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Small question about transistors and mosfets


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dyrodium
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Small question about transistors and mosfets

I think I am JUST starting to understand these things, but, before I go out and buy a few to experiment, I would like to check afew things...;

Transistors work by a small current flowing 2 of the legs, causing a larger current to flow (eg, 3v to 9v)

Mosfets Are like transistors, but handle larger currents, and can be used for speed control (somehow!).

I am currently studying my drill controlers for hints on how to make my own.

Any more important info (like what goes where) would be much welcome.

Thanks!
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Post Thu Feb 03, 2005 12:34 pm 
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Philip
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Mosfet info http://homepages.which.net/~paul.hills/SpeedControl/Mosfets.html
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Post Thu Feb 03, 2005 1:14 pm 
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colin



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I think some of your impressions are wrong.

"Transistors work by a small current flowing 2 of the legs, causing a larger current to flow (eg, 3v to 9v) "

I don't even follow that... "2 of the legs"?? "current flow" then you put a voltage in brackets?? that's not current

mosfets don't really handle larger currents...speed control can be done using either, using multiple methods.

I'm to lazy to type up a nice explanation on it...but i know spockie will give you some nice info

Post Thu Feb 03, 2005 7:20 pm 
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Spockie-Tech
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Well, thats a very big field to cover with a brief explanation.. Shocked

He's sort of close.

Transistors are current-controlled amplifiers,
Mosfets are voltage-controlled amplifiers.

with a Transistor (NPN type), the small current flow through the Base-Emitter junction determines how much current will be allowed to flow through the Collector-Emitter junction. Kind of like a water-valve that controls the flow of a large amount of water from a small amount of water flow.

with a Mosfet, the current flow through the Source-Drain junction is controlled by the voltage (electrical equivalent of water *pressure* (not flow)) on the gate, not the current through it.

Just to complicate things a little, since Mosfet gates behave like capacitors, they do require a brief current flow to charge them up or discharge them to the voltage you want, but ultimately, its the voltage that determines the flow through the mosfet.

Mosfets most certainly DO handle larger currents though.

The forward voltage-drop across the Collector-Emitter junction of a conventional Bi-Polar transistor severely limits the maximum current flow it can deal with since the power (heat) dissipation across a transistor is determined by the voltage drop across it multiplied by the current flowing through it.

The dual-junction voltage drop of a transistor means that even at moderate current flows (by our standards) a normal transistor will have a small voltage drop multiplied by a lot of current which means a lot of power lost (turned into heat) and it will cook quickly.

a Mosfet on the other hand has no junction voltage drops and a *very* small resistance when fully conducting, which allows much higher currents to flow through them before they start generating equivalent heat to a bipolar transistor.

The problem is MosFets are a lot trickier to control properly than transistors are.. you need to pay a lot more attention to keeping the voltage on the gate under tight control because of their capacitive characteristics.

Everyone nice and confused now ? Sorry, its not a simple subject..
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Post Thu Feb 03, 2005 8:11 pm 
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dyrodium
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Not too confused Smile . I just wanted more info on the topic so that I can experiment with them and actualy understand alittle about what's going on! I'm going to go out and buy a small flasher circuit now to put the methods into practice, thanks! Very Happy .

By the way, I wanted to know this stuff so that, in the not-so-distant future I can build my own circuits! I wanted help f rom my school, but the curiculum for yr ten electronics is fairly thin, I already know most of it! Very Happy
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Post Thu Feb 03, 2005 8:37 pm 
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dyrodium
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BTW colin, I REALLY didn't have much time to edit that post, so it was fairly inncorrect. 9v is voltage, not current. I think. Laughing
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Post Thu Feb 03, 2005 8:39 pm 
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ffej
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quote:
Originally posted by dyrodium:

By the way, I wanted to know this stuff so that, in the not-so-distant future I can build my own circuits! I wanted help f rom my school, but the curiculum for yr ten electronics is fairly thin, I already know most of it! Very Happy


Electronics in secondary schools is a joke . . . we had 4 different teachers over the space of 2 years, none of which had the slightest idea what was going on . . . 3 of them were woodwork teachers, and one was this crazy guy who spent too much time on the pipe. He would come to class and talk about how we should learn electronic controll of our brains, because all the rest was too easy . . . he also never shut up about base 10 being the downfall of the world, and how we should all have stuck on the mayan base 60 . . . .

One of the projets was to build a circuit of a CMOS IC . . . while everyone else was choosing stupid little gates that were expanded to a few toggle switches, I decided on the 555, and built the whole damn thing on veroboard with surface mount components. After I had it all made at 4am the morning it was due (im always a last minute person . . . ), it refused to work properly, and thanks to the sleep deprivation (and the flux fumes) I just decided to solder a few wires on and chuck a few lables on it, figuring I'd loose a few marks, but still pass. As it turns out, I got 20/20, because, although the teacher wouldnt admit it, he had no idea on how to test it, since it had no toggles or push buttons, or LED's on it . . .

BTW, the syllabus is pretty pathetic too . . .
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Post Thu Feb 03, 2005 10:13 pm 
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bigjimmy



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I know exactly how you guys feel

Post Sun Feb 13, 2005 8:43 pm 
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Ajax
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I was luck in some ways. The teachers I had did know a lot about electronics and I did not know more than them.

But what they had to teach I had taught myself at home.

I took one of my projects in to school and the teacher freaked when he saw 160V DC on the meter. (I was testing a power supply)

All I can suggest is that the TAFE's have some good night classes.

I did a course at Box Hill TAFE, were I built test equipment. Power supply, logic probe, Oscilloscope.

It was one of the best things I have done.
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Post Mon Feb 14, 2005 12:35 am 
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