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logic level mosfet

When looking at data sheets, what am I looking for to find mosfets I can drive with a microcontroller without having to use a transistor or other device to switch the FET, the typical ones I find listed that way on ebay are spendy and not to many of those. I know logic level means you can turn it on and off at 5V but differant manufactures use differant terms. Googling this got me some numbers but no real understanding of how to pick this info off the data sheet.

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Let me just say I was not recommending the IRF840 to you Ossippee but using it as an example. It's just one I come across a lot in mains rated products. I've pulled them out of dud appliance controllers and used them for low voltage stuff no problem however.
Probably more suitable in universal low voltage applications is the STP55NF06.  60V 55A. I can buy them for a little more than a buck locally. Probably cheaper in the US or China.
If you look at the datsheet it can easily handle the 55 amps at 5V gs. Thats if you've got something that big you want to switch.

http://arduino.cc/forum/index.php/topic,17215.msg125496.html#msg125496 stay away from IRFxxx's, find yourself some IRLxxxx :)

Merser: the Gate Source threshold is the voltage when the mosfet starts conducting. You only want to let a mosfet conducting at it's lowest Rds(on). A lot of mosfets start to conduct at a low voltage but the resistance/voltage drop of the mosfet will be high. That's the linear region of the mosfet. In that range the mosfet will get super hot in a short time. A lot of the cheap IRFXXX mosfets are like that, like the IRF840. Check the graphics in the datasheet, with a gate voltage of 4-5V it can only switch a little bit of current. They start conducting at +-4V gate voltage but become super hot.

If you heed the graph of Vgs vs drain current then you stay within the safe operating zone for the mosfet. For an Irf840 5Vgs  allows a load of 4 amps. That's not a small amount that is half it's rating.
Of course if you are switching a 50 amp load then you'll need a totally different mosfet and perhaps Chris's suggestion of a driver chip is a good idea.
I'm afraid I'm stuck in a small motor mentality here as by and large most projects on LMR don't use big motors.

I never bothered to look at graphs on data sheet, this has just saved me from many headaches, the visualization made by the graph makes complete sense to me, thanks agian!

 

I mean, more parts, less headaches. Man, I'm tired.

I did a lot of DIY motor drivers (big ones) before I ended up using (and loving) the Wild Thumper Controller. Put simply, MOSFETS and I have had a long history.

Here's my advice: Just get a driver chip. There are a million out there, super cheap, and they solve all your problems and fix everything. I used a TC4427CPA on one of my old driver boards as an example. Might have cost me a buck.

Sure, you are adding a part, but you are trading it for headaches.  

The parameter you are looking for is Gate Source Threshold Voltage. If you look at this datasheet here on page 2 you will see it is 2.5v-3v. On page 1 it gives absolute maximum ratings and the Gate source maximum volts is 20Volts.

I think Bajdi regarding Rds on, 10v is a standard rating but it doesn't mean they won't switch at a lower level. Look at a datsheet for an IRF840 and they omit the 4.5 Vgs rating but the Gate Source threshold is listed at 2-4 volts.

In the datasheet for the Si4900DY I linked to you will see that there is a very small difference of value in on resistance between the 4.5v and 10v ratings for Vgs.

 

Search in the datasheet for Rds(on). This is the resistance of the fet. This resistance varies according to the gate voltage. You only want to drive the mosfet at it's lowest resistance. The datasheet will give the resistance (Rds(on)) at a certain Vgs (gate voltage). When they only give a Rds(on) for a gate voltage of 10V you can't drive it directly from a mcu. Logic level mosfets have a low Rds(on) at a gate voltage of 4.5V.