Let's Make Robots!

Transistor terminal naming conventions.

I am trying to figure out how transistors work and have seen the three terminals on a bipolar transistor labelled base, collector, and emitter.

I am having a hard time understanding the purpose of the three, and would like it if someone could explain why they are named this way. I.e. what is basey about the base and what is the collector collecting?

At the moment they just look like three randomly chosen words.


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Right, I have read and taken on board all the info from the comments.  Thanks a lot.

Here is what I plan to do. I want to turn a motor on and off using a Netduino.   I think the motor will work best with its own power supply, hence the transistor.

Could someone please take a look at the circuit diagram below, and let me know if it is suitable and point out all my stupid mistakes. 

I imagine it will probably need some resistors too, but I don't understand where they should be or which strengths.  I think the motor itself will behave as resistor.

The 6V battery will be made of four AAs.  I choose this after reading this by Krumlink: http://letsmakerobots.com/node/28427

I think all the grounds need to be connected.

I read somewhere that it was a good idea to use a diode to protect the transistor from some kind of reverse current.  But I didn't understand in which situations this was nessacery or where it should go.

I am concerned that the transistor says "Current Ic Max:0.1A". while my motor ranges from 50mA to 700mA.  Does this means the motor will brake the transistor?

The components I am using are:

Transistor NPN BC547B - http://www.coolcomponents.co.uk/catalog/product_info.php?cPath=50_75&products_id=327

Motor - Solarbotics GM9 90 Degree - http://www.technobotsonline.com/solarbotics-gm9-90-degree.html


I see 3 "problems" with your design:

  • the BC547 is a signal transistor, yes it will blow. Go for a power darlington transistor, still NPN. One or two amps.
  • you should have a current limiting resistor between D0 and base. A rule of thumb is 1kohm, bit this really depends on the amplification (HFE) in your transistor.
  • yes a diode to short reverse spikes then stopping your motor will be a good idea. There seems to be two common ways to place that diode, either between emitter and collector on the transistor or across the two poles of your motor. I usually does it across the motor. pick a 1Amp shottky diode here. Connect it in the blocking direction, with the "ring" closest to your 6V battery.

A little background.

What Jassper said about emitting and collecting electrons is correct, but only for NPNs, not for PNPs, since they are wired electrically reverse to an NPN.

For why these terms were picked, we have to go back to the 1940s & 1950s. In those days it was common to think, not of electrons flowing from negative to positive, but electrons leap-frogging, producing the effect that holes (positive charges) flow from positive to negative. In reality, current flow is made up of flow in both directions, but it does not matter in practice as long as you are consistant within your own minds as to how you think of the current flowing.

[ "Hole flow" is the predominant effect, though. It is practically impossible for the same electron to make it all the way from one end to the other through all those atoms making up the wires in a circuit. Normally, an electron jumps from one atom to another, leaving a hole or positive charge in the atom it left. This in turn captures another electron from yet another atom, leaving a hole there. In this sense it is the holes or positive charges that "flow".]

Since I am a 1940s kid, I (originally) learned it as current always flowing from positive to negative, and also the first transistors were made as PNPs. NPNs were a bit harder to make at first, until the understanding of what was actually happening began to be understood better. That is why the arrows on transistors and diodes point the direction they do, from positive to negative. (If you are one of those who only think of electrons moving from negative to positive, then you must always consider the current is moving reverse to the way the arrows point. Consequently, even though advanced electronics has taught me that current is flowing in both directions at once (holes from plus to minus and electrons from minus to plus) I still find it easier to think in terms of positive to negative flow, so the arrows always point the right direction for me.

As I said, though, being consistant within your own thoughts is the key to understanding.

Now the first transistors started with a single germanium crystal, which they labeled the 'base' because it was physically the base of the device. The other two leads were made from tiny wire contacts (called "cat's whiskers" in those days) touching spots on the crystal. These were originally placed by hand, since touching different spots on the crystal gave better or poorer contacts & current flow. Next, they found that if they tried to pass current from one of the point-contacts to the other one, this current could be controlled --increased or decreased -- by a slight change in the voltage (ie the field charge) of the base crystal, since for the current to pass from one contact to the other, it had to go through the base to get to its destination.

As to calling either the emitter or the collector 'ground', this is an incorrect view, since it is possible to have different basic configurations with emitter grounded, collector grounded, or even base grounded. These three types of circuits give different output results, different amplification factors, different output impedances and have different input impedances, and so on.

I was going to draw out some different circuits, but this is a deep subject, so instead, I will leave that for your further study. You just need to find a good, comprehensive book (or webpage) explaining transistor theory.

I will leave you with this thought, though. If there is still a way I can be of assistance in explaining how transistors work, I will try to give a further answer.

The way I see it (and I'm sure to be corrected) is simple:

The Collector "Collects" electrons

The Emitter "Emits" electrons

The Base controls the electron flow between the Emitter and Collector. Like a "Military base Unit" controlling where the jets go and how many.


Yeah, I like it.  The military base thing makes sense. 

Because it's such a crazy idea, I will probably remember it!

OK, watched it. I'll be sure to go though all the videos over the next few days.

I still don't really understand the names though.  Take the emitter; wouldn't it have been simpler to name that ground?

maybe its because if u actually ground the emitter, it would render the transistor moot.
the emitter is the buisness end of the transistor, the thing you connect to whatever it needs
to manipulate the power for.

also you cannot rely on a transistor to ground your circuit, because it does not make a connection
all the time.
it switches on and off, and having it as the only ground, that would mean that when it is switched off,
it will break the circuit.

also where the power goes in, it does nothing to 'power' the device.
a transistor doesn't need power to operate unless and until it is being used,
unlike an IC which needs to actually needs to have power all the time in
order to function.



Here are a couple pages that may help you study up on transistors a bit:

http://www.mayothi.com/transistors.html <<This page is "mostly" correct, except for the bottom of the page where it says:

"Also note that the emitter is always tied to the fixed voltage (+12V or GND) when using transistors as switches."

The use of the word "always" makes this an untrue statement. They should have said, "usually" or "commonly".



Grounding the emitter makes the transistor moot? I can't figure what you mean there. There are many circuits where the emitter is grounded.

"the emitter is the buisness end of the transistor, the thing you connect to whatever it needs to manipulate the power for." --- Actually, the emitter is rarely used as an output. The only place it is used is in the common collector circuit, which is rarely employed in common usage.

"also you cannot rely on a transistor to ground your circuit, because it does not make a connection all the time. it switches on and off, and having it as the only ground, that would mean that when it is switched off, it will break the circuit."

Actually, in audio circuits a transistor is on all the time and never switches off but rather operates in the middle of its range. In switching circuits, however, it is the point of the circuit to turn things on and off. You want the circuit to allow or stop current flow to some device. (LED, motor, etc.) That is the point of a switch.

"also where the power goes in, it does nothing to 'power' the device." --I have no idea what you mean by this statement. sorry.

"a transistor doesn't need power to operate unless and until it is being used, " --This statement is true. Just like light bulbs, motors, IC chips or electric hot-plates, they do not need power if you are not using them.

"unlike an IC which needs to actually needs to have power all the time in order to function." Sorry, this is not exactly correct. If an IC chip is not being used, it can be turned off. Certain types of dynamic memories must remain under power so they do not lose their stored information, but any other ICs can be turned off at will.

well the point is that the emitter does not = ground, and what i said was
"you cannot rely on a transistor to ground your circuit"
by which i meant completing the circuit.

like in the diagram Garath posted, what completes the circuit is the line going to the capacitor.
if you take that away the circuit would be broken wouldn't it?

"I have no idea what you mean by this statement. sorry."
never mind, brainfart on my part there.

what i mean with ic's needing power is that they allow power to pass trough when not in use,
i was thinking of logic chips which have power going trough them all the time, but only do things
when they get a signal on an input pin.
as opposed to a transistor which is actually operated by current instead of signals.
so an IC actually has a ground pin, which could conceivably be the only thing connected to
ground, without having a broken circuit.

lol am i getting any closer?

anyhoo, thxz for the links, that electronics club one looks esepcially sweet.
in the list of transistors on there, which would you say are most similar to 3904/3906 ?