Newbie question re 2N2222 NPN transistor

Discussion in 'General Electronics Discussion' started by cadmus98, Aug 27, 2011.

1. cadmus98

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Aug 27, 2011
Hi,

I'm teaching myself electronics and am working through a book called "Make: Electronics". I'm a total beginner on this subject and am just starting to learn about transistors.

In a chapter on transistor switching it says, "...initially you just need to know that in this type of transistor [2N2222], the collector receives current, the base controls it, and the emitter sends it out."

On my breadboard, which is following the circuit diagram in the book, the 2N2222 is wired up so that the collector is connected to the more positive side of a series circuit, and the emitter the more negative side.

I was wondering why this was, because I was thinking that if the collector's role is to take in current and the emitter's role is to output it, then the collector should be wired to the more negative side and the emitter more positive. This because of direction of current flow, negative to positive. My LED is wired in like this, for example.

Can someone please tell me in very basic terms why this isn't so?

Tom

1,068
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Apr 8, 2011
3. jackorocko

1,284
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Apr 4, 2010
Last edited: Aug 28, 2011
4. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,449
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Jan 21, 2010
cadmus98, you'll find that the "arrows" shown on semiconductors show conventional current flow. That is, they show the direction of the current flowing from +ve to -ve.

This applies to LEDs too. If you check the datasheet or whatever to determine the anode and cathode end of your LED, you'll find the arrow part of the symbol points from +ve to -ve (or at least it does if the LED is to illuminate).

OK, we know that electrons move the other way, but when current was first considered we did not. It only became obvious when the first valve was manufactured. By then, the convention of current flowing from +ve to -ve was well established. In addition, it rarely makes any difference to your consideration except in some exotic situations where electrons can do things that holes cannot (thermionic valves are an example).

Bipolar transistors come in two varieties, PNP and NPN. THe polarity of all the voltages is reversed when comparing them. Your understanding works correctly for PNP transistors. If you draw one, you'll note that the electron flow is against the arrows though.

5. cadmus98

4
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Aug 27, 2011
Maybe. That's just a schematic thing, yeah? I read a textbook which said something to the effect that older diagrams were drawn with current flow from + to - and we call this conventional current flow, while on newer diagrams current flow is from - to +, which we call electron current flow.

If I understand rightly, electrons are said to have a negative charge and flow from negatively charged ions that have an excess of them (eg., my breadboard's negatively charged rail) to positively charged ions which have a deficiency of them (eg., my breadboard's positive rail).

It makes sense to me that my LED is wired up so that the negative lead is hooked into the side closer to the negative rail and the positive lead to the positive rail; as the current flow is from the negative rail to the positive rail, it flows through the LED from the negative lead to the positive.

I don't understand then why it is that my transistor's collector lead is wired into the more positive side of the same circuit, and the emitter the negative. Doesn't that imply that current is flowing both forwards and backwards in my circuit? Why doesn't it collect electrons from the side of the circuit they are coming from, and emit them to the side of the circuit they are going to?

Last edited: Aug 28, 2011
6. cadmus98

4
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Aug 27, 2011
Would this mean that if I am considering a circuit to follow electron current flow rather than conventional current flow, that the terms 'collector' and 'emitter' are back to front when using an NPN like the 2N2222?

Yeah, I learnt that about the diode. I'm starting to think of the markings more as trumpets than backwards arrows .

7. davennModerator

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Sep 5, 2009
DONT you will only confuse yourself. use conventional current flow and follow the
arrows on the device

Dave

8. jackorocko

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Apr 4, 2010
You do realize that electron flow and current flow are not the same thing. Energy is instant while electrons move at an agonizingly slow pace.

Electrons move from neg to pos but current flow is from pos to neg.

Google these, electron flow, hole flow, current flow

Try reading this: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/intrin.html

something has to make sense to you at some point.

Last edited: Aug 29, 2011
9. cadmus98

4
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Aug 27, 2011
Do you mean something different than conventional vs electron flow? (I know the difference between these two).

If you do, then no I did not know that.

10. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,449
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Jan 21, 2010
"Current" (by which we mean the movement of charge) happens at what is essentially the speed of light in that medium. OK, so current doesn't flow in glass, and copper is opaque, but there is an equivalent concept.

Electrons, on the other hand, move at a snail's pace.

When we're talking about electrons moving, they only go one way because they're physical objects (well, kinda).

When we refer to the movement of a charge, we can move it in either direction because it is a representation of a depletion or excess of electrons.

So, whilst when we talk about "current" flowing from +ve to -ve, what we are really doing is talking about the movement of charge, and for that, movement from +ve to -ve (conventional) is just as valid as -ve to +ve (electron). In neither case does the current actually measure the flow of electrons.

11. jackorocko

1,284
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Apr 4, 2010
Thanks for putting that into layman's terms. OP, reread the bold statement about 25 times until it sinks in.

Last edited: Aug 29, 2011
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