How Transistors Work -- The Learning Circuit 27

I'm fooling around on a breadboard with the GPIO pin from a RPi.

I attach the collector of a standard NPN BJT to the 3.3V power pin of the pi, the emitter to the Ground Pin. But when I command 3.3V out of the GPIO to the base, the transistor stays off.

But when I change the collector to the 5V pin, then the 3.3V from the GPIO pin activates the transistor.

Is there some minimum voltage difference needed between the collector and base in order for the transistor to activate?

Hi Matt,

As Jan says, the base resistor should not be omitted, and there is no minimum voltage between collector and base, so 3.3V can be used too.

These are from the 'output design tips' section here:

Raspberry Pi GPIO Explained

Thank you that link is very helpful! This is the set up I had that would not activate the transistor. I see what I did was not conventional, but why didn't it work?

Check Shabaz' schema 2.

You have 3 issues:

There's a 1K resistor in the emitter path. It is no longer a switch now, but a amplifier with negative feedback.

There is no resistor in the base path (say: 1 KOhm).

There is no series resistor for the LED (say: 330 Ohm)

Hi Matt,

The biasing is altered by the change of topology and resistances. Transistors have an analog behavior that does not extend linearly in every direction and topology, nothing is symmetrical when using them, they are not like mechanical switches which can have parts swapped onto either side etc. They need to be deployed in known patterns (this will be in the video, it's been a while since I've seen it, but I'm sure it is there), otherwise the behaviour changes.

You can see the voltage and current flowing in each of the three legs, to get a very good idea what is occurring, provided you're OK with Ohm's Law and diode drops, by building up this circuit in a simulator, e.g. Falstad. There is an example here, but you can modify it and monitor the behaviour: https://www.falstad.com/circuit/e-transswitch.html

Hi Matt,

Also, a really good book that covers it in an easy yet quite thorough way, is 'The Art of Electronics', either 2nd or 3rd edition is fine (a used 2nd edition may be cheaper, and it's just as good as the third edition). It contains many pattern layouts, all practical, but the authors explain them really well too.

shabaz  wrote:

 

Also, a really good book that covers it in an easy yet quite thorough way, is 'The Art of Electronics', ...

Page 81

image source: my copy. Fair use.

shabaz  wrote:

 

Also, a really good book that covers it in an easy yet quite thorough way, is 'The Art of Electronics', ...

Page 81

image source: my copy. Fair use.