Hi, today we're going to talk about diodes.
Now, I'm going to try to do my best to not get into the weeds too much on this and just
give a conceptual overview of what diodes are, why they're important, some important
aspects of the things like that.
And so hopefully Taylor will smack me around if I start getting too into the weeds.
And as it is, let's get right into it.
So a diode at its most basic is basically a one way valve for electricity.
So much like if you have a tube of water that has a flapper on it, and the water flows this
way, it pushes the flapper open and can flow just fine that way, but then comes back and
it closes the flapper and can't go back that way.
A diode does the same thing with electricity, it's pretty cool because in the way we model,
it in the way we think about it, ideally, if you have a voltage over it, a voltage that's
positive on the anode and negative on the cathode, that will make it so current can
just flow no problems, no inhibitions whatsoever.
Whereas ideally, if you have a positive voltage on the cathode and a negative voltage on anode,
it will block all current immediately.
And so in an ideal model, it'll look basically like this, your IV curve will show nothing
in the negative side and then an immediate infinite current, as soon as you hit anything
above zero volts as a positive voltage.
Now, the reality is, it doesn't quite work like that.
Reality is when you do have a reverse voltage, there's a little bit of leakage current, kind
of thinking of that flapper, probably have some water leaking out through the sides,
that's true.
Also, and we'll get into this a little bit more later.
If you have enough force pushing backwards, that flapper will break.
And you'll actually get a huge rush of current in the negative direction.
But even besides that, as we're going from that negative voltage, and we hit zero volts
and then start going above zero volts, there's actually little bit of pressure that needs
to be overcome before it can conduct in the forward direction.
So just as if that flapper in our model, needs a little bit of pressure to push it up and
out of the way, it's the same thing with a diode.
Now that forward pressure that forward voltage as it's called V's of f.
It depends on what diode you're working with.
So the normal typical diode is point seven volts, but you can get down to about point
three volts with the Schottky diode, and you can get quite a bit higher than one to four
to whatever volts if you're looking at a power diode or an LED, which is a subset of diodes.
So in reality, you have this kind of weird curve, that it's a slight negative current
when you have a negative voltage and then at zero volts, it's starts to slightly conduct
in the forward direction.
And then once you hit that V's of F, you have a not immediately infinite but highly spiking
current in the forward direction.
And that is a bit complicated.
It's hard to work with.
So usually when we're dealing with diodes, we'll deal with it in one of two models.
And that is either the assumption that there's absolutely no current anything below zero
forward volts and infinite current at anything above zero volts.
So to create a compromise between the ideal model, which is pretty unrealistic, and the
actual way that the diode works, which is pretty complicated, we also work with the
constant voltage diode.
Which is a compromise of sorts in that you still assume very simplistically that there
is absolutely no current from any negative voltage up to the forward voltage.
And then at that forward voltage, it immediately spikes to infinity.
So in reality, the only thing we're doing with the ideal model is shifting the point
in which the current goes from zero to infinity to the more realistic forward voltage that
you're actually expecting.
So one of the challenges about dealing with diodes like this is that it's nonlinear.
So if you're watching this video, most likely you are familiar with at least the basic of
circuits.
You've dealt with resistors capacitors and inductors and DC or AC whether or not you're
taking impedance, reactance, that sort of thing into account.
Those can all be modeled linearly.
And that is not something you can do with the diode.
So instead of just being able to say, Hey, I have this diode in here, and it's represented
by this number, you actually have to look at your circuit and make some assumptions,
saying, I think that this diode is going to be forward biased, the with the way all of
the voltages are going, I think that voltage is going to be higher there than right there.
And so let's assume that's forward biased.
And then that means either it's a short, or we're just going to act like there's a point
seven volt drop across it.
And then you have to solve everything.
And then you have to go back and check and say, Oh, actually, that was a negative voltage
across that, which means that that is reverse biased, which now I have to assume that is
an open and I have to redo the entire circuit.
So, it gets a little bit more complicated in that, unlike other things, you have to
make an assumption, solve everything, check your assumption, and if it's not right, change
your assumptions and redo it.
And you can do it multiple times.
And it can be a little bit frustrating, but once you get better with circuits and you
get more comfortable with it, it's going to become easier, but that is one of the biggest
challenges of working with diodes.
Even though conceptually they're pretty simple.
And you don't have some of the complications that you do with impedance and reactance,
you still have that nonlinearity that you have to deal with.
So that makes it pretty fun.
So let's go into a couple of things.
I have a couple different diodes here to one show how they look almost the exact same,
even though they're completely different.
And then I also want to go over a couple of the most important names... attributes!
That's the word I was looking for most important attributes of a diode.
So when you're looking at a data sheet, or you're trying to figure out if something's
going to work, make sure it actually does work for your circuit.
So first off, I just want to show right here I have a regular power diode.
And I have, I think, and this is part of the challenge here.
One of these is a Schottky diodes, I think these are Schottky diodes.
These are Zener diodes, and then these are lower power or signal diodes.
But if you look at them, they all look basically the same.
They're just slightly different sizes.
So it's incredibly important to note that there is writing on here, and we'll get some
close up shots of it.
And that is how you're going to know what exactly you're working with.
Because otherwise, if I don't put these back in the right bags, I'm going to be incredibly
confused later.
Right here I've got my general purpose diodes.
And these are something I'd use in a typical wave rectifier that's low power, something
that I only have a couple of milliamps.
Or probably could go up to an amp on these, I'd have to look up the datasheet, whereas
this one, I think, is a couple of amps.
So that's a general purpose versus power diode.
Now, as you look at these, these look almost the exact same, but they do completely different
things.
This is a Schottky diodes and these are Zener diodes.
And we've done some tutorials on Zener diodes, and they're pretty interesting.
So go and check those out, but we haven't done anything on Schottky diodes, but the
biggest thing I want to point out here is even though there are similarities in the
way that they're built, and there's a lot of physical similarities looking at it, you
need to read the fine print on the side to know exactly what they are.
And please be careful because they act very differently.
And if you put in a Schottky diode, when you mean to put an Zener diode things are going
to get really ugly.
So double check all that stuff.
If you want to learn more information about that, we're not covering that in this video.
So go look somewhere else.
But the biggest thing to note between all these is all diodes are basically two port
devices, you have one side where it goes in, one side where it goes out.
And also on all of these, despite being different sizes, all of them have a white band on one
side and that denotes the cathode.
So as we talked about that earlier, if this is forward biased, you will have the higher
voltage on this side and the lower voltage on this side.
Whereas if you go the other way, have higher voltage on the side, lower voltage on this
one side so current can only flow in that direction.
Okay, so let's go over a couple of the terms that you're going to need to know working
with these diodes.
And the first one is the forward voltage and that is basically knowing at what point when
you forward bias that diode it starts to conduct and that's really important.
Now the interesting thing is you also need to know the reverse voltage V's of R and you
think Well, what does that mean?
Because itt shouldn't be conducting it all in the back in the other direction.
But in reality, it does.
And that as I was talking about earlier, that's when the breakdown happens, that's when you
do so much pressure in the opposite direction that you break down and you start conducting
in the reverse direction.
So forward voltage, you want that to be as small as possible, typically, and reverse
voltage, you want that to be as big as possible.
Typically, unless you're working with Zener diodes.
Again, go check out our tutorials on Zener diodes, if you want to know what I'm talking
about with that.
And then really the only two big things that you really should know is related to those
and that's the forward current is knowing how much current you can put through it without
it burning out.
Like I mentioned earlier, this one is big, and it's a power diode.
And the point is, is that it can take several amps instead of just a couple hundred milliamps,
or just one amp.
Whereas if you get small signal diodes, they can only handle a couple of milliamps before
you burn it out.
So, again, ideally, in a perfect world, a diode can conduct as much current as you possibly
want but in reality, of course, that is not the case.
And just as we assume that there's absolutely no current when you are reverse biased, there
is.
And so we need to know that reverse leakage current or the reverse current.
And that's basically just what happens when you have a reverse bias on a diode, how much
of it is leaking through and all of these things, forward voltage, forward current,
reverse, those are trade offs.
If you want, what a better thing of one, if you want a better forward voltage, that usually
means that you don't have as good of a forward current and things like that.
So there's no such thing as the perfect diode.
It's all about trade offs and what you need for your application.
Okay, I think I went way too in depth on that.
And I'm blaming Taylor on this.
He was supposed to be the one to smack me down, if I went off rambling, and he didn't.
So this is all your fault, Taylor.
If you have any questions, we've done a lot of tutorials on pn junctions and semiconductor
devices, which explains how these things work.
So go check those out.
If those questions aren't answered by the tutorials, we already have jumped on the circuitbread.com,
leave a comment on the tutorial, leave it in YouTube or whatever and we'll get back
to you as soon as we can.
Hope you enjoyed this video hope it was interesting and informative.
If you liked it, give the video a like, subscribe your channel, all that good jazz and we will
see you in the next one.