polarity protection

keep your boards safe when power is flipped, plugged wrong, or hot-swapped.

fact

quick idea: if a user can plug it in, they can plug it in backwards. polarity protection makes the mistake non-fatal.

three common patterns

series diode — simplest, small current; costs you some voltage.
fuse + shunt diode — survives reverse briefly while the fuse opens.
“ideal diode” p-mosfet — very low loss; great for battery/usb devices.

quick chooser

supply	current	efficiency	best pick
coin cell / logic rails (≤100 mA)	low	not critical	schottky in series
5 v usb (≤1 a)	medium	medium-high	p-channel mosfet “ideal diode”
2–4s li-ion pack	high	high	p-mosfet (with gate zener) or ideal-diode controller
wall brick + barrel jack	varies	medium	fuse + shunt or p-mosfet
truly idiot-proof input	varies	low	bridge rectifier (~1.2–1.6 v silicon, 0.6–0.8 v schottky)

option 1 — series diode (schottky)

how it works: a diode in series passes forward power and blocks reverse. pick schottky for lower drop (~0.2–0.4 v at small currents).

loss: P ≈ I × V_F (e.g., 0.5 a × 0.3 v ≈ 0.15 w).
check I_avg, surge, and reverse voltage ratings.
good for tiny rails where a ~0.3 v drop is acceptable.

option 2 — fuse + shunt diode

how it works: a reverse-biased diode across the input conducts hard during a reverse connection, forcing current through the series fuse so it opens; normal polarity is unaffected.

caution

size the shunt diode to survive the surge until the fuse opens. ptc/polyfuses may trip slowly at low voltages.

option 3 — “ideal diode” with a p-mosfet

how it works: the body diode conducts at plug-in; then the fet turns on and drops only I² × R_DS(on). reverse input reverse-biases the body diode, so no current flows.

bench notes

orientation matters: source → input, drain → load; body diode points input → load. add a 9–12 v zener gate→source for 9–15 v systems or hot-plug events.

bridge rectifier

how it works: steers either polarity to the right rails using four diodes. the tradeoff is drop through two diodes.

fact

drop ≈ 2 × V_F (silicon ~1.2–1.6 v; schottky ~0.6–0.8 v).

tested patterns (mini cookbook)

5 v usb / 3.3 v logic (≤1 a) — p-mosfet ideal diode

p-fet: R_DS(on) ≤ 30 mΩ, V_DS ≥ 20 v, |V_GS| ≤ 20 v.
gate→source pull-up: ~100 kΩ; optional 100 Ω series in gate.
tvs at input (optional but nice): SMF5.0A.
examples: AO3407A, Si2301CDS, FDN306P; tiny load-switch ICs: TPS229xx; ideal-diode: MAX40200 (low current).

layout tips

try this

place the protector as the first thing after the connector; keep loop area tiny.

wide copper for fuse/shunt paths; short gate traces on mosfets.
add bulk (10–47 µf) and a 0.1 µf close to the load/regulator.

common mistakes

caution

easy pitfall: flipping the p-fet — the body diode will then conduct the wrong way.

no gate pull-up (floating gate).
regular silicon diode on 3.3 v rails → brownouts.
protecting ground instead of + rail (breaks usb/data references).

quick math

P_diode = I × V_F P_fet = I² × R_DS(on)

test plan

bench notes

current-limit your bench supply when testing reverse input; saves fuses and fingers.

power normally; measure drop across the protector.
briefly apply reverse with a current-limited supply; confirm no heat/smoke.
hot-plug several times and scope the load rail for dips/spikes.

parts & keywords

schottky series: SS14, SS24, BAT54, 1N5819
tvs (5 v): SMF5.0A / SMBJ5.0A
p-fets: AO3407A, Si2301CDS, FDN306P
controllers: LTC4412/4413, MAX40200