Which transistor for PWM over ESP 32 for a 3W LED?

Question

I want to use a transistor as a switch for a 3 watt LED (5V, 700mA.) The transistor is triggered/switched via an ESP32.

I want to dim the LED using PWM offered by the ESP32.

I need the transistor because the ESP32 does not deliver enough current (~40mA vs 700mA) on its pins and therefore an external power source must be used.

Furthermore, I have read that MOSFETs are better for fast switching, I suppose PWM is such a use case.

I am pretty new to the microcontroller game and therefore still confused by the sheer amount of different MOSFETs and BJTs.

Answer 1

All that you need to look for in a MOSFET in your case is the threshold voltage, which is the voltage at which the MOSFET starts turning on, and its R_DS(on) resistance which should be low enough not to cause substantial voltage drop and heating of the MOSFET.

You can simply look for "Logic level MOSFETs" which are guaranteed to be fully on at less than 5V on their gate electrode. If your ESP32 output only goes up to 3.3V, just look for the threshold voltage of less than 3V (or less than 2.8V, to be on the safe side).

The R_DS(on) for less than 1A of current can be as high as 100 mΩ without worrying about the MOSFET getting hot, and that's almost any MOSFET with a low enough threshold voltage - most of them will have less than 50 mΩ.