Welcome back. This is Dr. Ferri and we're continuing in our course on linear circuits. We're almost finished with this module. We wanted to finish it out, looking at some applications. In this particular module, we'll look at a lab demo: applications of capacitors. So far in this module, we have covered a lot of theoretical background on how to analyze circuits with capacitors and inductors in them. So, at this point we want to look at some physical applications of these quantities of capacitors and inductors. Let's start out with capacitors. Now we use capacitors all the time in the field of electrical and computer engineering. We use them in circuits because we like the behavior that they give to circuits. We like the reactive behavior. We like the transience that they cause in circuits. Now we also use them as miniature batteries, to hold charges. For example, in charge pumps, they're used as like little, miniature batteries. They're also used in memory in computers, the main memory, or DRAM, in a computer, has capacitors in them. Every bit is determined, every bit of information, is determined by whether a capacitor is charged or discharged. You got a one if it's charged. And a zero fits discharged. But those are all easy applications. Let's look outside the field of VC. In that case, the most common application uses sensors, capacitive sensors. Capacitive sensors work by varying the capacitance. If you look at this image here of a capacitor, this is what we've looked at before when we first started talking about capacitance. We showed that the capacitance is determined by this formula here. So it depends on the permittivity, the cross-sectional area of the plates, and the distance here between the plates. If you vary any one of those, you vary the capacitance. Another effect is by looking at the fringe effect. Which is where the electric fields bends around the outside or the edges of the capacitor. So as a common application of varying capacitance is a fluid level sensor. Fluid level sensor displaces the material between the plates from air to fluid. So as the fluid level grows, then the material between these plates changes. Which means that it's going to change this permitivity capacity or this permitivity concept. So in this lab demo, we will look at some physical examples, and applications of capacitance. Let's take a look at a capacitive keypad. Some capacitive touchscreens and keypads have a hard surface like this one. A finger close to the surface may act as a ground to reduce the local charge on capacitors. Or your finger may change the fringing effects by being near the edge of the plates. As a result, the effect of capacitance changes. Now some keypads have a squishy surface when you press down. And in these cases pressing down changes the distance between the plates. A similar phenomena happens in capacitive microphones. Let me show you one here. This is a capacitive microphone. It's very cheap microphone, and it's omni-directional, that means that it's sensitive to sound that hits it along the surface right here. Let me take one apart, so you can see the inside. So I took off the outer covering there. And the next it goes on, is one of the plates of the capacitor. So that's a capacitor plate. And then you've got a spacer. This red ring, o ring is a spacer there. And then to top it off, you've got the second plate right here. Now inside this plate, inside the round part, the outer edge of this round part is kind of hard, it keeps the spacer, but the inside is a very thin metallic flexible membrane. So when the sound waves hit this, that flexible membrane on the inside there, it goes in and out. And that means that you're going to have the distance between the plates change. And as the distance gets smaller, the capacitance increases. It's hard to measure capacitance directly. So instead we measure how a circuit response changes when we change the capacitance. To demonstrate that, let's look at this circuit. This is an RC circuit and let me show you what's going on here. We've got an a resistor and capacitor, in series with one another. Along this rail here is the ground, and this is the voltage source right here. So the voltage source goes into the resistor which is in series with the capacitor and that's connected to ground. I want to look at the voltage source, so my oscilloscope is in these green wires is showing the voltage source, and that's between, it's connected to the voltage source and to ground. And I also want to look at the voltage across the capacitor. So it's on this side of the capacitor and then to ground. Now I am showing this capacitor here. I dont have it hooked up right now. I wanted to be kind of flexible with it. It's connected to this other capacitor right there. Because it's in the same hole. This hole right here. So the two capacitors are connected right here. And then on the other side, I'm going to touch it to the other leg of the other capacitor. So when I have it loose it's open loop, and it has no effect on the circuit. When I touch the capacitor's legs like this, that means I'm increasing the capacitance, because remember, capacitors in parallel add. So I increase the capacitance, decrease it. Increase, decrease it. So it's as if I'm touching something. I'm touching the touch pad. I'm increasing the capacitance momentarily while I touch it, then I'll let go. So let's look at the response. Let's look at the screen. The screen shows the oscilloscope. Response, in green it show the voltage source. So I'm showing this with a square wave. Because what I'm really interested in doing is triggering the transient response. So I have to have a change in the voltage to the circuit to be able to see that transient response. Now if I increase the capacitance by touching that other capacitor to it, you could see suddenly the change. The time constant increases. Well the time constant is RC, so increasing the capacitance increases that time concept. The way I can detect the change in capacitance is I can put a separate circuit in there with a comparator. A comparator is a circuit I can build that checks when this voltage of the blue. So checks when this blue voltages or blue signal. The voltage cross capacitor reaches a certain level. For example I might want to see when it reaches this level that’s indicated by the red line. And as soon as it reaches that level, then I trigger a pulse. So if I look at the time difference between, when the voltage source goes high, that's a green line going high, and the blue line crossing that threshold line. It's longer time delay than in this case. And by that shift in time delay, I can tell when this capacitance has increased. In other words, I can tell when the keypad has been touched. And capacitive touchscreens work in the same way, but they also have spatial configurations. So you have to figure out where you have touched it on the touch pad. Not just when you touched it, but where you touched it. Another device that uses varying capacitance is a tuner. This tuner has these plates on it, and these plates actually are the capacitor plates. By rotating the device you change the distance between the plate or the overlap area of the plates. You can calibrate this to know the capacitance as a function of angle. So let me turn it. Right here, there's no overlap between the plates. And here, I start to get overlap and about half overlap. So I'm increasing the area of the capacitance plates. And that means I'm going to be increasing the capacitance. Here I've got 100% overlap. Not only that but if you look at the difference between these plates, they're now smaller because I've halved the distance. And then as I keep moving it around, I'm changing that overlap and that distance so I'm changing the capacitance. This is used in an old fashioned radio. So a dial was attached to this. As you change the dial, you were tuning the radio. You were tuning the frequency by changing that capacitance. Another device that works on the same principle is an antenna tuner. This is a case where it's got two sets of plates, these up here and these down here. And it's got a rotary dial here. So as I turn this, I'm making these plates in the center rotate around. At this point their half overlap the top ones and half overlap the bottom ones. If I keep rotating, at this point they completely overlap with the top plates and don't overlap with the bottom plates at all. So by changing the way I hook this up, I can change the capacitance a great deal. And this is used to tune an antenna. So in summary, we showed capacitive sensors such as touch pads and capacitive microphone and antenna tuners. And there are number people that helped to make this demo possible. And in the next lesson, we will go on to look at examples of inductance in physical applications. Thank you.
