Reverse Polarity Protection — Four Methods Compared

Reverse polarity protection is one of those things that seems over-engineered until the day you wire something backwards and lose a board. The JST connector that looks symmetrical until you notice it isn't. The barrel jack with the wrong pinout. The XT60 that slipped the wrong way in the dark. These all happen, and they all kill unprotected circuits.

Several methods handle reverse polarity with different tradeoffs. Understanding the differences helps you pick the right approach for your specific design.

Method 1: Series diode

The simplest approach. A diode in series with the positive supply rail — typically a Schottky for lower forward voltage drop. Current only flows when the supply is correctly polarised. With reverse polarity, the diode blocks.

Forward voltage drop: 0.2–0.4V for Schottky. This is essentially free power dissipation at load current. At 1A and 0.3V drop, that's 0.3W of heat in the diode.

Best for: 12V systems where a 0.3V drop is trivial, or any system with plenty of voltage headroom. Low-dropout regulators with tight input-output requirements don't pair well with series diodes.

Method 2: Polarity protection diode in parallel (shunt)

A series fuse plus a diode across the supply in the reverse direction. In normal operation, the diode is reverse-biased and does nothing. With reverse polarity, the diode conducts, creating a near-short that blows the fuse.

This gives a more definitive protection event (fuse blows, circuit disconnects) at the cost of a consumable. No forward voltage drop in normal operation.

Best for: systems where voltage drop is critical and you can accept the fuse-replacement overhead. Commonly used in automotive applications.

Method 3: P-channel MOSFET ideal diode

A P-channel MOSFET in series with the supply, with gate connected to ground via a resistor. With correct polarity, the MOSFET's body diode conducts initially, pulling the source above ground; the gate-source voltage turns the MOSFET on in its forward direction with very low on-resistance (RDS(on) typically 10–50mΩ). Essentially zero voltage drop at normal currents.

With reverse polarity, the gate-source voltage turns the MOSFET off. The body diode is reverse-biased (the drain is now positive relative to source). No current flows.

Power dissipation at 1A with a 50mΩ FET: 0.05W, vs. 0.3W for a Schottky diode. This is the standard approach for battery-powered designs where efficiency matters.

Best for: 3.3V and 5V systems where voltage drop matters, battery-powered designs, anything with significant current.

Method 4: Dedicated ideal diode IC

ICs like the LTC4359 or similar implement the ideal diode function with additional features: faster turn-on, better reverse current blocking, overcurrent limiting, and controlled startup. They drive a series N-channel MOSFET (which has lower RDS(on) than equivalent P-channel) and handle the gate drive automatically.

More complex and more expensive than the discrete P-channel circuit, but superior performance and additional protection features in one package.

Best for: high-current designs (above 5A), designs where you want combined reverse polarity and overvoltage/overcurrent protection in a single component, or professional designs where the discrete circuit's tolerances are insufficient.

Summary: for a 12V system, a Schottky diode is simple and fine. For a 3.3V/5V system with significant current, use a P-channel MOSFET. For high-current or high-precision applications, use an ideal diode IC.