Hi, and welcome back to another ElectroBOOM 101.
This time is about AC and DC. Oh, you think it's very straightforward, eh?
DC is this straight line and AC is this wiggly line? Well, you know nothing. Although...
it's not super inaccurate, but I can't finish the video now.
(Police Sirens) WARNING! WARNING! Bwah, it could use some echo.
I put some complex looking formulas on my video to look smart in case some professor or something is watching.
But frankly, in my line of work, I use the simple stuff to design complex circuits.
I want you to *grasp* the meaning of what I'm trying to say and really *feel* it in your heart.
And don't bother with formoolas (formulas) too much.
I talked to you about what voltage and current are in the past video, that the current is the amount of flowing
charges per second,
mainly electrons in a wire, and
voltage is the available energy per unit charge that moves charges and creates current.
Stop saying, "volt jolts, current kills!"
Okay? Now like water in a river the charges can flow
steadily, without any change. It's called DC, or direct current. In DC,
there is absolutely no change in the speed of flowing currents. No ripples. Nothing. Steady as a beating drum.
Beating drum is AC. That's DC. Nothing to it. It's the AC, or alternating
current, that's the mother of all evil.
That's when the flowing charges change direction and go back and forth with an alternating pattern,
which can be anything.
Of course, as long as it's changing forward in time.
It can't return in time. Time travel is impossible.
If you disagree, return in time, find me yesterday, and punch me in the eyebrow.
Now the most basic alternating waveform is a sinusoidal waveform, or sine wave, which looks like this.
Imagine you have a circle with a radius of one, and a dot that's moving around its perimeter at a fixed speed.
Now if you look at the shadow of the dot on the vertical axis and plot it in time,
you will have a sine wave with a magnitude of one. And as you all know, the city voltage is a sine wave. Now if
we scope it... uhm...
Which one was the live wire? Black is ground.
S***! F***!
Agh! White is neutral! Black is the color of a pirate flag. Black dangerous!
Here is our 120 volt AC.
The period of an AC signal is the time
it takes for one full pattern to finish, which in this case is 16.7 milliseconds.
And the frequency is the number of times this pattern repeats in one second, which is 60 Hertz in this case.
So naturally, frequencies shown with the lowercase f, with a unit of Hertz, is equal to one over
period, which is shown with a capital T, with a unit of second.
The sine is shown as sin of alpha, that gives you the value of sin between ±1. And alpha is the
angle of where your dot is, which is typically in Radian scale, where
360 degrees is equal to 2 pi radians. Now for a proper sine wave formula,
we have a times sine of Omega T plus Phi.
Like I said, a sine formula gives you a magnitude of 1. So, for example,
if you want to show a 100 volt signal, A would be equal to
100. For now,
let's assume Phi is 0. Omega is called the angular frequency, and
shows how fast your signal will oscillate. And T is time in
seconds. Now, as the time passes, the angle changes from 0 to
360 degrees and forward. But if Omega is bigger,
the change of angle per time is faster, and so higher frequency.
Omega is defined as 2 times π times f for frequency.
So for example, if frequency is equal to 1 Hertz, we have this formula. If time starts at 0 seconds,
we start at 0 degrees, and when time reaches 1 second, we reach 2π or 360 degrees.
Which means we have turned 1 turn in 1 second, or 1 Hertz.
You know what they say about AC? "It hertz"
yeugh yeugh yeugh yeugh
Now Phi is just a fixed angle we add here. Without Phi,
we always start at zero, but with Phi we can shift our waveform. For example, in case of three-phase line,
we have one phase that starts at zero, the other one that's shifted 120 degrees, like this, and
the third one that's shifted -120 degrees, like this.
They also call phi "phase-shift." All these formulas show
is that how the flow of current moves one way,
slows down, and returns. Or in terms of voltage, how the voltage changes in time, like you saw on the scope.
Wake up! [slap] There are still things to talk about here.
And you thought that's all there is about AC?
No, that was just a sine wave!
We have so many different AC patterns, like square wave, triangular wave, sawtooth wave, or anything in between.
Now, let me tell you a fascinating secret that only the engineers know.
[whispering] You're not gonna find a job anyways, so learn and do what you love very well.
[speaking normally] And the second thing is that any periodic wave form
can be written as the sum of a series of sine waves with different
magnitudes and frequencies. Any wave form. Here,
I put a spreadsheet together for you to add a series of sine waves and see what wave form we get. This one is for
the triangular wave form, with a specific
series formula. What you see here is the main sine wave.
If I add the next sine wave in the series, we get this. And then the next one, next one, next one, next one, next
one, next one...
It looks very much like a triangular wave. If I was able to go to infinity,
it would be a perfect triangular waveform. For the square wave,
we have a different series formula. And again,
I start from the main sine wave and add the next one. And next one, next one,
next one, next one, next one, next one, next one, next one, next one. And the more I can add, the more square it gets.
And here is the one for the sawtooth. And I add next one, next one, next one,
next one, next one, next one, next one...
And that's as far as I like to go.
They call all these sine waves that create a specific
waveform "harmonics," with the main harmonic being the lowest frequency sine wave.
Like for example, my voice has a main [starts speaking with melody]
frequency that can be the same as yours, but my voice is different than yours
[speaking normally] because my harmonics that are made by vibrations and resonances
through my own skull cavities and bones are different than yours.
Now thankfully, if your voice is garbage, it doesn't mean that you have an ugly face too.
It's just that your harmonics are s**t.
Similarly, all these waveforms can have the same main harmonic at the same frequency.
But their other harmonics are different. Now, you could refer to all these
waveforms by their magnitude, like a 1 volt peak sine wave or a square wave,
but it's kind of meaningless in terms of
electronic power delivery. Because, for example, a 1 volt
peak sine wave delivers a different power than a square wave or a triangular wave.
That's why for power lines, instead of using the peak voltage or current, we use
RMS or root mean square value. The root mean square value of a wave form is the voltage or current that can deliver the same power
as its equivalent DC voltage or current over a resistive load.
For example, a 1 volt RMS AC delivers the same power as a 1 volt DC
over a 1 ohm resistor.
That's why for the city power lines, the 120 volt AC or
240 volt AC are the RMS values. For sine waves, the peak value is equal to the RMS value
times square root of 2. But let's take a look at my car inverter, which converts a 12 volt off the car to
120 volt AC. And right now, it's running on my 12 volt battery. If I probe it...
Does it matter which wire I connect to?
Nope, because it's not connected to the mains anymore. So
there we go. See this is the waveform that my inverter is outputting, and my meter,
if you can see, still reads around 120 volt AC RMS, although the peak of this waveform is only around
140 volt, compared to the peak of the sine wave, which was
170 volts. This waveform, or the
city sine wave, or just 120 volt DC, can output the same power over the same resistor because their RMS values are the same.
Of course, the RMS of DC is the same as the DC value. Now, we are pretty much done with AC. But wait,
there's more! We don't have just AC or just DC. In fact, in almost all cases,
we have some of both added. An AC waveform, like this, can have a DC added to it, like this, that just
shifts it up and down.
It's like adding a fixed number to the AC wave. Now when you add the AC and DC together, the total
RMS is NOT the sum of the AC RMS plus the DC value.
It was a mistake I made a while back.
Made a bunch of smoke. The total RMS is how do you read this?
It is the square root of the AC RMS squared plus the DC squared.
Whatever. I'm done. But wait, there's more!!
Let me give you a general idea of what DC and AC are good for. DC is a constant.
It's ideal for power delivery to things like
circuits or lights or
DC motors and such, or to set or bias a circuit in a known state. But AC, it is also great for power
delivery. It is needed for power conversion. It can carry valuable information.
It can transfer power or information over wire or air. It is used for computing
complex information... AC is the base of every technology out there while DC is powering them.
Yep, now we're really done.
RECESS!
(beatboxing and dancing)
unibrow dance