Author Topic: mosfets???  (Read 2606 times)

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Offline cloneTopic starter

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mosfets???
« on: February 18, 2009, 12:35:29 AM »
ok.. so i am trying to understand the BASIC operation of a mosfet and what it is used for.

can anyone offer any help??? i've tried to google it but dont come up with anything i can make sense of.. there doesnt seem to be a simple explanation for it liek ther is with resistors and diodes and the like..

please help!!

Offline Trumpkin

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Re: mosfets???
« Reply #1 on: February 18, 2009, 09:27:09 AM »
From what I understand, it's like a transistor that can handle more current.
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Offline Soeren

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Re: mosfets???
« Reply #2 on: February 18, 2009, 04:38:51 PM »
Hi,

A MOSFET (not mosfet, it's an acronym and should be spelled in capitals). is a transistor, Metal Oxide Semiconductor, Field Effect Transistor.
What you refer to as just a transistor, is a BjT (Bipolar junction Transistor).
Both types comes in small signal units as well as high power units.
Regards,
Søren

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Offline Webbot

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Re: mosfets???
« Reply #3 on: February 18, 2009, 05:23:58 PM »
Presume you have Wiki'd http://en.wikipedia.org/wiki/Mosfet
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Offline cloneTopic starter

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Re: mosfets???
« Reply #4 on: February 18, 2009, 08:51:41 PM »
yes i wiki'd it... and it doesnt really say what it does.. well it might, but its swallowed up in alot of other technical mumbo

Offline Webbot

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Re: mosfets???
« Reply #5 on: February 19, 2009, 02:45:46 PM »
Ok in very simple terms then. A Power MOSFET (looks like a voltage regulator ie has a heatsink plate on the back) can be thought of as an on/off switch but, unlike smaller transistors, can handle much larger current.
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Offline ArcMan

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Re: mosfets???
« Reply #6 on: February 19, 2009, 03:26:16 PM »
Right.  Also, the MOSFET is turned on by applying voltage to the gate, whereas a BJT is turned on by passing current throught the base lead.  Almost no current is need to hold a MOSFET on.  That's the nature of a FET (Field Effect Transistor).

Offline cloneTopic starter

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Re: mosfets???
« Reply #7 on: February 19, 2009, 08:16:07 PM »
oh ok i get it now

Offline Metal Slug 2

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Re: mosfets???
« Reply #8 on: February 19, 2009, 08:49:13 PM »
Hello,

     Where would I be able to pick up a couple of 'general purpose' MOSFETs? (any chance for a sample?)  I intend to use one as a relay to power a 6v DC motor which runs on 4 AA batteries, which in turn will be controlled via my $50 robot board...where can i find schematics on how to wire the MOSFET up? ???  I have looked but my efforts have been fruitless...... :P

Thanks. :)

Offline Webbot

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Re: mosfets???
« Reply #9 on: February 19, 2009, 09:14:18 PM »
Hello,

     Where would I be able to pick up a couple of 'general purpose' MOSFETs? (any chance for a sample?)  I intend to use one as a relay to power a 6v DC motor which runs on 4 AA batteries, which in turn will be controlled via my $50 robot board...where can i find schematics on how to wire the MOSFET up? ???  I have looked but my efforts have been fruitless...... :P

Thanks. :)

One Power MOSFET will only allow the motor to spin in one direction. If you want the motor to go backwards as well as forwards you will need to build an H-Bridge. Search the forum, and google,  for schematics.
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Offline Metal Slug 2

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Re: mosfets???
« Reply #10 on: February 19, 2009, 09:51:36 PM »
I only need the motor to spin in one direction, so no h-bridge will be required.  I am just having trouble finding the proper MOSFET for my intended use...when i search i end up with a ton of different types of MOSFETS along with a bunch of technical specs which i can only minimally understand.... :-\

Offline cloneTopic starter

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Re: mosfets???
« Reply #11 on: February 19, 2009, 09:53:05 PM »
lol ya metal slug that was my problem too..

Offline ArcMan

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Re: mosfets???
« Reply #12 on: February 20, 2009, 03:06:04 PM »
OK.  I know what you mean.  There's a million MOSFETs out there.  I use the HRF3205 N-channel MOSFET in my H-bridge design.  Quite popular.  Just connect the MOSFET between your motor and ground (MOSFET Drain to motor, MOSFET Source to ground).  Connect the other motor lead to Positive.  Connect the MOSFET gate to your controlling source.  Thing is, you need at least 8V Vgs to turn the MOSFET all the way on, so you'll need an NPN transistor between the micro and the MOSFET.

Offline Metal Slug 2

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Re: mosfets???
« Reply #13 on: February 20, 2009, 04:17:49 PM »
OK.  I know what you mean.  There's a million MOSFETs out there.  I use the HRF3205 N-channel MOSFET in my H-bridge design.  Quite popular.  Just connect the MOSFET between your motor and ground (MOSFET Drain to motor, MOSFET Source to ground).  Connect the other motor lead to Positive.  Connect the MOSFET gate to your controlling source.  Thing is, you need at least 8V Vgs to turn the MOSFET all the way on, so you'll need an NPN transistor between the micro and the MOSFET.

Awesome, Thanks! ;D

Its just to bad that only 4 days ago I ordered stuff from digikey....now im gonna have to pay for shipping again, there goes 8 bucks out the window.  ArcMan, would you by any chance know which NPN transistor i should pickup also? (from Digikey...they have about 6000 different kinds....)

Thanks.
« Last Edit: February 20, 2009, 04:23:21 PM by Metal Slug 2 »

Offline Webbot

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Re: mosfets???
« Reply #14 on: February 20, 2009, 06:43:40 PM »
For just everyday (low current stuff) I tend to use a 2N2222
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Offline ArcMan

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Re: mosfets???
« Reply #15 on: February 21, 2009, 11:13:50 AM »
Yep.  A 2N2222 or 2N3904 will do the job just fine.  Use a common-emitter configuration to boost the output voltage of the micro.  Google common-emmiter transistor circuit if you're not familiar with this.
What parts did you order?  They may work just fine.  Just about any N-channel enhancement-mode MOSFET would work in the circuit I describe.
« Last Edit: February 21, 2009, 01:18:57 PM by ArcMan »

Offline Razor Concepts

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Re: mosfets???
« Reply #16 on: February 21, 2009, 11:16:18 AM »
While you're at it, get a bunch of the 2n2222, 2n3904 and 2n3906 transistors - they will come in handy as they are the most common transistors.

Offline Soeren

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Re: mosfets???
« Reply #17 on: February 21, 2009, 02:10:02 PM »
Hi,

Where would I be able to pick up a couple of 'general purpose' MOSFETs? (any chance for a sample?)


The FDP8870 from Fairchild is not really a general purpose device, but a good choice for your app. whatever size your motor is.
It handles up to 30V max. and a current of up to 156A with a R_ds_on of  4.1mΩ (i.e. 0.0041Ω).
You should be able to get a few samples if you use a proper email (don't use Hotmail, Gmail etc.)

http://www.fairchildsemi.com/pf/FD/FDP8870.html
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Metal Slug 2

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Re: mosfets???
« Reply #18 on: February 21, 2009, 09:14:31 PM »
Ive been searching and Digikey is out of stock on the 2n2222 transistors so i am going with mouser this time...

I looked up the 2n2222, and 2n3904 (I have a pack of 2n3906's), but im not sure which transistors are the right ones..... :-\

http://ca.mouser.com/Search/Refine.aspx?Keyword=2N2222

http://ca.mouser.com/Search/Refine.aspx?N=254459&Keyword=2n3904&FS=True

Hi,

Where would I be able to pick up a couple of 'general purpose' MOSFETs? (any chance for a sample?)


The FDP8870 from Fairchild is not really a general purpose device, but a good choice for your app. whatever size your motor is.
It handles up to 30V max. and a current of up to 156A with a R_ds_on of  4.1mΩ (i.e. 0.0041Ω).
You should be able to get a few samples if you use a proper email (don't use Hotmail, Gmail etc.)

http://www.fairchildsemi.com/pf/FD/FDP8870.html



Thanks for the tip, but I think ill just buy the MOSFET.  Ill get a couple of these MOSFETS and a few of what ArcMan suggested.

Thanks. :)

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