So it appears I know how a lot of things work but I can never actually MAKE them work when I build them. Here's about the most simple NPN switching circuit you could imagine. I was just fooling around with this Breadboard Companion(tm) thing which has a 555 IC on it (which I might use for a PWM circuit) which controls an output pin, called Clk, which has a duty cycled controlled by a trim pot on the board. Here's my circuit: Code: 470 LED |----/\/\/\/------>|--------||| Clk | 5v 5k C / o-----------/\/\/\/-----------B| E \ | | | o 12v See anything wrong with it? No? Yeah, neither do I. The LED switches on and off just fine, with the exact same duty cycle as the LED on the companion PCB. But here's what I don't get - The voltage drop across the NPN transistor is around 8 or 9 volts...leaving the collector output to be only 3 to 4 volts. What's with the huge voltage drop? The base voltage input voltage is around 3-5v and the emitter input is always 12v. How the hell does the collector output manage to be less than the base input? I thought these things were supposed to amplify. I've been using 2N3904s, 2N333s and a dozen other NPN transistors that have been festering in my basement for some 20 or 30 years. Most of them work, but they all act the same way. Now maybe I'm missing some very important concept about NPN transistors here, but I'm quite confused as to why this isn't working how I think it should be working. I even flipped the transistor, reversing C and E, which of course made it act funny and get quite warm quite quickly. So...what am I doing wrong here? I've ruled out that it's the transistor itself and I've tried many input voltages (used a trim pot) so the only thing I have left to blame it on is my incompetance. This isn't a practical circuit for anything but I'd like to know what I'm doing wrong before I go trying to design something useful...thanks.
You're basically switching the transistor on and off and the LED is happy because it gets to steal a little power and turn itself on, using a few volts and leaving your voltage drop over the transistor being 8 or 9v. What I don't understand is why your emitter is at 12v all the time. I'd expect that to be 0v and tied to ground with the collector being at 12v and then using the transistor to switch the current on and off and using an emitter follower to power the LED. hmmm funkeh. I'm not as good with transistors as I thought I was. Now I'm confused Someone help me out here!
The transistor's upside down in your drawing if it's NPN. Swap 12V & 0V (and turn the led round). That spare should almost all be across the LED resistor, under 0.4V across the transistor if the LED is on the collector side. If the transistor has been connected wrong, it could be b0rked. However, if the transistor was connected emitter-follower, the emitter voltage would be (5V - 0.7V - Voltage drop over 5k base resistor), whatever the collector supply, so with 12V supply and 5V clock the transistor would drop over 7.7V. Guess that's what you've done. See here.
Remember, The easiest way to remember transistor directionality is that the collector collects energy, and the emitter emits it. (ignoring the fact thaty electrons really flow from negative to positive, It's just like the arrow on a diode.)
Whatever the actual case, there's a shift in positive charge in the conventional way we're used to edit: BTW I think the dude just flipped his schemmo around (and therefore got C and E mixed up)
What you show is an emitter follower circuit, but (as cpemma already mentioned) with C and E switched on the transistor, so it probably has a poor gain. Your high voltage drop on the transistor is normal for an emitter follower because the load (LED and resistor in series) is always one diode drop (0.7V) below the base voltage. If you have 5V on the base you would have 5V-0.7V=4.3V on the load. That leaves 12V-4.3V=7.7V drop on the transistor. Because of the poor gain your base current may be considerable (several mA) so the 5kOhm resistor will lower the actual base voltage even more.
I thought the collector was called the collector because it collected the electrons emitted from the emitter inside the transistor? Or maybe I'm confusing transistors with tubes. Anyways, thanks for the help guys, I think I've got it now. I did a lot of reading on various sites, Wikipedia was especially good, but I could only learn how things work, I could never seem to find a page that says: HEY, HERE IS HOW YOU MAKE A DAMN SWITCH. Apparently I didn't check the electronics thread...thanks for that, by the way, cpemma.
You were entirely right. Emitter emits eletctrons (or holes in the case of PNP) and collector collects them.
A fact that goes completely against conventional wisdom, though it is correct. In reality, the emitter actually collects electrons, and the collector emits them, at least with NPN transistors, which account for the overwhelming majority of transistors that are made. Transistor terminology was apparently written by someone who did not understand the difference between electron flow and conventional energy flow (search for it, there are a couple of threads about it). Just think about the issue a little less, and you'll be fine.
Wolfe is right. The less you think about it the more sense it will make. Be grateful you don't work in telecommunications. all our power is positively grounded.
