Selecting a MOSFET

[jgauthier]

Hello,

I am having some trouble selecting a MOSFET, and maybe I need more than one.
The scenarios is a I have an MCPU that will be switching a GPIO at 3.3V. I originally wanted to switch a MOSFET to actuate a solenoid. The solenoid may be 12V, or it may be 24V. It's not completely decided yet, but I am trying to find a MOSFET capable (at 12V is needs to have and Id=5A, at 24V Id=2.5A). The issue I am running into is that many MOSFETs with a Vgs(th) <= 3.3V also have a much higher Vgs, sometimes 10V, or even upwards of 20V and 30V. My understanding of Vgs is that closer you get to Vgs the more "on" the MOSFET is. But I can only provide 3.3V.

I'm wondering if this is a scenario where I can find one MOSFET to switch 12V to have enough across Vgs on a secondary MOSFET so the MOSFET can be fully activated and the solenoid fully actuated.

Advice welcomed. Thanks!

 

[Bluejets]

Drive your mosfet gate via a small properly biased transistor from the micro 3v3 output.

 

[PETERDECO]

We use IRLZ44N logic level MOSFET for our 3V circuits. A direct replacement is STP55NF06L.

 

[Harald Kapp]

many MOSFETs with a Vgs(th) <= 3.3V also have a much higher Vgs, sometimes 10V, or even upwards of 20V and 30V.

Are you maybe looking at V_GSmax? That is the max, Gate-Source voltage the MOSFET can tolerate without being destroyed. This voltage is not a parameter to be used for designing normal operating conditions.
When you look at e.g. the datasheet of the IRLZ44N as proposed by @PETERDECO , you see that this MOSFET is good for almost I_DS = 30 A at V_GS = 3.3V:

 

[jgauthier]

Thanks everyone. I believe my issues was an understanding that as Vgs was increased so was ID. And while that is true, I need to look at the characteristics graphs to determine a MOSFET will actually fit my needs. I appreciate it, everyone.

 

[Harald Kapp]

as Vgs was increased so was ID

That is true, as you can see from the graphs, but not to more than I_DS = (V_supply-V_DS)/R_load.
Once V_DSis saturated (minimum value reached), increasing V_GS will no longer increase I_DS.