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Controlling load from 3.3v logic: FET vs BJT vs darlington pair vs H-bridge?

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poodyp:
I've been trying to find the answer to this for several days now but keep running in circles...

I'm working on a project with a 3v motor, a microcontroller running at 3.3v and a battery at 3.7v. Every time I find someone suggesting one solution, I'll search for info and find someone else suggesting a different solution, and I don't know what the "best" one for my situation is.

Will a 2N7000 with a Vgs(th) of 3v max be fully saturated at 3.3v? Will my micro be able to source enough current (25ma) to saturate a transistor?

I'm quite confused at this point. Any help is greatly appreciated.

Psi:
As i read the datasheet.... It says the Vgs(th) is normally 2.1V but could be up to a max of 3V. So at this voltage the fet is able to turn fully on.

However there is a graph in the datasheet that shows that a 3V Vgs(th) will only work for the lowest of drain current.
It looks like at 2A current and 25deg C the Vgs(th) needs to be 8.5V ??

Seems rather high to me

jahonen:
It usually is best to look at specified Rds(on) figures and see if they are specified for the gate-source-voltage one is using. Vgs(th) is the voltage where there starts to be some conduction, in practice you'll need quite a bit more to open the MOSFET fully. But it also depends what are your current requirements?

In this case, it seems that there are no less than 4.5 volts specified so 2n7000 is not perhaps the best for the purpose. For example, FDN327 (120 milliohms @ 1.8 V Vgs) might be better, or, then the BJT.

Regards,
Janne

mikeselectricstuff:
In many cases there are several equally good solutions - depends on the load.
MOSFETs are usually the most efficient as there is no static drive current.
2n7000 is a pretty poor MOSFET - there are zillions with  much better specs, especially when driving from 3.3V.

Zero999:
How much current does the motor need?

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