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MOSFET Switch (low-side)

@electrace/mosfet-switch@1.0.0 · CC-BY-4.0
vload_max 30 Vi_max 3 Avctrl 3–5.5 V
The actual schematic inside this block — every part is explained below.

MOSFET Switch (low-side)

THE answer to "how do I control a 12 V thing from a 3.3 V pin." The GPIO moves the gate; the FET does the heavy lifting; the load sits between your supply and sw.

  • Q1 — a logic-level N-channel MOSFET on the load's low side (between load and ground). Gate high → channel on → current flows. The gate draws essentially zero steady current, which is the whole magic: microamps of GPIO effort controlling amps of load.
  • R1 (100 Ω) — the gate isn't free to switch: it's a small capacitor, and charging it instantly rings. The series resistor rounds that edge off.
  • R2 (100 k) — during reset/boot your GPIO floats. Without the pulldown the gate drifts, and a drifting gate means a half-on FET warming itself across your load. This resistor makes "undefined" mean "off."

Why low-side? It's the simple, robust topology: the FET's source sits at ground, so your 3.3 V gate drive is measured against 0 V — full enhancement, no level-shifting tricks. The tradeoff: the load's "ground" terminal isn't true ground while off. When that matters (shared-ground sensors), you need the high-side version.

Exposes: ctrl (GPIO), sw (connect the load's LOW side here; load's high side goes to its supply), gnd.

⚠ The two classic failures: a non-logic-level FET (half-on at 3.3 V → cooks at load current), and an inductive load with no flyback diode (turn-off kick avalanches the FET). The checks flag the second when the load declares itself inductive.

Exposed nets

●ctrlin · signal
●gndin · gnd
●swout · signal

Inside this block

Q1
n-fet-logic
the switch — a logic-level N-FET that pulls the load's low side to ground when the gate goes high
R1
100
gate series resistor — tames the current spike into the gate capacitance and stops ringing
R2
100k
gate pulldown — keeps the FET firmly OFF while the GPIO is floating (reset, boot, unplugged)

Limits & gotchas

⚠vgs.note 0 — Use a LOGIC-LEVEL FET: it must be fully on at your GPIO voltage. A '10 V gate' FET driven from 3.3 V is half-on — it works on the bench and then burns up at real current.
⚠load.note 0 — Switching an inductive load (motor, relay, solenoid)? Add a flyback diode across the load or the inductive kick at turn-off will avalanche the FET.
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