(upbeat music)
- All right, today in our "Circuits 101" series,
we are going to be introduced to capacitors.
So capacitors are something you will see everywhere
and they can be used in a lot of different ways.
But first I'm gonna talk
about the more simple way that they're used in DC circuits.
Jump into the AC way that circuits are used
kind of provide what, for me,
is a good analogy of how they work.
And then we will jump
into how they're most commonly used in a circuit
with two circuit examples here, some practical examples.
So we'll jump into that at the end.
But first, let's talk about capacitors.
And capacitors are simply two plates
held very closely together
and then they act kind of as electrical storage.
And the interesting thing is that these capacitors
and in DC act as an open, you can put a DC voltage
over and no current will pass whatsoever.
So very briefly
I'd just like to say when you are doing a DC analysis
you can look at a capacitor and just say,
oh I can pull that out.
I don't even need to worry about that.
Anything in series with that capacitor, you can take it out.
There's no current flowing through it.
It won't have any effect on the voltage in a DC circuit.
And so, it actually simplifies the circuit quite a bit
by sometimes you're removing big swaths
of the circuit so you don't even have to analyze them.
And that's on the DC side.
And I'm not even going to do a circuit example
for that because it's so simple.
In a DC analysis, if you see a capacitor,
that and anything in series
with it can just get rid of and you are done.
Now with AC, capacitors and circuits
can act a little bit strange, and that is
because their impedance, their resistance changes depending
on the frequency and the capacitance of the capacitor.
And that is something we will get into
and also think about the different ramifications of that.
The impedance of capacitors can actually be modeled
with the equation or the number one J omega C.
And so the J is simply an imaginary term since I in,
electrical engineering is current, we use J instead
of I like mathematicians and other people do.
So that J is simply an imaginary term
and that will affect phase shift and the leading
and the lagging and all that sort of stuff
that we aren't going to worry about in this tutorial.
Even though on the practical example
you will see an example of it, but just right now know
that that has something to do with the phase
the power factor, all that stuff that we will get
into more when we are doing AC circuit analysis.
The omega is simply the frequency of the AC circuit,
and if you remember,
omega is simply two pi F or the frequency.
So omega is just yes, two pi F, there we go.
And then C obviously is the capacitance.
And so you can actually model
and when you're doing AC analysis,
you can model a capacitor with that one over J omega C.
Now if we just take that
and we look at it and we see omega remembering
that that's directly linearly related to frequency,
let's take a DC value zero and put it in there.
And then we see no matter how big
or how small the capacitor is
that impedance is going to be infinity
because it's one over J times zero times the capacitance.
So you can see right there
from the mathematical equation that ah,
it makes sense that at DC, impedance is infinity.
And of course math always describes reality
not reality com describing math or something like that.
That's kind of a pet peeve of mine.
But that is a great way to think of it intuitively.
If you ever forget like, wait, what?
How does this act?
Just remember one over J omega C.
If omega is very small or zero
then your impedance is gonna be incredibly high.
And on the inverse, if that omega is very high,
then no matter how big the capacitance is or whatever,
then your impedance is going to be very, very small.
And of course, they're interrelated
and it's that middle area where things get muddied
and things get a little bit more complicated.
But you can look at it with the extremes of DC,
you basically have impedance of infinity, infinite frequency
whatever that would be would mean zero impedance.
And then it's just
in the middle that you get this sliding scale
that you have to calculate to get more precise measurements.
So I've talked a little bit about how capacitors work
with DC and AC and how they can block DC
and pass through AC, but let's just think a little bit more
about what it is going back to that idea
of its simply two plates very, very close to each other.
Now what is happening, you can consider it this way
and there is gonna be some argument
and some people that may disagree with me
but this works as a metaphor or an analogy
and we won't worry about the physics of it
because I think this works fairly well.
But with those plates very, very close together
if you have a positive voltage and a negative voltage
then you're going to get a lot of electrons
on one side of the plate and a lot of positive charge
or lack of electrons on the other side of the plate
and they're kind of attracting each other
and they want to go past, but there's a gap
between them that they literally cannot jump.
But with this negative plate collecting the electrons
you can almost think of it as a bucket.
So if you have a circuit and it's going along
and you have a bucket, it can store that charge for you.
So this is a very interesting thing
because you'll hear leading and lagging sometimes
in AC circuits with capacitors and inductors
and they'll say that capacitors have a leading current.
So capacitive circuits lead with their current.
And for the longest time I just, I couldn't remember
and I'd have to either wrote, memorize it and hope
that it stayed in my brain long enough for a test
or just look it up every single time.
But finally, the way that it finally clicked
for me that I'm like, oh yes, this makes sense
is using water as an analogy.
So imagine you unhook the hose
from the end and suddenly the water starts rushing out
but there's actually still pressure in there
at first as the water starts rushing out
because it's still being squeezed in the bucket
and squeezing the hose and the current goes out.
And it isn't until the current flows
for a bit that the pressure starts to drop
or the voltage starts to drop
and the voltage drops after the current is going to drop.
So that is the best way to think of it
is that the voltage moves
after the current because you have that bucket
that what is literally just a plate that collects electrons
in our analogy is a bucket that can collect charge.
And so the water flows in, fills the bucket
and then comes up to pressure.
And then once it flows out
the water flows out and then the pressure drops.
So the voltage is behind the current meaning
that a capacitive circuit, the current leads the voltage.
And that's one of the reasons why it's confusing
because you'll hear that in a capacitive circuit,
it is a leading and you have to remember
that that's in reference to the current,
the current leads the voltage.
And so hopefully,
that analogy makes it a little bit more clear
and will help you understand that why
in a capacitive circuit you have the current first
and then the voltage first going in and then going out.
So that's an analogy of how it leads,
how the current leads the voltage.
But we can discuss very briefly
how a capacitor actually works
and why it doesn't let things pass through in DC,
but why it lets things pass through with AC.
And it's interesting because there's been a lot
of talk on this couple of years ago
I did a lot of research into this
and there were some people who I generally very much agreed
with but didn't in this case.
And that's totally fine.
I came up with a way that I'm not sure
if it's correct in a physics standpoint
but it makes sense to me
and it makes sense to my understanding
of how electricity works. So if it works for you, great.
If not, well, I'm sure you'll let me know.
The way I think of a capacitor is there's like a diaphragm
in the middle and when you have current flowing,
it kind of pushes and it pushes that diaphragm to the side
but it doesn't actually move anywhere.
You don't get any flow,
it's just kind of like trying to push.
But if you are to take that and pull that away
and then push from this side,
that diaphragm moves the other way.
And then if you go back and forth, back and forth,
even though no fluid is passing through that diaphragm,
it's wiggling back and forth and you're transferring power.
And so that's what I think,
with this you are not necessarily literally moving electrons
from one side of the capacitor to the other side
even though from a quantum mechanic's point
of view of view that totally can happen.
I think what happens is
that you get the current flowing one way
and you get that pressure pushing one way
and transferring that power through that diaphragm
and then it goes back the other way
in an AC circuit and pushes it back.
And so that's the way I visualize it
in my mind is basically you have something pushing one way
and then you reverse the voltage and it pushes the other way
and it's back and forth, back and forth.
And even though no electrons are passing through
you are still getting that transfer
of power from one side to the other.
Again, a physicist may walk in here
and smack me around, but it makes sense.
And as far as I can tell
it works for understanding how all of this works.
Okay, so let's talk about two configurations
that are most common with a capacitor.
And the first way is called
either blocking coupling or AC coupling.
And basically this is when you have a capacitor
in series with a load and then it blocks the DC
which is where it gets the blocking, or it couples the AC
which is where you can say, alright
I'm not totally connecting these two circuits
I'm only coupling the AC portion of it
because once you put that capacitor in series with it
it will stop all of the DC current
but it'll pass the AC current.
So that's one of the configuration.
The second way that you use a capacitor
is called bypassing shunt or decoupling capacitor.
And that is when you put the capacitor
in parallel with your load.
And basically that means it will take an AC signal
and shunt it away from your load
because if you have an AC signal, it will want to drop
through the capacitor versus going through your load.
And it decouples things
because it can remove the AC signal that would instead
be going to your load, it will be shunted to ground.
So that's something that I think is best illustrated
by showing a couple of circuit diagrams
and showing how it acts in real life.
So with that, I am going to do the first one first,
the AC coupling and going to show how
as you increase your frequency,
you will get more signal passed through.
And then I'll go over and check
out the decoupling or the shunt configuration
and we'll jump over to the CellScope right now and do that.
So let's start with the blocking
or coupling or AC coupling circuit.
So here we start with a capacitor in series with a resistor.
And as you can see
the signal has to flow through the capacitor
and then through the resistor to ground.
Now on my CellScope, you can see I have a yellow line
and that is basically the input to the capacitor.
And then my blue is actually my voltage
over the resistor or over the load.
And at the moment you can tell
that my frequency is about 10 kilohertz
which is close enough to DC
that you can only see the faintest of wiggles there.
Note that right now my divisions are both at one volt
so this is the same scale right here.
When we are doing other things,
we may not be able to see it,
we may need to change the scale
but here we are with a practically DC circuit
and we are not getting much through the capacitor
but let's immediately jump this
from 10 kilohertz up to 100 kilohertz.
All right and already you can see
a little bit more wiggle there
just it's passing a little bit, not a ton,
but you're getting a little bit.
But we will probably see a much bigger jump
because we're moving faster
from 100 kilohertz up to one megahertz.
So as we go up, ah
you're seeing those blue lines that is getting more defined
in the peak to peak voltage is changing
and getting much, much bigger.
So we are now at one megahertz and you can tell
that obviously we are getting, oh, what is that?
About a quarter, maybe a little bit less
of the voltage being passed through.
But as we go from one megahertz up to 10 megahertz,
we see it now it's about half.
So as we continue up, you will see that this gets closer
and closer to passing the entirety of the signal.
So we are now at 100 megahertz
and those are almost the exact same.
So right now at 100 megahertz,
we're basically treating this capacitor as a closed,
it's a a short circuit
but also when I was talking about the phase shift
and the leading in the lagging, you are seeing that now
because you are seeing they're not directly lined up.
And that is again
because the capacitor is charging in discharging
as we oscillate between positive and negative.
And that is causing a lag in the response.
And so this is very, very important in AC circuits.
And one of the main things you do is calculate
the phase shift from the peak of one to the next.
And that is something that we will get
into quite a bit in our circuits two series tutorials
but we aren't going to get into right now.
One last thing I'd like to go
over while discussing the block blocking or coupling
or AC coupling is right now I don't have any DC offset.
So let's take this offset and you'll see the input shifting
and you'll see the output staying basically the same.
So my input is now offset by two volts DC
but my output has remained completely unchanged.
I'm just having triggered troubles today
and if I shift down the other way,
I will once again mess up my trigger
but show how the output remains unchanged.
And again, that is exactly what you would expect
with this blocking where we block the DC
and AC coupling where we allow the AC to go through.
Alright, so let's now move on to the next one.
Okay, for the bypassing shunt or decoupling capacitor
what I have set up is instead of just your load
with the capacitor in series with it, I set
up a capacitor in series with another much smaller resistor
in parallel with the load resistor.
So in this case we have
that 100 omer resistor that is simply there
to make it so we don't get too much current
through the capacitor and burn it out.
But that is all it's doing is to limit that a little bit.
And many, many times you don't want that resistor
because you want as much current to flow
through that capacitor as possible.
Now I have it set up
so it's basically the same here we are measuring the input
into the load and then I'm measuring with the blue
the voltage over that 100 ohm bypass resistors just so
that we can measure to see how much is going through there.
And again, we're looking at about one megahertz
and we're not seeing too much going through this resistor
but as we go up, we'll notice something interesting
that even though I have a waveform generator
it won't be able to produce enough current
to maintain this two volt peak to peak.
So what will be happening
is as I am increasing the frequency
enough current will be shunted through this 100 ohm resistor
and this capacitor to cause the overall voltage to decrease.
And so we'll basically see this in action.
So right now we're at one megahertz
let's bump it up to 10 megahertz
and it so far seems to be doing pretty well.
But then once we jump it up to 20, still doing all right
now we're starting to see things get weird
and it's can no longer produce the current,
now at 60 megahertz jump up to 70
and it's dropping a huge amount.
So let's change the scale a little bit here.
So I'm gonna jump
that up to 200 millivolts per division instead
of two and then change our scale this direction.
So you can still see that it looks pretty clean
but it has served its purpose of making it
so that any current that would normally be going
through the or any high frequency
or voltage surges that would normally go
through the load are now going
through this bypassing or shunt capacitor.
And that is why you will see capacitors placed right next
to the power supply on ICs and things like that
because one it's not only helping
when there is a voltage drop
but if there's a voltage spike that will go through
and get shunted through the capacitor
to ground versus going into the load itself.
Okay, so that is an example
of the bypassing shunt or decoupling capacitor.
And we originally did the blocking
or coupling or AC coupling capacitor circuit and that is it.
That's all we need to worry about.
You've learned how
in a DC circuit you can just remove the capacitor
you don't even need to worry about it.
Hopefully gave you a couple of ideas
on how a capacitor works physically and hmm,
just some visualization,
so it's a little bit more intuitive for you
and hopefully the blocking and the shunting
and the AC coupling, all that stuff makes a lot more sense
since we did the physical real example.
If this was helpful, please give this video a like,
subscribe to our channel
and we will see you in the next one.
(upbeat music)
Hey, we hope you enjoyed the video.
Did you know that circuitbread.com has more than tutorials?
One of the other many things that we have
are several excellent open source textbooks
that benefit from our search tools, highlighting,
super fast page changes and keyboard friendly navigation.
Go check 'em out.
(upbeat music)