Voltage Across NPN BJT With Inverted LED Output Oscilloscope Measured By Electronzap Electronics

So now, in this video we're gonna look at the voltage across the transistor again. But last time we looked at it pretty much as a switch. So it wasn't quite an on/off switch. It could go completely on and completely off, but there's also a middle ground, where the LED was kind of dim.

So now we have another LED here. And so this led is actually going to be on fully when the transistor is off fully. And when the transistor is on fully then the LED will be off fully. So the LED is going to be inverted from. What the transistor is doing, but again, we're interested in the voltage across the transistor. So zooming in, you can see the trim pot right? There, there's, a resistive element from the top pin to the bottom.

Pin I pretty much assume. You know, this then there's a middle pin that's, a wiper that slides across the resistive element as I turn this. But the wiper connects right there. We have a 10 kilohm resistor to the 10 kilohm trim pot. So take 2 ohm resistors going to the base of a transistor is that.

Usually a pretty good resistance to use right there so that's the middle pin of this 2n 3904. So the left pin is the emitter middle pin base, reg 10 collectors, if I turn it this way, it's like the schematic there emitter at the bottom base in the middle collector on top. So I'll put the base to the 10 kilohm resistor. Emitter is down there up here.

We have the LED so that's the line lead. The anode of the LED to the collector short lead. The cathode is going to ground right there. Just as the emitter. We. Also have this jumper here, which goes to the cathode of the red LED short lead the anode, the line lead of the LED up.

One spot is going to a 1 kilo, ohm resistor. So that we can go up to about 12 volts if we want to there's a resistor they're going to VCC and I actually have the power supply at 9 volts right now. So now we are ready to take our measurement. But before we do let's, look at the power supply. So it's off right now.

That's. Why both LEDs are off. And what we're going to do first is limit. The. Current that this outputs from sixty William Center to let's go to 20 because that's all the LEDs can handle and almost all the current will be going through the LEDs.

So we got twenty. We don't have to worry about burning out. The LEDs, if we had, twenty William saw that's a good indication that we are drawing more current than we expect. Just the power supply is limiting it.

So any case we got the nine volts there if I hit the power button. Now you can see both LEDs turned on that's, because the. Transistor is off. So the green LED is on the transistor is off. So now the transistor is on.

And the green LED is off the main thing you'll. Notice, here we have seven William with the green LED off. And when the green LED turns on, we actually have less current so that's because the current going through the LED was going through the transistor earlier, and it was having a much easier time. So a more current could flow through altogether right there. So we can also raise this to 12 volts since we have. A 1 kilo, ohm resistor, protecting both of the LEDs in this case.

And again, we have 7 mil amps there with the green LED lit and 11 when the green LED is off so let's, zoom in and I, look at the voltages. So we have an idea what happens based on the LEDs lighting up we're gonna look at the voltage across the transistor. So we can put the blue one anywhere along the negative rail there. We could put it to the jumper, put it there we're going to set it down there. So that it's out of the way and then the red.Once I'm going to put to where the collector of the transistor and the anode, the lying lead of the green LED Connect, right there. And there you can see there is no voltage so that's, why the green LED is lit up.

Current cannot go through the resistor that led it does go through there as you can see, it's lit up. But then it goes right to ground, it's, a direct connection to ground through the transistor. Current cannot go through the green LED let's, turn the trim pot. So we got to get less than 0.7 volts.

To the base of the transistor. And now the green LED turned on, and you can see that we have about three volts across the LED, which makes sense it's, a green LED. If we go across both the red LED and the green LED, unfortunately, we knocked it out I. Think it'd be easier just to grab one of these make sure I put it in the right way. We should have five volts right there as you can see. And we could also look I don't want to go in there, though we could look at the voltage across the resistor.

If we wanted. To so those videos about voltage I'll do extra voltage measurements. So I'm just going across the resistor, not the resistor in LED there you go. So we got a little more than seven if I go across the resistor and LED. You can see that it is higher right there. So any case may take away though is the transistor, so I went back to ground, but with the red jumper, the wrong jumper. And these green LEDs are really bright it's kind of blinding me there.

We go much better, so we'll, go back and look at those. Voltages again to the collector of the transistor and then the ground with that one. So again, it's almost exactly zero. If I lower the voltage to the base. Now, the transistor is off completely, and it's, just the voltage built up across the green LED.

And if I turn this up so that the transistor is conducting as well as it Kim there's, just a slight a voltage build up there, but that practically none and nowhere near the three volts needed to get the green LED lit up. So any case this is one of my. Voltage videos so I'm to be a little obsessive about voltage hope you still enjoyed though check out one of the other videos that are posted click like subscribe, the belle-isle that I put links in the description, check them out I, donate to Patreon. If you can, that helped out the most make sure you turn off your meters and stuff when you're done -, and the power I will see ya in the next video.