Reverse-Polarity Protection Circuits: Series Diode, Schottky, P-FET, and Why Test Pads Sit Outside Them

Most circuits that run off a user-replaceable battery include something to survive the battery going in backwards. Three topologies dominate, and each buys its protection with a different cost. ## Series diode The simplest approach: one diode in series with the positive rail. Correct polarity forward-biases it and power flows; reverse polarity blocks. The cost is the forward voltage drop — roughly 0.7 V for an ordinary silicon diode, which is a serious loss on a 3 V rail, and continuous dissipation of that drop times the load current. A Schottky diode lowers the drop to roughly 0.2–0.4 V at modest currents (rising toward 0.7 V near its rated maximum), which is why Schottkys are the default choice in small battery devices. Schottkys leak more in reverse and tolerate less reverse voltage than silicon diodes, which matters for the blocking case. ## P-channel MOSFET A P-channel MOSFET placed in the positive rail, source to the battery and gate to the load side, protects with far less loss. With correct polarity the gate sits below the source, the channel enhances on, and conduction happens through a channel resistance that can be milliohms — a drop far smaller than any diode. With the battery reversed, the gate and source sit at the same potential, the device stays off, and its intrinsic body diode is reverse-biased and blocks. The trade-off is cost, board area, and a gate that must be protected against overvoltage. This is the standard choice wherever the diode drop or its dissipation is unacceptable. An ideal diode controller IC is the same idea with an active gate driver and a lower drop still. ## Parallel (crowbar) diode A diode across the supply, cathode to positive, does nothing under correct polarity and shorts the reversed supply. It relies on a series fuse or on the source's own limits to clear the fault. It's cheap and drop-free but converts a polarity error into a blown fuse, and it only works when the source can actually be current-limited or fused. ## Two reasons "it has reverse protection" still fails **Placement.** Protection sits between the battery contacts and the supply rail — that is its whole job. Every point downstream of it, including the VCC test pad, programming header, and any exposed pad on the rail, is *unprotected by construction*. Soldering an external supply to a downstream test pad bypasses the protection device entirely and feeds the silicon directly. A meter reading megohms one way across VCC and GND is often the protection diode correctly blocking — while the damage sits behind it. **Energy rating.** A protection part is chosen against an assumed worst-case fault. A Schottky sized for a coin cell's tens of milliamps has no answer for a cell that can push amps; it fails short and then the fault reaches the load anyway. See Battery Fault Current: Why the Cell You Pick Decides Whether a Mistake Is Survivable. The consequences of the protection not covering your connection point are covered in Reverse-Polarity Damage in ICs: The Hot Chip and the Diode-Asymmetry Test. Connector-level versions of the same problem appear in Modular PSU Cables Are Not Interchangeable: A Common Cause of Drive Death.