Inductor kickback 2 of 2

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. This is what we use transistors for, and a transistor is a rather small, delicate device.

So if I go in here and I somehow cause a huge spike in voltage to occur, very, very likely I'm going to destroy that transistor that's used in this position right here as an electronic switch. 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 inductor, and the magnetic energy that's stored around this inductor is going to force that current to flow even if we open this switch. In the case here, we saw when we open a mechanical switch, it sparks right across there. 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 an IV curve. Here's V, here's I, and it looks like this. The current—I'm just going to sketch this—the current is zero when the voltage on the diode is negative, and somewhere around here, 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. When I have a positive voltage, this is about 0.6 volts; something like that could be 0.5, could be 0.7. 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—a very, very small value. So I'm going to take advantage of this diode device to help me with this problem I have over here with the current.

The way we do that is 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. 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, 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 a hundred thousand volts or something; that's where it was headed. Well, we're not going to let it get there, so I'm going to take that away.

As soon as this switch opens, this voltage right here is headed up; this is going up hard. But we're not going to let it get very high, and that's the job of this diode. So as soon as this voltage here gets to about six tenths 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 0.6 volts, right?

This is at 3 volts, this is at 3.6 volts, so V diode is 0.6 volts, and that puts us about here on the curve. What's going to happen is this current that we had here, this current that was flowing down through this inductor, is going to do what? It's going to flow this way through the diode, and that's this portion of the curve right here.

It'll keep going like that; look at that! We provided a current path for this inductor that's not going to 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 going to be between 0.6 and, say, 0.8 volts.

So the highest this is going to get is 3.6 to 3.8 volts, and typically, a transistor down here will easily withstand this kind of voltage on its terminals. This is called the effect of having this voltage go way north; it's called kickback. That's kickback from an inductor; this diode is a protection diode.

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, it could be—you can 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 want to keep in mind whenever you have an inductor and you're going to switch it on and off. You basically want to design in a diode like this to protect your circuit from those unstoppable inductor currents.