The transistor as a switch

Thread Starter


Joined Dec 31, 1969
Do you have the polarity incorrect here? Current flows from + (long bar of the battery symbol) to - , thus in your example "(a) mechanical switch, (b) NPN transistor switch, (c) PNP transistor switch." the pnp and npn examples are the wrong way round.

Mike G


Joined Mar 24, 2008
You are talking conventional flow, which is actually backwards from reality. It was invented first, so as an old standard it dies hard. Electrons (the negative charge carrier) flow from the negative terminal to positive. The early chapters in the AAC text book explain this.

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Last edited:


Joined Nov 28, 2008
I agree with the first poster.

Current flows from positive to negative because it is a convention adopted earlier.

Convention is convention.

I have no trouble understanding and accepting direction of current flow and electron flow to be opposite of each other.

However, it is misleading to draw arrow of electron flow and refer it as current flow in the associate text of a circuit schematic.




Joined Mar 24, 2008
But convention is wrong in this case, we are talking reality. If you're going to teach something from scratch, you might as well go with physics vs. something invented by Ben Franklin. In many cases it doesn't matter, but in some it does. Why relearn a standard because the original is flawed but used anyhow?