Reverse Polarity Protection Circuit
An arrangement that prevents damage to the circuit if the positive and negative supply terminals are connected the wrong way round. It addresses the most common wiring mistake in the field; on an unprotected power stage, reverse connection leaves permanent damage within seconds.
The source of the damage is the MOSFET body diodes in the H-bridge: with reversed polarity these diodes become forward-biased and open a current path between the supply and ground. The semiconductors can be damaged in the time it takes the fuse to blow.
Three common methods:
- Series diode: simple and cheap, but it constantly drops 0.4–0.7 V. At 15–30 A this means 6–20 W of heat.
- P-channel MOSFET: provides low voltage drop and losses. In 24 V systems a Zener is needed so that the gate-source voltage does not exceed the typical ±20 V limit.
- Relay or fused bridge: mostly on the wiring side, as coarse protection.
Even when electronic protection is present, the safest approach is to check the connection: verify the polarity with a multimeter before tightening the terminals, use different colours for the positive and negative cables, and position the terminal labels so they remain readable after installation.
Context where this term is used: DC Motor Driver Selection Guide