Switching a motor, LED strip or relay from a microcontroller pin is one of the most common jobs for an N-channel MOSFET — and one of the most common places to pick the wrong part.

Vgs(th) is not the turn-on voltage

The gate threshold voltage, Vgs(th), is the gate voltage at which the MOSFET only just begins to conduct — typically specified at a tiny current such as 250 µA. A part with a 2 to 4 V threshold is not fully on at 4 V.

The figure that matters is Rds(on) at a stated gate voltage. Look in the datasheet for the Rds(on) rows and find the one that matches the gate voltage you actually have.

Logic-level MOSFETs

A logic-level MOSFET has its Rds(on) specified and guaranteed at low gate voltages such as 4.5 V, and often 2.5 V. That is what you need when driving the gate straight from a 5 V or 3.3 V microcontroller pin.

Two examples from our catalog:

  • AO3400A (SOT-23, 30 V): Rds(on) is specified at gate voltages down to 2.5 V. It can be driven directly from 3.3 V logic and suits loads of a few amps.
  • IRFZ44N (TO-220, 55 V, 49 A): Rds(on) is specified at 10 V gate drive, with a 2 to 4 V threshold. From a 5 V pin it is only partly on and will run hot under heavy load; from 3.3 V it is unsuitable. Drive it with a gate driver or a 10 to 12 V gate supply.

Check the heat

When fully on, the MOSFET dissipates:

Power = I² × Rds(on)

At 3 A through 50 mΩ, that is 0.45 W — a lot for a SOT-23 package, fine for a TO-220. Rds(on) also rises as the part heats up, so leave margin.

Selection checklist

  1. Vds rating: at least 1.5 to 2 times your supply voltage. Inductive loads create spikes.
  2. Rds(on) at your gate voltage: not at 10 V if you only have 3.3 V.
  3. Continuous current: well above your load current, after considering heat.
  4. Package: can it shed the power you calculated?
  5. Gate charge: matters if you are switching fast (PWM at tens of kHz). A microcontroller pin can only supply a few milliamps, so high-gate-charge parts switch slowly and heat up.

The supporting components

  • Gate resistor (about 100 Ω) between the pin and the gate, to limit the current spike and damp ringing.
  • Pull-down resistor (about 10 kΩ) from gate to source, so the MOSFET stays off while the microcontroller is resetting.
  • Flyback diode across any inductive load — motors, relays, solenoids — to protect the MOSFET when it switches off.

Browse Transistors and filter by type, voltage, current and package, and pick up the resistors for the gate while you are there.