Reverse polarity protection you'll thank yourself for

Why it matters

You will plug in a power supply backwards. Not today, maybe not this month, but eventually a 12 V barrel jack will meet your board at 3 a.m. and the polarity will be wrong. If you designed in reverse polarity protection, you'll swap the cable and move on. If you didn't, you'll be ordering new ICs.

The cost is a diode or a MOSFET and maybe ten minutes of layout. The return is every board you build after the first one.

Series diode: simple, lossy, good enough

A Schottky diode in series with your positive rail is the fastest path to protection. When the supply is correct, current flows through the diode. When it's backwards, the diode blocks and nothing dies.

The catch is voltage drop. A 1N5819 will eat 0.4 V at 1 A, which matters if you're running a tight LDO headroom budget. It also dissipates power—0.4 W in that example—so you'll want a diode rated for twice your expected current and enough thermal mass to not cook itself.

For low-current designs under 500 mA, this is fine. Pick a Schottky with a forward voltage under 0.5 V at your max current, check the datasheet's thermal resistance, and move on.

P-channel MOSFET: efficient, slightly fussy

If you're pushing more than an amp or your margin is thin, use a P-channel MOSFET as a high-side switch. Connect source to your positive input, drain to your board's supply rail, and gate to ground through a resistor. When polarity is correct, the MOSFET's body diode conducts briefly, pulling the gate below the source and turning the channel on hard. Rdson is low—often under 50 milliohms—so you're losing millivolts instead of half a volt.

When polarity is reversed, the body diode blocks and the MOSFET stays off. Your board sees nothing.

Pick a MOSFET with a gate threshold voltage well below your minimum supply and an Rdson that keeps dissipation under 0.1 W at full load. A 20 V, 30 mOhm part like the DMG2305UX works for most 12 V hobby projects. Add a 100k pull-down on the gate so the MOSFET doesn't float during power-up.

The load-switch IC option

If you want reverse protection, overcurrent limiting, and soft-start in one package, use a load switch IC like the TPS22916. These integrate the MOSFET, gate driver, and often a current limit in a single SOT-23 or DFN. You'll pay a dollar instead of thirty cents, but the layout is cleaner and you get extra features.

This is overkill for a one-off bench supply, but if you're building more than five boards or handing them to other people, the integrated protection is worth it.

Testing

Before you trust it, test it. Power the board normally and measure voltage drop across your protection device. Then reverse the supply and confirm nothing on the board sees voltage. Use a current-limited bench supply for the reverse test so you're not counting on your protection alone.

If you're using a MOSFET, check that the gate voltage swings correctly and that Rdson is close to the datasheet value under load. A bad gate resistor or insufficient Vgs can leave the channel partially on, turning your protection into a heater.

Document it

Your bill of materials should list the protection component and its ratings. If someone rebuilds your board with a different diode or MOSFET, they need to know what you sized it for. Include the expected voltage drop and power dissipation so the next person doesn't replace your 1 A Schottky with a 100 mA part and wonder why it's smoking.

Reverse polarity protection is cheap insurance. Pick the topology that fits your current and margin, test it once, and forget about it until the day it saves you.