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FETs and pin names

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Gregary Boyles

Tue Apr 28 2015, 02:27PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

One thing about these has perplexed me.

Why is the pin you connect to Vcc called the drain and the pin you connect to GND the source?

It seems opposite of what it should be, i.e. the drain drains the current to GND and the source gets it from Vcc.

Teodor Zafiroski

Tue Apr 28 2015, 02:42PM

Registered Member #48007 Joined: Wed Jun 18 2014, 01:00PM
Location: Prilep, Republic of Macedonia
Posts: 33

The way we think the current flows isn't the way that electrons flow, it is the opposite, so electrons go from a negative to a positive potential, the FET's pin names are given by the direction of electron flow... Hope you understand...

Gregary Boyles

Tue Apr 28 2015, 04:30PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

Teodor Zafiroski wrote ...

The way we think the current flows isn't the way that electrons flow, it is the opposite, so electrons go from a negative to a positive potential, the FET's pin names are given by the direction of electron flow... Hope you understand...

Yeah I understand the difference between conventional current and electron movement, i.e. it was decided when they had no idea what electricity was.

Except that the pin naming with NPN transistors at least seems intuitive to me.

hen918

Tue Apr 28 2015, 05:02PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

You just have to remember it's N-channel, so it sources from the negative.

klugesmith

Tue Apr 28 2015, 05:41PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

... the pin naming with NPN transistors at least seems intuitive to me.

Don't the "main path" pin names for BJT's and MOSFETS sort of match each other?
Electrons flow from emitter to collector, or source to drain, in the "N" devices.
Holes flow from emitter to collector, or source to drain, in the "P" devices.

Yeah I understand the difference between conventional current and electron movement, i.e. it was decided when they had no idea what electricity was.

Not so fast. Electrons were discovered in 1895. Before that there was a complete system of electrical units, the same ones we use in the 21st century. Electric lights, telephones, and streetcars were part of everyday life. Most electric power was not only AC, it was already three-phase. None of those technologies care about how charge moves in conductors, why currents in magnetic fields experience forces, or why magnetic flux changes induce voltages. Neither Maxwell's laws nor electric circuit theory depend on these so-called electrons.

It's good to see this thread discussing electric current in the conventional sense. We need to "flip" and think about electrons only when looking inside certain components, such as the transistors in OP.

Antonio

Wed Apr 29 2015, 01:12AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

The names are really due to the "correct" direction of the current, with electrons moving. The same in an NPN bipolar transistor, where the emitter is the negative terminal and the collector the positive. But really, practically nothing in the science of electricity, or electronics, depends on the existence of electrons. It's actually very difficult do devise and experiment that proves that electrical current is a flow of electrons, and even that electrons exist.

Ash Small

Wed Apr 29 2015, 02:33AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Surely the sputtering inside a vacuum tube is conclusive proof of electrons?

But I agree. for most stuff you don't need to consider them.

Gregary Boyles

Wed Apr 29 2015, 05:39AM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

Antonio wrote ...

The names are really due to the "correct" direction of the current, with electrons moving. The same in an NPN bipolar transistor, where the emitter is the negative terminal and the collector the positive. But really, practically nothing in the science of electricity, or electronics, depends on the existence of electrons. It's actually very difficult do devise and experiment that proves that electrical current is a flow of electrons, and even that electrons exist.

I believe it has been fairly recently discovered that the concept of electrons literally flowing along the wires from - to + is also false. The free electrons in metal atoms don't travel far at all, but rather it is a kind of wave (as in an ocean wave) within the sea of free electrons that flows from - to +.

Eleccentric

Thu Apr 30 2015, 12:15AM

Registered Member #33460 Joined: Tue Aug 27 2013, 06:23PM
Location: Seattle
Posts: 46

It is my understanding that the motions of individual electrons within a DC circuit are more chaotic than is apparent by the seeming smoothness of the current. An example I recall from undergrad physics stated that if an ordinary household lighting circuit were powered by DC, the time it takes any individual electron to pass from the circuit breaker, through the house, and back again is as much as a few hours. The path taken by a single electron involves nearly as much backward motion as it does forward. When the circuit is first energized, there is an electromagnetic wave that propagates down/around the conductor, exerting force on the electrons in the load, and thus delivering power (almost) instantaneously, despite the sluggish pace of the electrons themselves.

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