Switching loads from an ESP32 or Arduino with a MOSFET

The standard circuit: a low-side N-channel MOSFET

For DC loads up to a few amps, the switch goes between the load and ground: the load sits between the supply’s + and the MOSFET’s drain, the source goes to ground, and the pin drives the gate. This is called low-side switching, and it lets a 3.3 V pin control a 12 V or 24 V load, because the gate voltage is measured from the source, which is at ground.

Gate resistor and pull-down as in our transistor switch calculator. Use a pin that is not a strapping pin (see the mistakes below).

Choosing a logic-level MOSFET

One line in the datasheet decides whether a MOSFET works from your pin: RDS(on) maximum at a stated VGS. The threshold voltage VGS(th) is not the voltage at which the MOSFET is on: it is where it barely starts to conduct, at 250 µA or 1 mA. Between the threshold and the lowest VGS with a guaranteed RDS(on), the datasheet promises nothing.

• From an ESP32 (3.3 V): pick a MOSFET with RDS(on) guaranteed at 2.5 V. Espressif specifies the high output level as at least 0.8 × VDD with the pin sourcing current; a gate draws almost nothing once charged, so it sees close to 3.3 V, but 2.5 V leaves margin.
• From an Arduino Uno or Nano (5 V): RDS(on) guaranteed at 4.5 V (or 4 V) is enough.
• “Logic level” in a name or a shop listing is not a guarantee. The IRLZ44N, sold as logic-level for decades, is guaranteed from 4 V: fine at 5 V, not at 3.3 V.

Maximum values at 25 °C from the makers’ datasheets. The 3.3 V currents keep a SOT-23 near a quarter of a watt, for a small board (see the heat section); the 5 V currents are the highest our calculator allows in free air at 25 °C, with 25 °C of margin below the maximum junction temperature, as on its part pages. AOS lists the AO3400A and AOD4184A as in full production, Infineon the IRLZ44N as active. Examples, not endorsements. On any alternative check RDS(on) at or below your gate voltage, VDS with margin over the supply (30 V for 12 V is common), VGS max, and the package’s thermal resistance. Parts sold under the same name by second-source makers have their own datasheets: read the one for the part you have.

Heat: conduction loss and the package

A MOSFET that is fully on behaves like a small resistor. It dissipates P = I² × RDS(on), and the junction warms by P × θJA above the air around it. RDS(on) also rises with temperature, to about 1.5 times at 125 °C, so a hot MOSFET gets hotter; our calculator raises it with the temperature until the two agree.

P = I² × RDS(on)(Tj) + P(switching)
Tj = Ta + P × θJA

An AO3400A carrying 2 A from a 3.3 V pin: 2² × 0.048 = 0.19 W at 25 °C, a little more once warm. With 5 kHz PWM it dissipates 0.26 W in all and its junction reaches about 58 °C: comfortable. The Si2302CDS in the same spot reaches about 104 °C: within its rating, but too hot to leave inside a box. The SOT-23 figures assume the makers’ test boards with a patch of copper on the drain; on a bare footprint they run hotter.

• SOT-23: on a small module, with less copper than that, keep it to about a quarter of a watt: about 2 A for an AO3400A, 2.2 A for an IRLML6344, 1.5 A for an Si2302CDS. Our calculator’s part pages use the makers’ figures and allow more (3.4 A for the AO3400A).
• TO-220 in free air: about 62 °C/W, so 1 W raises the junction by about 60 °C. Above 1.5 to 2 W add a heatsink; the calculator gives the heatsink’s °C/W.
• TO-252 (DPAK): it uses the board as its heatsink: AOS gives 55 °C/W for the AOD4184A on a square inch of copper.

Pick the board, the MOSFET and the load: the calculator gives the verdict, the heat, the junction temperature, the diode and better options when a part doesn’t fit.Open the transistor switch calculator

PWM: frequency, gate charge and switching loss

LED dimming: any frequency above about 1.25 kHz avoids visible flicker. On the original ESP32, ledcAttach(pin, 5000, 13) gives 5 kHz with 13 bits of resolution (Arduino-ESP32 3.x); analogWrite() uses 1 kHz and 8 bits. An Arduino Uno’s analogWrite() runs at 490 Hz on pins 3, 9, 10 and 11 and 980 Hz on pins 5 and 6, with 8 bits.

Motors and pumps: the coil smooths the current, but frequencies in the audible range whine. About 20 kHz is above hearing: on an ESP32 that leaves 11 bits; on an Uno, Timer1 on pin 9 or 10 reaches 20 kHz with TOP = 799, about 9.6 bits.

Switching loss. At every edge the MOSFET passes through a half-on state, and the pin moves the gate charge Qg with only about 20 mA. An IRLZ44N (48 nC) driven from an Uno pin takes up to about 2.4 µs per edge. At 490 Hz that costs at most 42 mW; at 20 kHz, with a 3 A motor, up to 1.73 W, enough for the calculator to ask for a heatsink. Both are upper bounds: the calculator counts the whole Qg, while the half-on state lasts through only part of it. The real loss is lower, but at 20 kHz still more than the conduction loss. Fast PWM wants a MOSFET with a small gate charge, or a gate driver.

Pick the chip and the frequency: the calculator gives the duty resolution, the exact frequency and the code for Arduino, ESPHome and the SDKs.Open the PWM calculator

When to add a gate driver

Two cases call for one: a MOSFET that is not guaranteed at your pin’s voltage but suits the load (a 3 A motor from a 3.3 V ESP32), or PWM fast enough that a pin can’t move the gate charge quickly. A gate driver takes the logic signal and drives the gate with an amp or more from a 10 to 12 V supply. For the 3 A motor at 20 kHz from an ESP32, our calculator finds that an IRLZ44N driven at 12 V by a gate driver dissipates 0.52 W and stays near 57 °C.

From Microchip’s and TI’s product pages. On any alternative check that the input threshold is below your logic high, that the supply range covers the gate voltage you want, and keep the gate below 80 % of the MOSFET’s VGS maximum: 12 V suits a ±20 V gate, not a ±12 V one.

A bipolar transistor can stand in for a slow job: a small NPN that pulls the gate to ground, with a resistor pulling it up to 12 V, gives a 12 V gate swing from a 3.3 V pin. It turns the MOSFET on only as fast as the pull-up resistor charges the gate, so it suits on/off switching and low PWM frequencies, not 20 kHz. It also inverts the logic. While the pin is low or floating (during reset, boot and flashing, or with the board unpowered), the pull-up turns the MOSFET on and the load runs: the opposite of what the 10 kΩ pull-down does in the standard circuit. For a load that must stay off by default, add a second NPN stage to invert the logic back, or use a non-inverting gate driver such as the TC4427 with 10 kΩ from its input to ground.

High-side switching with a P-channel MOSFET

Some loads must keep their ground: a module whose ground is shared with the microcontroller through a data line, or a device you want to cut from its positive supply. Then the switch goes on the + side. A P-channel MOSFET sits with its source on the supply; a resistor from gate to source keeps it off, and pulling the gate down turns it on. A 3.3 V pin can’t do that directly on a 12 V rail (its high level is still 8.7 V below the source, so the MOSFET never turns off), so a small NPN does the pulling.

With an AO3401A on 12 V and 1 A, the calculator keeps the gate at about 9 V with that divider, and the MOSFET dissipates 62 mW (open the example). The logic is not inverted: pin high, load on. Any small NPN works as the level shifter: 2N3904, BC547 or an SMD MMBT3904.

From the makers’ datasheets (AOS lists the AOD4185 in full production). A P-channel MOSFET is never a “logic-level” question here: the gate swing comes from the load supply, so check VGS max against that supply.

Typical loads, and which diode

Diodes as our calculator picks them: 1N4148 (75 V, 150 mA average), 1N4007 (1000 V, 1 A), 1N5819 Schottky (40 V, 1 A), SS34 Schottky (40 V, 3 A). With PWM the diode conducts on every cycle, so use a Schottky rated for the load current.

Size a motor’s MOSFET and diode for the stall current, not the running current: check the motor’s datasheet, or measure the winding resistance and divide the supply voltage by it. A plain diode across a solenoid or relay makes it release a little more slowly; if the release time matters, the valve or relay maker’s application notes give the alternatives.

The IRF520 module trap, and better modules

The common red “IRF520 MOSFET driver module” connects its signal pin almost straight to the gate of an IRF520. That part’s threshold can be up to 4 V and its RDS(on) is guaranteed only at 10 V. From an ESP32 it may not turn on at all (our calculator says it stays off); from an Arduino it conducts only partly (not guaranteed at 5 V) and heats up with a few amps.

Modules built around the AOD4184A (often sold as “D4184” modules) are a real improvement from 5 V: 9.5 mΩ guaranteed at 4.5 V. From 3.3 V they are not guaranteed either, since the AOD4184A’s threshold goes up to 2.6 V and no RDS(on) is given below 4.5 V. Whatever the listing says, read the marking on the MOSFET and look it up.

Worked examples

A. A 12 V LED strip, 2 A, dimmed by an ESP32

AO3400A on GPIO25 with 100 Ω and 10 kΩ, no diode, PWM at 5 kHz with 13 bits. The calculator: 0.26 W of heat (42 mW of it from switching), junction about 58 °C at 25 °C ambient, 109 mV lost across the MOSFET. An IRLML6344 in the same place: 0.2 W, 45 °C. In the code, 4096 of 8192 is 50 %.

const int STRIP = 25;

void setup() {
  ledcAttach(STRIP, 5000, 13);
}

void loop() {
  ledcWrite(STRIP, 4096);
}

Open this example in the transistor switch calculator, then change the strip current or the PWM frequency.Example A in the calculator

B. A 12 V pump, 3 A, from an Arduino Uno

IRLZ44N on pin 9 with 100 Ω and 10 kΩ, an SS34 Schottky across the pump, analogWrite() at 490 Hz. The calculator: 0.29 W, at most 42 mW of it from switching, about 43 °C, no heatsink needed. An AOD4184A in DPAK, entered as a custom part, gives 88 mW of conduction loss: custom parts carry no gate charge, and its 17 nC (maximum at 4.5 V) adds at most 15 mW at 490 Hz. The same IRLZ44N from an ESP32 is not guaranteed: use a gate driver there, or an AO3400A if the pump draws under 2 A.

Open example B: Arduino Uno, IRLZ44N, 3 A pump at 490 Hz.Example B in the calculator

C. A 12 V solenoid valve from an ESP32

The valve’s coil measures 24 Ω, so it draws 0.5 A and 6 W at 12 V. An AO3400A with a 1N4007 across the coil, switched on and off (no PWM): 12 mW in the MOSFET, essentially cold. A 12 V relay coil of about 70 mA is even easier: the calculator picks a 1N4148 for it.

Open example C: ESP32, AO3400A, 0.5 A valve with its diode.Example C in the calculator

Common mistakes

• No common ground. The supply’s − must connect to the board’s GND, or the gate voltage means nothing.
• A floating gate at boot. Without the 10 kΩ pull-down, the load can twitch on at power-up or during flashing, while the pin is still an input.
• A strapping pin. On an ESP32, a 10 kΩ pull-down on GPIO0 holds it low at reset and the board starts in download mode; GPIO2, 5, 12 and 15 are strapping pins too, and some boards show activity on GPIO5, 14 and 15 during boot. GPIO4, 13, 16 to 19, 21 to 23, 25 to 27, 32 and 33 are safer choices (not 16 and 17 on WROVER modules, which use them for PSRAM), as our ESP32 pinout shows.
• A missing diode on a motor, pump, valve or relay. The voltage spike at turn-off can exceed the MOSFET’s VDS and destroy it, often after working for a while.
• Trusting VGS(th). A threshold of 1 to 2 V doesn’t make a MOSFET work at 3.3 V; RDS(on) at your gate voltage does.
• A MOSFET on the high side driven straight from the pin. An N-channel on the + side needs a gate above the supply; a P-channel on 12 V never turns off from 3.3 V.
• Powering the load through the board. The dev board’s 5 V pin and its traces are not meant for amps: wire the load supply straight to the load and the MOSFET.

Safety

• Never switch mains with these circuits. The MOSFETs here are for low-voltage DC. For 230 V or 120 V loads use a certified relay module, a certified solid-state relay or a ready-made smart plug, and leave mains wiring to someone qualified where your rules require it.
• Fuse the low-voltage supply. A 12 V supply that can give 10 A will push it into a short; a fuse sized for the wire protects it.
• Heat. A TO-220 at 2 W in free air runs well above 100 °C at the junction and burns fingers at the tab; mount it on a heatsink and keep it away from plastic.
• Stall and inrush. Motors, pumps and capacitive loads draw several times their running current at start: size the supply, the wire and the MOSFET for it.