Hello, this is Doctor Ferri. This particular lesson is a lab demo on
RC circuits. And just to set it up, I want to explain
what we're looking at. We're going to be looking at an RC
circuit. We'd like to be able to look at the
transient response. In order to do that, and measure it, we
have to keep generating a transient response.
It's hard to catch it if we only see it once.
So, we're going to keep generating and triggering a transient response.
And the way we're going to do it, if you look on the left, it's as if I have two
circuit sources. One with the polarity this way and the
other with the polarity that way. So, if I'm looking at it, when it's, when
the switch is connected this way, I will have one voltage source.
And when the switch is connected this way, I will have the opposite voltage
source. And in order to generate that in a
physical system, I can use a function generator to give me a square wave.
So looking at the left, a square wave is equivalent to, just switching back and
forth. And then, you generate the square wave.
So, the function generator is going to generate a square wave, and then we'll be
looking at the voltage across the capacitor.
The lab demo is on RC Circuits. This experiment is to demonstrate the use
of some very common instruments used in electrical engineering.
And to use those instruments to measure the characteristics of an RC circuit.
If we have a circuit and we want to measure voltages, we have to be able to
input that into our, our measurement device.
And in this case, we'll be using an, a national instruments myDAQ, which is the
data aquisition board. These wires allow us to output voltage
from the myDAQ or circuit. So, these two are going to be output
voltages. And it also allows us, if you look at the
side of this, and that leaves different channels.
So, these are my output voltages to my circuit.
Like it acts like a source, and these are two input voltage channels.
And those allow me to measure two different wave forms, voltage wave forms.
So, for example, let me set this up so that, I want to output.
So, that's my source voltage. I'm outputting a voltage, and then I want
to measure it. This is channel zero.
I want to measure the exact same thing that I'm outputting.
So, if I go to my computer, I'm running software on the computer.
And the software allows me to take those measured signals and to display them on
what's known as an oscilloscope. And I'm also using a function generator.
A function generator generates a signal. I can output a, a sine wave, a triangular
wave, a square wave, and the oscilloscope allows me to measure it.
Since I've got the circuit set up, let me go ahead and run these.
So, this is outputting a 100 Hertz sine wave.
That's clicked, that's a sine wave. Let me change this to 2 volts peak to
peak. And then if I run the oscilloscope, let
me adjust the edge. The trigger and the edge, so that it
doesn't move around so much. The oscilloscope is displaying that sine
wave across this scale, the horizontal scale.
It's a time-based time division of five milliseconds per division, so that's 5
milliseconds per division. We've got a period of about, looks like
two divisions, so 10 milliseconds. Well, 10 milliseconds and 100 Hertz, it
matches. 100 hertz has a 10 millisecond period.
And the vertical is the voltage, so this is set at one volt per division.
And I've got two volts, peak to peak, which is what I asked for.
Now I can take the function generator, and I can adjust the frequency, and I'm
measuring the output signal. So, I'm, I'm just measuring my source
signal. I can change the amplitude.
Okay. I'm going to set that back to 2, and I
can also just adjust the DC offset. So, I can raise or lower it.
Go ahead and set that back to zero. So, oscilloscopes and function generators
are very, very common. And a lot of times, they come in big,
their own self contained instruments and they could cost thousands of dollars.
This one I'm using is actually relatively cheap.
It's $175, or in that range. Around $200 which is cheap in comparison.
So, I'm going to build an RC circuit, and actually, let's look at the RC circuit I
want to build. Let's start with a capacitor, this
capacitor, this is what a real capacitor looks like.
They actually come in a lot of different shapes and sizes.
This is a common one and this has a code on there, 104.
The 104 stands for 1,0, and then four zeros following times 10 to the minus 12,
and that's in farads. If I convert that to microfarads, it
would be 0.1 microfarads. So, the code 104 corresponds to 0.1
microfarads. I want to put that in a circuit I've got
a, a source voltage, a resistor, and series with capacitor.
And I'm going to call this V sub c is my voltage I'm trying to measure.
So, I'm going to build this, this circuit.
The source is from the myDAQ, and I'm going to be measuring this voltage.
So the first thing it does is, from the source goes to resistor.
And then, from the resistor, it goes to the capacitor.
And then, from the capacitor, it goes back to the source.
So, what I've just hooked up here is the source voltage.
And I've also hooked up one of my channels in my oscilloscope to measure
that source voltage. So, I just want to see what the source
voltage looks like. Now with the other channel, I want to
measure the voltage across the capacitor. So I'm taking this wire, and measuring
across that capacitor. So, the orange wire's there correspond to
measuring of V sub c. And they are going to be recorded on
channel one on my oscilloscope. So, going back to my oscilloscope and
function generator, let me set up this to run.
Let me set it up with a square wave, and let me run this.
What I'm seeing is the green, which is my source voltage, that's on channel zero.
Channel one, let me enable that. It's in blue.
It is my voltage across capacitor. They look like they're almost on top of
one note here, so let me change my screen right here.
Now, I want to change my frequency. Oh, I need to change this to a1 Now I see
a difference of a 0. Channel zero is the source voltage,
channel one is the capacitor voltage. And I see that I get the characteristic
exponential growth on here. If I start from this voltage and I go up
to this voltage, then this is what we see on typically on RC circuits.
A response of an RC circuit. If I want to measure the time constant,
let me expand this time scale a little bit bigger.
A little bit easier to see. I want to measure this time constant.
To measure a time constant, use the oscilloscope.
I can use these cursors. The cursors, I set at a certain point
here, and that's at this time division. And then, I go over and set this one.
Now, recall that the time constant is roughly about 2 3rd of the way there.
So if I look at that, this is, this is roughly about 2 3rd of the way.
That DT tells me the time difference between the first cursor and the second.
That's a hundred microseconds, a hundred microseconds is 0.1 milliseconds.
And 0.1 milliseconds is the, the value of RC.
This particular case, the R is a thousand, and the capacitor is 0.1
microfarad. And the combination is 0.0001, or 1 mili,
0.1 milliseconds. And that's what I'm measuring right here.
So this experiment, we've looked at typical instruments that electrical
engineers use. We've built an RC circuit on there, and
we've shown how to measure the time constant of an RC circuit.
If I change the R or the C in this circuit, I'm going to be changing that
time constant. Thank you.
In summary, we've looked at an oscilloscope to measure and record
voltage signals versus time. A function generator allows you to input
voltage, voltage signals into a circuit. Inputting a square wave into the circuit
allows you to capture the RC circuit transient behavior and to measure the
time constant. In the next lesson, we will look at
analyzing RL circuits.