# Understanding n-MOSFET specs

I'm using an FQP30N06L to dim a 1W LED being driven by a constant current supply. This MOSFET has a $V_{GS}$ threshold of 2.5V, which is why I picked it since I'm driving it from a 3.3V PWM pin on an Arduino. It works fine when the drain-source voltage is under 6V or so. When it gets to 9V, it stops turning on. I always understood N-channel MOSFETS to be the way to go in this scenario precisely because they can be driven from a low voltage relative to drain (as opposed to having to drive down from a high one for a P-channel). What am I missing? What parameter should I be looking at to understand?

$V_{GS}$ chart for reference:

Schematic of how I'm using it for reference:

simulate this circuit – Schematic created using CircuitLab

In the schematic above, M1 is getting stuck off, but I can turn it on by bringing Gate to Drain

You need resistors in series with the LEDs.

That is not a 3.3V logic-level MOSFET and cannot reliably be driven fully on with 3.3V. It is specified at 5 and 10V so it's best to drive it with at least 5V. If you don't have 5V available, you might want to buy a different MOSFET.

All Vgs(th) guarantees you is that with 2.5V you'll have at least 250uA flowing with gate tied to drain.

Rds(on) is guaranteed to be < 45m$\Omega$ with 5V drive at 25'C (a fair bit higher when hot).

Your graph above is a typical characteristic and not something you can depend on except perhaps the general shape.

• sigh. thanks. also - LEDs are 1W, driven off a constant current supply, hence no resistors. Really irritating since SparkFun advertises these as logic level. Trust but verify, I guess. Dec 23, 2017 at 0:13
• @kolosy "LEDs are 1W, driven off a constant current supply" um that can make a huge difference to what the mosfet is going to do to... Dec 23, 2017 at 0:19
• @Trevor care to elaborate? I did mention the 1W part in the post. Will add the CC part. Dec 23, 2017 at 0:30
• @kolosy if you have a constant current driver trying to deliver NmA down that chain you may find discharging the gate drain capacitor rather difficult. Dec 23, 2017 at 0:32
• Well, 5V is logic level, just not yours. The schematic symbol you used is for a 9V constant voltage supply. There is a different symbol for a constant current supply. CC supply may be why you are seeing strange things happen. Dec 23, 2017 at 2:04