- [Voiceover] So, the problem
with allowing this spark to happen across here is if
this is not a mechanical switch, we can build switches out of
electronic devices, as well. But this is what we use transistors for. And a transistor is a rather
small, delicate device. And so, if I go in here
and I somehow cause a huge spike in voltage to
occur, very, very likely I'm gonna destroy that
transistor that's used in this position right here
as an electronic switch. So, the way to deal with
this is to provide a place for this current to go. Here's this current that's
flowing through this conductor, and the magnetic energy that's
stored around this inductor is gonna force that current to flow, even if we open this switch. And in the case here we saw when we opened the mechanical switch, it
sparks right across there, so the inductor wins,
the inductor current wins in that case. And it can be very damaging to whatever is in this position down here. The best way to solve this is
with a device called a diode. And this is another semiconductor device, it's not a switch, but a diode has, a diode has an IV curve. Here's V, here's I. And it looks like this. The current, I'm just gonna sketch this, the current is zero when
the voltage on the diode is negative. And it's somewhere around
here that the current goes up like that. So, when we have a diode, this
is the symbol for a diode, like that, and that's plus and minus V. And this is the current through the diode. So, positive current and
positive voltage is in this part of the graph over here, and
when I have a positive voltage, this is about .6 volts,
something like that, could be .5, could be .7, when
you have a positive voltage on this, then the current starts to flow freely through this. If this voltage goes
negative, so that means that this voltage is higher than this voltage, then the current actually
goes pretty much to zero, very, very small value. So, I'm gonna take advantage
of this diode device to help me with this
problem I have over here with the current, and the
way we do that is we do this. We hook up a diode pointing
in this direction here. Pretty distinctive. But anytime you have a circuit
that has an inductor in it, so it's a solenoid, or
a motor, or a relay, this is a way you can protect the devices driving your coil. So, let's look at what
happens here when we push this push button down and let it go again. When we started out, we
had no current flowing, the push button was open,
so there's no current going through here, and
there's no voltage difference across here, both these
points are at three volts. And so, there's no
voltage across the diode. And that's that point right
there, no current, no voltage. So, all the currents are
zero, and the voltage across the diode is zero, so there's no current. Nothing's happening. Now we push the button down. And a current starts to
flow through the inductor, and there's three volts
across the inductor. So, let's look at that,
there's plus three volts here, and there's zero volts down here. So, there's three volts
on this side of the diode, this is the side where it doesn't conduct. That three volts represents
negative three volts across the diode. Let me mark the diode voltage on here. Here's minus, plus, V diode. And we'll call this V diode. So, this is a V diode of negative three, so we're operating over here
out at negative three volts on the diode, and the current
through the diode is zero, there's no current flowing over here. All of this current is going down and going through the
inductor, and doing whatever this inductor is supposed to do. Okay, now we open the switch. Now we open the switch. And as you recall a minute
ago, what happened was this voltage right here,
this voltage went big. This went plus big, and we had like, 100,000 volts or something. That's where it was headed. Well, we're not gonna let it get there, so I'm gonna take that away. So, this voltage here, as
soon as this switch opens, this voltage right
here, this is headed up. This is going up, hard. But we're not gonna let it get very high, and that's the job of this diode. So, as soon as this
voltage here gets to about 6/10ths of a volt higher than right here. So, when this voltage gets to 3.6 volts, what that means is that the V diode... Now equals about .6 volts, right? This is at three volts. This is at 3.6 volts,
so V diode is .6 volts. And that puts us about here on the curve. And what's gonna happen is
this current that we had here, this current that was flowing
down through this inductor is gonna do what? It's gonna go this way. It's gonna flow this
way through the diode, and that's this portion
of the curve right here. And it'll keep going like that. Look at that, we've
provided a current path for this inductor that's not
gonna destroy what's down here. The highest voltage this will get to is whatever voltage it climbs
to on this diode curve here, which is gonna be between
.6 and point, say .8 volts. So, the highest this
is gonna get is point, 3.6 to 3.8 volts. And typically, and typically
a transistor down here will easily withstand
this kind of a voltage on its terminals. So, this is called, the
effect of having this voltage go way north is called kickback. That's kickback from an inductor. This diode is a protection diode, and basically what it does
is it supplies a pathway for that inductor current
when this voltage goes above the top end of the inductor. So, whenever you build something
that has an inductor in it, and it could be, you could build a circuit that drives a motor, or an actuator, or a solenoid, or something like that, or part of a robot, this
is one of the circuits you wanna keep in mind. Whenever you have an inductor
and you're gonna switch it on and off, you basically,
you wanna design in a diode like this to protect
your circuit from those unstoppable inductor currents.