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  • Okay.

  • In the last video, we saw how a transistor is gonna try to There's gonna allow a collector current called.

  • I said, see, you allow a collector current to flow that is equal to the base current.

  • That that may happen to be flowing Isobe times some large number, like 100.

  • So it's gonna allow this collector current is large collector current to a flow that is 100 times whatever the base current that's flowing.

  • And so let's we want to use that in this circuit that we have here.

  • So the circuit we have that switching way have some sort of signal coming in here that switching from zero volts to five volts.

  • Ah, but it's not enough current here to power and led adequately.

  • So if I go and I look at the current, uh, if I switch my my multi meter over here to look at current and I look at the current here where the led would go, um, I need to be on DC milli amps.

  • You can see it's switching from 2.3 millions, Not not a whole lot.

  • So I have this 0.3 million current that switching on and off, and I want to use that to driving led.

  • But of course, if I put the led directly in there as we saw before, the lady is not very bright at all, because we just don't have enough current to drive it so But we can take that current.

  • And we can run that current through this transistor from the base to the emitter.

  • And the transistor will then allow a current to flow from the collector to the emitter.

  • That is 100 times that which you know a point.

  • I guess this is This is Ah.

  • In our case, this would be 0.3 million pes.

  • And so it would allow a collector current that is up to, uh, 30 million amps to flow.

  • Um, when this space current is flowing and of course, an led requires, um, you know, typically 20 million pes.

  • So this collector current, you know, the transistor would allow plenty of current to flow more than the led needs, so the transistor would go into saturation, basically, just allow current to flow through it, um, without without impeding it at all.

  • So if we come back to our circuit here.

  • Let's look at just the basic circuit we had, which was a down here we have a five old supply that is able to provide plenty of current, um, with the five volts going through the led through the resistor and then back to ground in the conventional flow.

  • So if I want to put a transistor into this circuit that will switch this circuit on and off based on the current that may or may not be flowing on this signal wires, let's see how we would do that.

  • So they come back to look at my schematic here.

  • We've got the transistor and the transistors going to switch the current flow on or off, depending on this base current.

  • And so I just need to put it in Siri's with the the led circuit so I can have my five old supply up here and actually let me do this.

  • Let me just do this in yellow.

  • I could do my five old supply up here, and then I'm going through my led and then I have my resist er, of course, because I'm I always normally want a resistor with an led, and this resistor happens to be 220 homes.

  • And then I will take this and go through the transistor before I go to ground.

  • And so right now, if I look at the circuit that I have hooked up, I have this part and it hooks directly to ground, So the transistor is not there.

  • So what I what I want to do is insert this transistor so that the transistor can then switch this circuit on and off.

  • So when the bass currents flowing, the transistor will allow current to flow through the circuit.

  • And when the bass currents not there when this voltage isn't there, then the transistor will not allow this to flow in the led will turn off into the led will, then give us an indication of whether the base current is flowing or not.

  • And because the transistor allows, um, lots of current to flow through the collector to admit her the led should be nice and bright.

  • So let me just hook this up, and then we'll come back to figure out how we get how we get this base current flowing.

  • So over here I have my circuit and basically what I want to do is I want to hook the transistor up, um, somewhere in in the circuit.

  • And so if I look at my transistor, the left lead is the emitter.

  • The middle is the base, the right is the collector.

  • And so in this case, I want the emitter which is on the left, to be connected to my ground.

  • The base will will leave unconnected.

  • We'll deal with that in in a moment.

  • And then the collector I want connected to the resister which goes to the led, which goes to the positive voltage.

  • So really, what?

  • I'm what I'm doing is I just wantto break my circuit here, put the transistor in and then connect the ground over here.

  • So now what we have is we have we started, started the top here we started the positive.

  • Here we have positive voltage coming in, going through the led, going through the resistor, going into the collector of the transistor.

  • The base, of course, is unconnected right now and then the emitter of the transistor then goes back to ground.

  • And so if we come back, we look at our circuit.

  • This is exactly what we have hooked up right now.

  • And of course, the led you saw was off right now.

  • And that's because there's no current flowing from the base to the emitter.

  • So how do we get a current flowing from the base to the emitter?

  • Well, we have that that five volt, Um uh, signal that we're that we've been playing with, um that is able to give us a small amount of current.

  • So let's just hook that up.

  • So we have our little mystery circuit over here, and it has that five volts signal coming to us, and then it also has its own ground, which will just go and connect to our ground over there.

  • So basically, we'll just connect this mystery signal between the base and the emitter like this, and it should, when it's when it is on current will flow from the base to the emitter in this direction.

  • And when that current is flowing, it will cause more current to flow up to 100 times more current to flow from the collector to the emitter.

  • And that should allow current to flow through our led and resistor.

  • And we should see the led come one.

  • So let's give it a try.

  • So again we'll hook the ground part of our little signal up to our ground and also up to the emitter.

  • And then we want the signal to flow from the base to the emitter.

  • So we'll just hook this up to the base and you can already see.

  • The led is flashing and it's flashing quite brightly and again.

  • That's because the current flow through the led is coming from our nice five volt power supply through the led through the resistor through the transistor, but only when the transistor is also seeing a small amount of current flowing from its base to its emitter.

  • And so hopefully you can see all of that.

  • I'll zoom in just a little bit in case you can't just to take a closer look at the circuit when you slide things over here so you can see.

  • And of course, I'll get rid of, well, actually get rid of all the stuff we had up on the top here, just so we aren't confused with what we're looking at and zoom in a little bit more.

  • And so what's going on just to just to be just to clarify.

  • Here we've got the voltage coming in is going through the led through the resistor into the collector of the transistor.

  • And then when the transistor is on, it will switch.

  • It'll allow that current to flow from the collector, which is the right pin to the emitter, which is the left pin, which is then connected to our ground.

  • So that completes the circuit.

  • Um, and then the led is on when current is flowing from the base to the emitter.

  • So when there's current from going through this signal, then it turns the led on.

  • And so here we're basically allowing our signal to turn.

  • This led on and off, even though this signal originally was not giving us enough.

  • Current are still not giving us enough current to actually turn the led on, and hopefully this makes sense why transistors are often called current amplifiers because we're in some sense, amplifying the current that we're getting from here to.

  • Ah, much larger value of current that's going through the led now, and also why transistors might be referred to a switch is because we're using or, more specifically, transistors are often referred to his current controlled switches, and so we have a small current here that's controlling the transistor, which is switching on and off are led.



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B1 中級

用半導體來解決我們的問題|數字電子技術(10的8個問題 (Using a transistor to solve our problem | Digital electronics (8 of 10))

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    林宜悉 發佈於 2021 年 01 月 14 日