Electronics > Beginners
Misunderstanding Mosfets - Solenoid Driver?
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kazzle101:
Hello
I want check I'm doing this right, and that I'm not being daft thinking the smaller SOT-23 mosfet will be suitable.

I am wanting to drive a few 12V 2.5W solenoids using an Arduino clone - Teensy 3.2, that runs 3v3 logic. The solenoid draws about 420mA at 12V when on.

Looking trough my box of stuff I have found two N-Mosfets that seem to do what I want:
SI2338: Max: 30V at 6A in a SOT-23 package
STD25NF: Max: 100V at 25A in a D-PAK package

Now I've always believed that to switch the transistor fully on, I should look for the Gate Threshold Voltage Vgs(th) and that this should be below that being used on the Gate - the 3.3V logic. For the SI2338 this is Min:1.2V, Max:2.5V and the STD25 Min:2V, Max:4V

I've made up the circuit on breadboard as in the badly drawn diagram, and switched between the transistors. Both appear to perform equally well, the solenoid clacks on and off as expected, nothing gets warm and on my cheap oscilloscope the signal looks square, I can't see any ramping.

Am I OK to use the SI2338? Or have I missed something?

Datasheets:
SI2338: https://www.vishay.com/docs/67877/si2338ds.pdf
STD25NF: http://www.farnell.com/datasheets/1689940.pdf

Thanks
Karl.
aheid:
The SI2338 should be fine. You'll want a max threshold voltage that's significantly lower than your drive voltage. Since you only need it to pass some ~0.5A, you can cut a bit closer.

Look at the first plot on the page of plots, I_D vs V_DS. At 3V V_GS it's fairly limited how well it conducts, while the higher V_GS values cause it to saturate at far higher drain currents.

Compare with the same plot for STD25 (figure 3). The lowest curve is for 4V V_GS and shows it saturating at less than 2A. Given that you'd drive it at 3.3V you can see how marginal that would be.
Psi:
Looks fine to me too
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