23,250 capacitance is a tuner. This tuner has these plates on it, and 113 00:09:23,250 --> 00:09:26,460 these plates actually are the, the capacitor plates. 114 00:09:26,460 --> 00:09:30,080 By rotating the device, you change the distance between the plate, or the 115 00:09:30,080 --> 00:09:34,030 overlap area of the plates. You can calibrate this to know the di, 116 00:09:34,030 --> 00:09:37,389 the capacitance as a function of angle, so let me turn it. 117 00:09:37,389 --> 00:09:40,770 Right here, there's no overlap between the plates. 118 00:09:40,770 --> 00:09:44,910 And here, I start to get overlap. And about half overlap, so I'm increasing 119 00:09:44,910 --> 00:09:49,260 the area of the capacitance plates. And that means I'm going to be increasing 120 00:09:49,260 --> 00:09:52,640 the capacitance. Here, I've got 100% overlap. 121 00:09:52,640 --> 00:09:56,820 Not only that, but if you look at the difference between these plates, they're 122 00:09:56,820 --> 00:10:00,790 now smaller because of, I'm half the distance. 123 00:10:00,790 --> 00:10:06,420 And then as I keep moving it around I'm changing that overlap and that distance, 124 00:10:06,420 --> 00:10:12,660 I'm changing the capacitance. This is used as in an old-fashioned 125 00:10:12,660 --> 00:10:15,440 radio. So a dial was attached to this as you 126 00:10:15,440 --> 00:10:18,310 tuned. You'd change the dial, you were tuning 127 00:10:18,310 --> 00:10:23,370 the radio, you were tuning the frequency, by changing that capacitance. 128 00:10:23,370 --> 00:10:27,830 Another device that works on the same principle Is an antenna tuner. 129 00:10:27,830 --> 00:10:30,590 This is a case where it has two sets of plates. 130 00:10:30,590 --> 00:10:34,790 These up here and these down here. And it's got a rotary dial here. 131 00:10:34,790 --> 00:10:41,780 So as I turn this, I'm making these plates in the center, rotate around. 132 00:10:41,780 --> 00:10:45,090 At this point they're half overlap the top ones, and half overlap the bottom 133 00:10:45,090 --> 00:10:49,980 ones. If I keep rotating, at this point, they 134 00:10:49,980 --> 00:10:53,570 completely overlap with the top points and don't overlap with the bottom points 135 00:10:53,570 --> 00:10:56,530 at all. So by changing the way I hook this up, I 136 00:10:56,530 --> 00:11:02,573 can change the capacitance a great deal, and this is used to tune antenna. 137 00:11:02,573 --> 00:11:09,857 So, in summary, we showed capacity sensors; such as, touch pads and 138 00:11:09,857 --> 00:11:13,964 capacitive microphones and antenna tuners. 139 00:11:13,964 --> 00:11:21,179 And, there are a number of people that helped to make this demo, possible and in 140 00:11:21,179 --> 00:11:26,930 the next lesson, we will go on to look at examples of inductive. 141 00:11:26,930 --> 00:11:30,230 Inductance in, physical applications. Thank you.