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I have a circuit that you can see on upverter.

I've been using this for some time and have built a few of these, but I'm hooking one up now and it's not working. I troubleshot and narrowed it down to the fact that the MOSFET is simply not coming on with the 3.3V that is on the GPIO pin on my Raspberry Pi 2. If I give it 5V (manually) it activates fine. I switched the MOSFET out for another (same model) I have and it still fails the same way.

The datasheet says that it should have 2-4V for the GS voltage to turn it on, so I'm expecting 3.3V to work (and I've seen it work a few times). Any idea why it would be requiring 5V?

You can see in the upverter circuit diagram that my load is a little toy monkey that is essentially a DC motor with 2 AA batteries. I run wires to that circuit to effectively turn it on with my RP2.

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The data sheet says that it is guaranteed to start to turn on (250uA) with 2-4V. So there is no guarantee you will not get a unit with high threshold voltage, and even if the threshold voltage happens to be 3.3V, 250uA is not exactly fully turned on for most purposes.

Of course you might get lucky and find a particular unit that was closer to 2V so it sort-of works, but it's really bad practice to depend on luck rather than datasheet guaranteed values. This particular part is only guaranteed at 10V Vgs (0.1 ohm Rds(on)). '

Use a part that is specified at no more than 3.3V (1.8V and 2.5V parts are available) and you'll not have these problems. Most such parts are in surface mount packages, however there are a few in TO220 packages.

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  • \$\begingroup\$ The tip is to look for guaranteed [low] Rdson values like "RDS(ON) = 120 mOhm @ VGS = 1.8 V" for example atop of the "Features" of fairchildsemi.com/datasheets/FD/FDN327N.pdf ST's marketing seems a bit more deficient in outlining such things in their datasheets. \$\endgroup\$ – Fizz Oct 4 '15 at 0:03
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    \$\begingroup\$ The datasheet of another recent/good example (you found) is organized in the same manner "RDS(ON) < 85mΩ (VGS = 1.8V)" aosmd.com/res/data_sheets/AO3414.pdf \$\endgroup\$ – Fizz Oct 4 '15 at 0:10
  • \$\begingroup\$ Actually ST's datasheets might have these too, e.g. st.com/web/en/resource/technical/document/datasheet/… I think they don't put the guaranteed voltage/Rdson combo at the top for the "standard" 10V (or more) parts. \$\endgroup\$ – Fizz Oct 4 '15 at 0:18
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    \$\begingroup\$ That circuit also drops the input voltage down to ~2.5V because of the 10K and 33K resistors. I would reduce the 10K resistor to 1k or less to maximize the drive to the FET. \$\endgroup\$ – Kevin White Oct 4 '15 at 0:36
  • \$\begingroup\$ @KevinWhite, I thought of that. I think because the MOSFET acts as nearly an open then it still ends up dropping most of the voltage. \$\endgroup\$ – Jeremy Foster Oct 4 '15 at 4:11

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