If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:7:26

Video transcript

- [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.