Electronic – MOSFET choice for 12v LED Strip and 3.3v Logic

3.3vledled stripmicrocontrollermosfet

I'm currently planning a project on a low-cost WiFi-enabled RGB LED
controller and could use some help in looking over my parts list and
general concept.

What I'm wanting to do is a build a relatively low-cost, WiFi-enabled
RGB LED (strip) controller. In the end it should be controlled via an
Android App which is already being developed in my free time.
For the WiFi module I want to use the infamous ESP8266 board, namely an
ESP-07.

We are talking about normal, RGB, LED Strips (5050 SMD in my case) – nothing fancy.

The plan is to flash it with nodemcu firmware (can't provide a link, but googling "nodemcu" will show what it is about, but it doesn't really matter for the sake of the question),
write a simple TCP server implementation which allows to manipulate 3
GPIO pins' duty cycle over the network. I have no problems related to
the software or the programming of either the MCU, the App or anything
else. What I'm unsure about are the electronics involved here. I've come
to understand that I want to use transistors or MOSFETs to drive the LED
Strips 12v using the 3.3v logic of the ESP8266. From what I know, I
should choose a FET that can be switched fast (requirement for PWM) and
has a maximum gate threshold voltage below 3.3 Volts.
Based on these requirements and the availability at my supplier I've
chosen the IRFZ44N
and I want
to wire it directly to the three GPIO outputs of the Microcontroller.

Is
there any reason for this being a bad idea? Should I maybe add a diode?
I don't think so since LEDs are resistive load and not inductive (so
there shouldn't be voltage spikes or any of the likes, but I'm a layman
when it comes to electronics, far more involved in programming..)
Please tell me if there are any pitfalls to this plan and thank you for
taking the time to read through my plan.

Best Answer

The FET you link to is not suitable for your application. Taking a look at the "typical characteristics" table, we see this:

enter image description here

Vgs(th) is listed as between 2 and 4 volts - already it could be higher than our supply voltage, but not especially the "conditions" column on the right: the specification is given with only 250 microamps of current. Your LEDs, presumably, will draw a lot more than that.

To figure out if the FET is suitable, you need to know how much current you plan to draw, then look for the graph plotting "typical output characteristics":

enter image description here

Look for a line corresponding to the gate voltage you plan to use - in this case, there isn't one low enough - and look for where the curve flattens out. If that's at a current level greater than the amount of current you need to supply, you're good. Alternately, look at the "typical transfer characteristics":

enter image description here

This plots maximum current as a function of gate voltage, so you can trace a line from your MCU's gate voltage - 3.3 volts - to determine what the maximum current level at that gate voltage will be. Again, in this case, the plot doesn't actually go low enough to evaluate that on this particular FET.

Keep looking - there are FETs that meet your requirements - and remember that everything depends on how much current you plan to conduct.

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