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 transient that they, that they cause in circuits.
Now we also use them as miniature batters to hold charges.
For example in charge pumps they're used this 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've got a one if it's charged.
And a zero if it's discharged. But those are all ECE applications.
Let's look outside the field of ECE. In that case, the most common application
is as 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, 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 field bends around the outside or the edges of the capacitor.
So as a common application of varying capacitance is a fluid level sense.
Fluid level sensor displaces the material between the plates from air to fluid.
So as a fluid level grows, then the material between these plates changes
which means that it's going to change this, this permittivity capacity.
Or this permittivity constant. 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 touch screens 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 in the capacitors.
Or your finger may change the fringing effects by near the edge of the plates as
a result the effective capacitance changes.
Now some, 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 a very cheap microphone. It's omnidirectional.
That means it's sensitive to sound that hits it in along this 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 that goes on is one of the
plates that capicator. So that's a capacitor plate.
Then you've got a spacer. It's a red ring, o-ring, is a space in
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 it, 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 here.
On the inside there 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 RC, a resistor and capacitor In series with one another.
Along this rail here is the ground. And this is the the voltage source right
here. So the voltage source goes into the
resister 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, you know 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. [SOUND].
Now I'm showing this capacitor here. I don't have it hooked up right now.
I wanted to be kind of flexible with it. It's, it's connected to this other
capacitor right there, because it's in the same hole.
This hole right here, so this, the two capacitors are connected right here, and
then on the other side [SOUND]. 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's, it has no affect 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 a touch pad. I'm increasing the capacitance
momentarily while I touch it, then I let go.
So let's look at the response, let's look at the screen.
The screen shows the oscilloscope, response.
In green is shown 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 this circuit to be able to see that transient response.
Now if I increase the capacitance by touching that other capacitor to it, you
can see, suddenly the change. The time constant increases, while the
time constant is RC, So increasing the capacitance increases that time constant.
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, that checks when this voltage of the blue.
So checks when this blue voltage, or blue signal, the voltage crosses 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 the green line going high. And the blue line crossing that, that
threshold line, it's longer time delay than in this case.
And by that shifted time delay, I can tell When this capacitance is increased.
In other words I can tell when the keypad has been touched.
And touchscreens work, capacitive touchscreens work in the same way, but
they also have spacial 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, 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 di,
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 of, I'm half the distance.
And then as I keep moving it around I'm changing that overlap and that distance,
I'm changing the capacitance. This is used as in an old-fashioned
radio. So a dial was attached to this as you
tuned. You'd 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 has 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 they're half overlap the top ones, and half overlap the bottom
ones. If I keep rotating, at this point, they
completely overlap with the top points and don't overlap with the bottom points
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 antenna.
So, in summary, we showed capacity sensors; such as, touch pads and
capacitive microphones and antenna tuners.
And, there are a number of people that helped to make this demo, possible and in
the next lesson, we will go on to look at examples of inductive.
Inductance in, physical applications. Thank you.