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Lesson 13 - DC motor, transistor question
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Lesson 13 - DC motor, transistor question

by chimerea on Tue Oct 22, 2013 9:45 am

Hello all,

Last week, I was reading lesson 13's instructions about DC motors. Being all new in electronics, my mind was triggering questions related on the function of a transistor in an electric circuit. After some studying on how transistors are made and how they work, I still cannot understand the setup of the circuit board.

The one question is coming from this schematic:
http://learn.adafruit.com/assets/2349

If I understand correct, the southern side of the NPN transistor symbol is the emitter side, west is base and north is collector. But why is the emitter connected to the ground? Doesn't it 'reverse bias' the base/emitter junction? It looks like one stream of electrons move from the base to the emitter and the other stream of electrons move from the collector to the emitter?
But considering the following drawing on NPN transistors, it would make more sense if the emitter side was connected to the digital I/O pin and the base to the ground, so electrons move from emitter to base and emitter to collector:

Image

I am obviously missing some information or misunderstanding the basics, but can't figure out what it is.
HELP, I'M LOST!!!

Much appreciations in advance!
chimerea
 
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Re: Lesson 13 - DC motor, transistor question

by adafruit_support_mike on Tue Oct 22, 2013 12:56 pm

Bipolar transistors are some of the hardest components to really understand. There are entire books about just that.

In direct answer to one of your questions: connecting the base to a positive voltage and the emitter to GND forward biases the base-emitter diode in an NPN transistor. The P-side of the diode is positive relative to the N-side. The collector-base diode is the one that's reverse biased.

That probably doesn't get you any closer to understanding how BJTs work though.

A complete explanation of how BJTs work -- even omitting the math -- runs several pages.. I've tried. Rather than dumping all that on you at once, let me break the information into pieces by giving partial answers and asking if you need more background to understand those.

For starters, BJTs work because:

- Each of the two PN junctions is basically a diode.

- The depletion zones for the base-emitter and base-collector diodes overlap.

- The overlapping part of the collector and emitter depletion zones is much larger than the bulk conductive region at the base end of the base-emitter diode.

- The base-emitter diode is forward biased, and thus conductive.

- The base-collector diode is reverse-biased.

- The reverse-biased base-collector diode steals carriers from the emitter before they can get to the base.


If your knowlege of semiconductors is where mine was when I asked the kinds of questions you're asking, you understood about 60% of that.. but the 40% you didn't understand includes most of the key terms, like 'depletion zone'.

So.. let's step back to a slightly less deep-and-complicated question:

How well do you understand diodes, PN junctions, and depletion zones?
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Re: Lesson 13 - DC motor, transistor question

by chimerea on Tue Oct 22, 2013 6:26 pm

Hi Mike,

First off, big thanks for your reply! I used to believe that, by connecting the emitter to the ground pin, the free electrons from the N type emitter would move to the ground, whereas it should move in the opposite direction towards the base and eventually establish an emitter-to-collector electron flow. But when you mentioned positive voltage, I shamefully realized that the ground pin should be considered as a reference point and not an earth pin that just 'dumps' electrons. At least... by looking at it this way would make it more understandable why the NPN was connected like it was in lesson 13's schematics. I have consulted three transistor books that are reasonable OK to begin with and get some basic understanding on PN junctions and isolated/depletion regions, but I guess I need to pay more attention on the very, very basics of electronics first. :oops:
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Re: Lesson 13 - DC motor, transistor question

by adafruit_support_mike on Wed Oct 23, 2013 1:36 am

As I said above, there's a lot to learn before BJTs really begin to make sense. You can use them by memorizing a few basic rules:

- For NPN transistors, the emitter should be closer to GND than the base. For PNP transistors, the emitter should be closer to VCC than the base.

- In normal operation, the voltage at the emitter will be about 0.65v closer to the rail than the voltage at the base.

- The amount of current flowing from collector to emitter will be whatever it takes to make the previous rule true for whatever resistance is between the emitter and the rail.

- The voltage at the collector will be whatever is appropriate for the rule above and whatever resistance is between the collector and the opposite rail.

Learning why all that happens takes a lot more work.
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